hypothesis example ap bio

1.1 The Science of Biology

Learning objectives.

In this section, you will explore the following questions:

• What are the characteristics shared by the natural sciences?
• What are the steps of the scientific method?

Connection for AP ® courses

Biology is the science that studies living organisms and their interactions with one another and with their environment. The process of science attempts to describe and understand the nature of the universe by rational means. Science has many fields; those fields related to the physical world, including biology, are considered natural sciences. All of the natural sciences follow the laws of chemistry and physics. For example, when studying biology, you must remember living organisms obey the laws of thermodynamics while using free energy and matter from the environment to carry out life processes that are explored in later chapters, such as metabolism and reproduction.

Two types of logical reasoning are used in science: inductive reasoning and deductive reasoning. Inductive reasoning uses particular results to produce general scientific principles. Deductive reasoning uses logical thinking to predict results by applying scientific principles or practices. The scientific method is a step-by-step process that consists of: making observations, defining a problem, posing hypotheses, testing these hypotheses by designing and conducting investigations, and drawing conclusions from data and results. Scientists then communicate their results to the scientific community. Scientific theories are subject to revision as new information is collected.

The content presented in this section supports the Learning Objectives outlined in Big Idea 2 of the AP ® Biology Curriculum Framework. The Learning Objectives merge Essential Knowledge content with one or more of the seven Science Practices. These objectives provide a transparent foundation for the AP ® Biology course, along with inquiry-based laboratory experiences, instructional activities, and AP ® Exam questions.

Teacher Support

Illustrate uses of the scientific method in class. Divide students in groups of four or five and ask them to design experiments to test the existence of connections they have wondered about. Help them decide if they have a working hypothesis that can be tested and falsified. Give examples of hypotheses that are not falsifiable because they are based on subjective assessments. They are neither observable nor measurable. For example, birds like classical music is based on a subjective assessment. Ask if this hypothesis can be modified to become a testable hypothesis. Stress the need for controls and provide examples such as the use of placebos in pharmacology.

Biology is not a collection of facts to be memorized. Biological systems follow the law of physics and chemistry. Give as an example gas laws in chemistry and respiration physiology. Many students come with a 19th century view of natural sciences; each discipline is in its own sphere. Give as an example, bioinformatics which uses organism biology, chemistry, and physics to label DNA with light emitting reporter molecules (Next Generation sequencing). These molecules can then be scanned by light-sensing machinery, allowing huge amounts of information to be gathered on their DNA. Bring to their attention the fact that the analysis of these data is an application of mathematics and computer science.

For more information about next generation sequencing, check out this informative review .

What is biology? In simple terms, biology is the study of life. This is a very broad definition because the scope of biology is vast. Biologists may study anything from the microscopic or submicroscopic view of a cell to ecosystems and the whole living planet ( Figure 1.2 ). Listening to the daily news, you will quickly realize how many aspects of biology are discussed every day. For example, recent news topics include Escherichia coli ( Figure 1.3 ) outbreaks in spinach and Salmonella contamination in peanut butter. On a global scale, many researchers are committed to finding ways to protect the planet, solve environmental issues, and reduce the effects of climate change. All of these diverse endeavors are related to different facets of the discipline of biology.

The Process of Science

Biology is a science, but what exactly is science? What does the study of biology share with other scientific disciplines? Science (from the Latin scientia , meaning “knowledge”) can be defined as knowledge that covers general truths or the operation of general laws, especially when acquired and tested by the scientific method. It becomes clear from this definition that the application of the scientific method plays a major role in science. The scientific method is a method of research with defined steps that include experiments and careful observation.

The steps of the scientific method will be examined in detail later, but one of the most important aspects of this method is the testing of hypotheses by means of repeatable experiments. A hypothesis is a suggested explanation for an event, which can be tested. Although using the scientific method is inherent to science, it is inadequate in determining what science is. This is because it is relatively easy to apply the scientific method to disciplines such as physics and chemistry, but when it comes to disciplines like archaeology, psychology, and geology, the scientific method becomes less applicable as it becomes more difficult to repeat experiments.

These areas of study are still sciences, however. Consider archaeology—even though one cannot perform repeatable experiments, hypotheses may still be supported. For instance, an archaeologist can hypothesize that an ancient culture existed based on finding a piece of pottery. Further hypotheses could be made about various characteristics of this culture, and these hypotheses may be found to be correct or false through continued support or contradictions from other findings. A hypothesis may become a verified theory. A theory is a tested and confirmed explanation for observations or phenomena. Science may be better defined as fields of study that attempt to comprehend the nature of the universe.

Natural Sciences

What would you expect to see in a museum of natural sciences? Frogs? Plants? Dinosaur skeletons? Exhibits about how the brain functions? A planetarium? Gems and minerals? Or, maybe all of the above? Science includes such diverse fields as astronomy, biology, computer sciences, geology, logic, physics, chemistry, and mathematics ( Figure 1.4 ). However, those fields of science related to the physical world and its phenomena and processes are considered natural sciences . Thus, a museum of natural sciences might contain any of the items listed above.

There is no complete agreement when it comes to defining what the natural sciences include, however. For some experts, the natural sciences are astronomy, biology, chemistry, earth science, and physics. Other scholars choose to divide natural sciences into life sciences , which study living things and include biology, and physical sciences , which study nonliving matter and include astronomy, geology, physics, and chemistry. Some disciplines such as biophysics and biochemistry build on both life and physical sciences and are interdisciplinary. Natural sciences are sometimes referred to as “hard science” because they rely on the use of quantitative data; social sciences that study society and human behavior are more likely to use qualitative assessments to drive investigations and findings.

Not surprisingly, the natural science of biology has many branches or subdisciplines. Cell biologists study cell structure and function, while biologists who study anatomy investigate the structure of an entire organism. Those biologists studying physiology, however, focus on the internal functioning of an organism. Some areas of biology focus on only particular types of living things. For example, botanists explore plants, while zoologists specialize in animals.

Scientific Reasoning

One thing is common to all forms of science: an ultimate goal “to know.” Curiosity and inquiry are the driving forces for the development of science. Scientists seek to understand the world and the way it operates. To do this, they use two methods of logical thinking: inductive reasoning and deductive reasoning.

Inductive reasoning is a form of logical thinking that uses related observations to arrive at a general conclusion. This type of reasoning is common in descriptive science. A life scientist such as a biologist makes observations and records them. These data can be qualitative or quantitative, and the raw data can be supplemented with drawings, pictures, photos, or videos. From many observations, the scientist can infer conclusions (inductions) based on evidence. Inductive reasoning involves formulating generalizations inferred from careful observation and the analysis of a large amount of data. Brain studies provide an example. In this type of research, many live brains are observed while people are doing a specific activity, such as viewing images of food. The part of the brain that “lights up” during this activity is then predicted to be the part controlling the response to the selected stimulus, in this case, images of food. The “lighting up” of the various areas of the brain is caused by excess absorption of radioactive sugar derivatives by active areas of the brain. The resultant increase in radioactivity is observed by a scanner. Then, researchers can stimulate that part of the brain to see if similar responses result.

Deductive reasoning or deduction is the type of logic used in hypothesis-based science. In deductive reason, the pattern of thinking moves in the opposite direction as compared to inductive reasoning. Deductive reasoning is a form of logical thinking that uses a general principle or law to predict specific results. From those general principles, a scientist can deduce and predict the specific results that would be valid as long as the general principles are valid. Studies in climate change can illustrate this type of reasoning. For example, scientists may predict that if the climate becomes warmer in a particular region, then the distribution of plants and animals should change. These predictions have been made and tested, and many such changes have been found, such as the modification of arable areas for agriculture, with change based on temperature averages.

Both types of logical thinking are related to the two main pathways of scientific study: descriptive science and hypothesis-based science. Descriptive (or discovery) science , which is usually inductive, aims to observe, explore, and discover, while hypothesis-based science , which is usually deductive, begins with a specific question or problem and a potential answer or solution that can be tested. The boundary between these two forms of study is often blurred, and most scientific endeavors combine both approaches. The fuzzy boundary becomes apparent when thinking about how easily observation can lead to specific questions. For example, a gentleman in the 1940s observed that the burr seeds that stuck to his clothes and his dog’s fur had a tiny hook structure. On closer inspection, he discovered that the burrs’ gripping device was more reliable than a zipper. He eventually developed a company and produced the hook-and-loop fastener often used on lace-less sneakers and athletic braces. Descriptive science and hypothesis-based science are in continuous dialogue.

The Scientific Method

Biologists study the living world by posing questions about it and seeking science-based responses. This approach is common to other sciences as well and is often referred to as the scientific method. The scientific method was used even in ancient times, but it was first documented by England’s Sir Francis Bacon (1561–1626) ( Figure 1.5 ), who set up inductive methods for scientific inquiry. The scientific method is not exclusively used by biologists but can be applied to almost all fields of study as a logical, rational problem-solving method.

The scientific process typically starts with an observation (often a problem to be solved) that leads to a question. Let’s think about a simple problem that starts with an observation and apply the scientific method to solve the problem. One Monday morning, a student arrives at class and quickly discovers that the classroom is too warm. That is an observation that also describes a problem: the classroom is too warm. The student then asks a question: “Why is the classroom so warm?”

Proposing a Hypothesis

Recall that a hypothesis is a suggested explanation that can be tested. To solve a problem, several hypotheses may be proposed. For example, one hypothesis might be, “The classroom is warm because no one turned on the air conditioning.” But there could be other responses to the question, and therefore other hypotheses may be proposed. A second hypothesis might be, “The classroom is warm because there is a power failure, and so the air conditioning doesn’t work.”

Once a hypothesis has been selected, the student can make a prediction. A prediction is similar to a hypothesis but it typically has the format “If . . . then . . . .” For example, the prediction for the first hypothesis might be, “ If the student turns on the air conditioning, then the classroom will no longer be too warm.”

Testing a Hypothesis

A valid hypothesis must be testable. It should also be falsifiable , meaning that it can be disproven by experimental results. Importantly, science does not claim to “prove” anything because scientific understandings are always subject to modification with further information. This step—openness to disproving ideas—is what distinguishes sciences from non-sciences. The presence of the supernatural, for instance, is neither testable nor falsifiable. To test a hypothesis, a researcher will conduct one or more experiments designed to eliminate one or more of the hypotheses. Each experiment will have one or more variables and one or more controls. A variable is any part of the experiment that can vary or change during the experiment. The control group contains every feature of the experimental group except it is not given the manipulation that is hypothesized about. Therefore, if the results of the experimental group differ from the control group, the difference must be due to the hypothesized manipulation, rather than some outside factor. Look for the variables and controls in the examples that follow. To test the first hypothesis, the student would find out if the air conditioning is on. If the air conditioning is turned on but does not work, there should be another reason, and this hypothesis should be rejected. To test the second hypothesis, the student could check if the lights in the classroom are functional. If so, there is no power failure and this hypothesis should be rejected. Each hypothesis should be tested by carrying out appropriate experiments. Be aware that rejecting one hypothesis does not determine whether or not the other hypotheses can be accepted; it simply eliminates one hypothesis that is not valid ( see this figure ). Using the scientific method, the hypotheses that are inconsistent with experimental data are rejected.

While this “warm classroom” example is based on observational results, other hypotheses and experiments might have clearer controls. For instance, a student might attend class on Monday and realize she had difficulty concentrating on the lecture. One observation to explain this occurrence might be, “When I eat breakfast before class, I am better able to pay attention.” The student could then design an experiment with a control to test this hypothesis.

In hypothesis-based science, specific results are predicted from a general premise. This type of reasoning is called deductive reasoning: deduction proceeds from the general to the particular. But the reverse of the process is also possible: sometimes, scientists reach a general conclusion from a number of specific observations. This type of reasoning is called inductive reasoning, and it proceeds from the particular to the general. Inductive and deductive reasoning are often used in tandem to advance scientific knowledge ( see this figure ). In recent years a new approach of testing hypotheses has developed as a result of an exponential growth of data deposited in various databases. Using computer algorithms and statistical analyses of data in databases, a new field of so-called "data research" (also referred to as "in silico" research) provides new methods of data analyses and their interpretation. This will increase the demand for specialists in both biology and computer science, a promising career opportunity.

Science Practice Connection for AP® Courses

Think about it.

Almost all plants use water, carbon dioxide, and energy from the sun to make sugars. Think about what would happen to plants that don’t have sunlight as an energy source or sufficient water. What would happen to organisms that depend on those plants for their own survival?

Make a prediction about what would happen to the organisms living in a rain forest if 50% of its trees were destroyed. How would you test your prediction?

Use this example as a model to make predictions. Emphasize there is no rigid scientific method scheme. Active science is a combination of observations and measurement. Offer the example of ecology where the conventional scientific method is not always applicable because researchers cannot always set experiments in a laboratory and control all the variables.

Possible answers:

Destruction of the rain forest affects the trees, the animals which feed on the vegetation, take shelter on the trees, and large predators which feed on smaller animals. Furthermore, because the trees positively affect rain through massive evaporation and condensation of water vapor, drought follows deforestation.

Tell students a similar experiment on a grand scale may have happened in the past and introduce the next activity “What killed the dinosaurs?”

Some predictions can be made and later observations can support or disprove the prediction.

Ask, “what killed the dinosaurs?” Explain many scientists point to a massive asteroid crashing in the Yucatan peninsula in Mexico. One of the effects was the creation of smoke clouds and debris that blocked the Sun, stamped out many plants and, consequently, brought mass extinction. As is common in the scientific community, many other researchers offer divergent explanations.

Go to this site for a good example of the complexity of scientific method and scientific debate.

Visual Connection

In the example below, the scientific method is used to solve an everyday problem. Order the scientific method steps (numbered items) with the process of solving the everyday problem (lettered items). Based on the results of the experiment, is the hypothesis correct? If it is incorrect, propose some alternative hypotheses.

• The original hypothesis is correct. There is something wrong with the electrical outlet and therefore the toaster doesn’t work.
• The original hypothesis is incorrect. Alternative hypothesis includes that toaster wasn’t turned on.
• The original hypothesis is correct. The coffee maker and the toaster do not work when plugged into the outlet.
• The original hypothesis is incorrect. Alternative hypotheses includes that both coffee maker and toaster were broken.
• All flying birds and insects have wings. Birds and insects flap their wings as they move through the air. Therefore, wings enable flight.
• Insects generally survive mild winters better than harsh ones. Therefore, insect pests will become more problematic if global temperatures increase.
• Chromosomes, the carriers of DNA, are distributed evenly between the daughter cells during cell division. Therefore, each daughter cell will have the same chromosome set as the mother cell.
• Animals as diverse as humans, insects, and wolves all exhibit social behavior. Therefore, social behavior must have an evolutionary advantage.
• 1- Inductive, 2- Deductive, 3- Deductive, 4- Inductive
• 1- Deductive, 2- Inductive, 3- Deductive, 4- Inductive
• 1- Inductive, 2- Deductive, 3- Inductive, 4- Deductive
• 1- Inductive, 2-Inductive, 3- Inductive, 4- Deductive

The scientific method may seem too rigid and structured. It is important to keep in mind that, although scientists often follow this sequence, there is flexibility. Sometimes an experiment leads to conclusions that favor a change in approach; often, an experiment brings entirely new scientific questions to the puzzle. Many times, science does not operate in a linear fashion; instead, scientists continually draw inferences and make generalizations, finding patterns as their research proceeds. Scientific reasoning is more complex than the scientific method alone suggests. Notice, too, that the scientific method can be applied to solving problems that aren’t necessarily scientific in nature.

Two Types of Science: Basic Science and Applied Science

The scientific community has been debating for the last few decades about the value of different types of science. Is it valuable to pursue science for the sake of simply gaining knowledge, or does scientific knowledge only have worth if we can apply it to solving a specific problem or to bettering our lives? This question focuses on the differences between two types of science: basic science and applied science.

Basic science or “pure” science seeks to expand knowledge regardless of the short-term application of that knowledge. It is not focused on developing a product or a service of immediate public or commercial value. The immediate goal of basic science is knowledge for knowledge’s sake, though this does not mean that, in the end, it may not result in a practical application.

In contrast, applied science or “technology,” aims to use science to solve real-world problems, making it possible, for example, to improve a crop yield, find a cure for a particular disease, or save animals threatened by a natural disaster ( Figure 1.8 ). In applied science, the problem is usually defined for the researcher.

Some individuals may perceive applied science as “useful” and basic science as “useless.” A question these people might pose to a scientist advocating knowledge acquisition would be, “What for?” A careful look at the history of science, however, reveals that basic knowledge has resulted in many remarkable applications of great value. Many scientists think that a basic understanding of science is necessary before an application is developed; therefore, applied science relies on the results generated through basic science. Other scientists think that it is time to move on from basic science and instead to find solutions to actual problems. Both approaches are valid. It is true that there are problems that demand immediate attention; however, few solutions would be found without the help of the wide knowledge foundation generated through basic science.

One example of how basic and applied science can work together to solve practical problems occurred after the discovery of DNA structure led to an understanding of the molecular mechanisms governing DNA replication. Strands of DNA, unique in every human, are found in our cells, where they provide the instructions necessary for life. During DNA replication, DNA makes new copies of itself, shortly before a cell divides. Understanding the mechanisms of DNA replication enabled scientists to develop laboratory techniques that are now used to identify genetic diseases. Without basic science, it is unlikely that applied science could exist.

Another example of the link between basic and applied research is the Human Genome Project, a study in which each human chromosome was analyzed and mapped to determine the precise sequence of DNA subunits and the exact location of each gene. (The gene is the basic unit of heredity represented by a specific DNA segment that codes for a functional molecule.) Other less complex organisms have also been studied as part of this project in order to gain a better understanding of human chromosomes. The Human Genome Project ( Figure 1.9 ) relied on basic research carried out with simple organisms and, later, with the human genome. An important end goal eventually became using the data for applied research, seeking cures and early diagnoses for genetically related diseases.

While research efforts in both basic science and applied science are usually carefully planned, it is important to note that some discoveries are made by serendipity , that is, by means of a fortunate accident or a lucky surprise. Penicillin was discovered when biologist Alexander Fleming accidentally left a petri dish of Staphylococcus bacteria open. An unwanted mold grew on the dish, killing the bacteria. The mold turned out to be Penicillium , and a new antibiotic was discovered. Even in the highly organized world of science, luck—when combined with an observant, curious mind—can lead to unexpected breakthroughs.

Reporting Scientific Work

Whether scientific research is basic science or applied science, scientists must share their findings in order for other researchers to expand and build upon their discoveries. Collaboration with other scientists—when planning, conducting, and analyzing results—is important for scientific research. For this reason, important aspects of a scientist’s work are communicating with peers and disseminating results to peers. Scientists can share results by presenting them at a scientific meeting or conference, but this approach can reach only the select few who are present. Instead, most scientists present their results in peer-reviewed manuscripts that are published in scientific journals. Peer-reviewed manuscripts are scientific papers that are reviewed by a scientist’s colleagues, or peers. These colleagues are qualified individuals, often experts in the same research area, who judge whether or not the scientist’s work is suitable for publication. The process of peer review helps to ensure that the research described in a scientific paper or grant proposal is original, significant, logical, and thorough. Grant proposals, which are requests for research funding, are also subject to peer review. Scientists publish their work so other scientists can reproduce their experiments under similar or different conditions to expand on the findings.

A scientific paper is very different from creative writing. Although creativity is required to design experiments, there are fixed guidelines when it comes to presenting scientific results. First, scientific writing must be brief, concise, and accurate. A scientific paper needs to be succinct but detailed enough to allow peers to reproduce the experiments.

The scientific paper consists of several specific sections—introduction, materials and methods, results, and discussion. This structure is sometimes called the “IMRaD” format. There are usually acknowledgment and reference sections as well as an abstract (a concise summary) at the beginning of the paper. There might be additional sections depending on the type of paper and the journal where it will be published; for example, some review papers require an outline.

The introduction starts with brief, but broad, background information about what is known in the field. A good introduction also gives the rationale of the work; it justifies the work carried out and also briefly mentions the end of the paper, where the hypothesis or research question driving the research will be presented. The introduction refers to the published scientific work of others and therefore requires citations following the style of the journal. Using the work or ideas of others without proper citation is considered plagiarism .

The materials and methods section includes a complete and accurate description of the substances used, and the method and techniques used by the researchers to gather data. The description should be thorough enough to allow another researcher to repeat the experiment and obtain similar results, but it does not have to be verbose. This section will also include information on how measurements were made and what types of calculations and statistical analyses were used to examine raw data. Although the materials and methods section gives an accurate description of the experiments, it does not discuss them.

Some journals require a results section followed by a discussion section, but it is more common to combine both. If the journal does not allow the combination of both sections, the results section simply narrates the findings without any further interpretation. The results are presented by means of tables or graphs, but no duplicate information should be presented. In the discussion section, the researcher will interpret the results, describe how variables may be related, and attempt to explain the observations. It is indispensable to conduct an extensive literature search to put the results in the context of previously published scientific research. Therefore, proper citations are included in this section as well.

Finally, the conclusion section summarizes the importance of the experimental findings. While the scientific paper almost certainly answered one or more scientific questions that were stated, any good research should lead to more questions. Therefore, a well-done scientific paper leaves doors open for the researcher and others to continue and expand on the findings.

Review articles do not follow the IMRAD format because they do not present original scientific findings, or primary literature; instead, they summarize and comment on findings that were published as primary literature and typically include extensive reference sections.

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• Authors: Julianne Zedalis, John Eggebrecht
• Publisher/website: OpenStax
• Book title: Biology for AP® Courses
• Publication date: Mar 8, 2018
• Location: Houston, Texas
• Book URL: https://openstax.org/books/biology-ap-courses/pages/1-introduction
• Section URL: https://openstax.org/books/biology-ap-courses/pages/1-1-the-science-of-biology

© Jan 8, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.

AP® Biology

The chi square test: ap® biology crash course.

• The Albert Team
• Last Updated On: March 7, 2024

The statistics section of the AP® Biology exam is without a doubt one of the most notoriously difficult sections. Biology students are comfortable with memorizing and understanding content, which is why this topic seems like the most difficult to master. In this article,  The Chi Square Test: AP® Biology Crash Course , we will teach you a system for how to perform the Chi Square test every time. We will begin by reviewing some topics that you must know about statistics before you can complete the Chi Square test. Next, we will simplify the equation by defining each of the Chi Square variables. We will then use a simple example as practice to make sure that we have learned every part of the equation. Finally, we will finish with reviewing a more difficult question that you could see on your AP® Biology exam .

Null and Alternative Hypotheses

As background information, first you need to understand that a scientist must create the null and alternative hypotheses prior to performing their experiment. If the dependent variable is not influenced by the independent variable , the null hypothesis will be accepted. If the dependent variable is influenced by the independent variable, the data should lead the scientist to reject the null hypothesis . The null and alternative hypotheses can be a difficult topic to describe. Let’s look at an example.

Consider an experiment about flipping a coin. The null hypothesis would be that you would observe the coin landing on heads fifty percent of the time and the coin landing on tails fifty percent of the time. The null hypothesis predicts that you will not see a change in your data due to the independent variable.

The alternative hypothesis for this experiment would be that you would not observe the coins landing on heads and tails an even number of times. You could choose to hypothesize you would see more heads, that you would see more tails, or that you would just see a different ratio than 1:1. Any of these hypotheses would be acceptable as alternative hypotheses.

Defining the Variables

Now we will go over the Chi-Square equation. One of the most difficult parts of learning statistics is the long and confusing equations. In order to master the Chi Square test, we will begin by defining the variables.

This is the Chi Square test equation. You must know how to use this equation for the AP® Bio exam. However, you will not need to memorize the equation; it will be provided to you on the AP® Biology Equations and Formulas sheet that you will receive at the beginning of your examination.

Now that you have seen the equation, let’s define each of the variables so that you can begin to understand it!

•   X 2  :The first variable, which looks like an x, is called chi squared. You can think of chi like x in algebra because it will be the variable that you will solve for during your statistical test. •   ∑ : This symbol is called sigma. Sigma is the symbol that is used to mean “sum” in statistics. In this case, this means that we will be adding everything that comes after the sigma together. •   O : This variable will be the observed data that you record during your experiment. This could be any quantitative data that is collected, such as: height, weight, number of times something occurs, etc. An example of this would be the recorded number of times that you get heads or tails in a coin-flipping experiment. •   E : This variable will be the expected data that you will determine before running your experiment. This will always be the data that you would expect to see if your independent variable does not impact your dependent variable. For example, in the case of coin flips, this would be 50 heads and 50 tails.

The equation should begin to make more sense now that the variables are defined.

Working out the Coin Flip

We have talked about the coin flip example and, now that we know the equation, we will solve the problem. Let’s pretend that we performed the coin flip experiment and got the following data:

Now we put these numbers into the equation:

Heads (55-50) 2 /50= .5

Tails (45-50) 2 /50= .5

Lastly, we add them together.

c 2 = .5+.5=1

Now that we have c 2 we must figure out what that means for our experiment! To do that, we must review one more concept.

Degrees of Freedom and Critical Values

Degrees of freedom is a term that statisticians use to determine what values a scientist must get for the data to be significantly different from the expected values. That may sound confusing, so let’s try and simplify it. In order for a scientist to say that the observed data is different from the expected data, there is a numerical threshold the scientist must reach, which is based on the number of outcomes and a chosen critical value.

Let’s return to our coin flipping example. When we are flipping the coin, there are two outcomes: heads and tails. To get degrees of freedom, we take the number of outcomes and subtract one; therefore, in this experiment, the degree of freedom is one. We then take that information and look at a table to determine our chi-square value:

We will look at the column for one degree of freedom. Typically, scientists use a .05 critical value. A .05 critical value represents that there is a 95% chance that the difference between the data you expected to get and the data you observed is due to something other than chance. In this example, our value will be 3.84.

Coin Flip Results

In our coin flip experiment, Chi Square was 1. When we look at the table, we see that Chi Square must have been greater than 3.84 for us to say that the expected data was significantly different from the observed data. We did not reach that threshold. So, for this example, we will say that we failed to reject the null hypothesis.

The best way to get better at these statistical questions is to practice. Next, we will go through a question using the Chi Square Test that you could see on your AP® Bio exam.

AP® Biology Exam Question

This question was adapted from the 2013 AP® Biology exam.

In an investigation of fruit-fly behavior, a covered choice chamber is used to test whether the spatial distribution of flies is affected by the presence of a substance placed at one end of the chamber. To test the flies’ preference for glucose, 60 flies are introduced into the middle of the choice chamber at the insertion point. A ripe banana is placed at one end of the chamber, and an unripe banana is placed at the other end. The positions of flies are observed and recorded after 1 minute and after 10 minutes. Perform a Chi Square test on the data for the ten minute time point. Specify the null hypothesis and accept or reject it.

Okay, we will begin by identifying the null hypothesis . The null hypothesis would be that the flies would be evenly distributed across the three chambers (ripe, middle, and unripe).

Next, we will perform the Chi-Square test just like we did in the heads or tails experiment. Because there are three conditions, it may be helpful to use this set up to organize yourself:

Ok, so we have a Chi Square of 48.9. Our degrees of freedom are 3(ripe, middle, unripe)-1=2. Let’s look at that table above for a confidence variable of .05. You should get a value of 5.99. Our Chi Square value of 48.9 is much larger than 5.99 so in this case we are able to reject the null hypothesis. This means that the flies are not randomly assorting themselves, and the banana is influencing their behavior.

The Chi Square test is something that takes practice. Once you learn the system of solving these problems, you will be able to solve any Chi Square problem using the exact same method every time! In this article, we have reviewed the Chi Square test using two examples. If you are still interested in reviewing the bio-statistics that will be on your AP® Biology Exam, please check out our article The Dihybrid Cross Problem: AP® Biology Crash Course . Let us know how studying is going and if you have any questions!

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Excel at Science

• May 9, 2020

How to Answer Experiment Questions on AP Biology FRQ

Updated: Sep 22, 2023

On the AP Biology exam, the first section is multiple-choice and the second section is a set of 8 FRQs (free response questions), in which you may be given an experiment setup or asked to design an experiment yourself. Many students find the FRQs challenging because experimental design is not a specific chapter in the AP Biology textbook.

In order to answer these questions well, you need to put on your scientist’s hat and think about it as if you were running the experiment. The best way to demonstrate this is to walk through some examples of experiments. First, we will discuss the guidelines and terminology used for designing and running experiments in biology.

An experiment should always be based on a hypothesis, something that you believe might be true and that you want to test. If there is no hypothesis, there is no purpose for the experiment. Often, the hypothesis is an association between a factor and a result of interest. Some examples are:

Sunlight and plant growth

Mutation in bacteria and resistance to an antibiotic

A particular drug and decreased blood pressure

Soil acidity and flower color

Let’s take the second example, a particular mutation in bacteria and resistance to a specific antibiotic. There are so many different aspects of bacteria and the environment they live in. How can we determine that one particular trait (in this case, a mutated gene) is responsible for antibiotic resistance?

This is why scientists use controlled environments for their experiments. They can control for all factors ( keep them the same) across all experimental groups except the suspected factor, the gene mutation. Each experimental group has a different treatment or condition. In a control group, there is no special treatment. The control group serves as a baseline to compare the other groups to. The diagram below illustrates this:

Notice that all other factors (bacterial strain, concentration of nutrients, concentration of antibiotic added, etc.) are kept the same.

Another term often used in experimentation is null hypothesis . This is different from the scientific hypothesis! Many students get confused by that. The null hypothesis is more of a statistics term and it states that there will be no significant difference observed among the different experimental groups. Scientists usually hope to reject the null hypothesis , which means they do observe a real difference, supporting their scientific hypothesis . This will all become more clear when we walk through some examples.

Example Problems:

2017 FRQ - #2 Bees and Caffeine Experiment

This question involves an experiment about bees and the nectar they encounter while pollinating flowers. The scientists want to understand the role of caffeine on the bees’ memory.

The question gives a table showing the results of the experiment, shown below. It includes a control group and test group (caffeine). It also shows the probability of the bees returning to a recently visited nectar source. This probability is used to represent the bees’ short-term and long-term memory.

As you read through the question and think about the experiment, you should consider the set of questions below. Just consider them, no need to write them down. They will help you plan out your responses to the actual problem:

What are these scientists testing in this experiment? In other words, what is their scientific hypothesis ?

What is the independent and dependent variable?

What is the difference between the control and test group? What’s the purpose of the control group? Note that sometimes there is more than one test group. Here, we only have one, which is the caffeine treatment group.

What is the null hypothesis ?

How could the experimental data be represented graphically?

What do the +/- values mean in each of the data cells?

If you are able to answer all those questions, you will have no trouble with this problem. So let’s answer them:

The scientific hypothesis is that exposure to caffeine is associated with the bees’ memory.

The independent variable is the treatment, which is exposure to caffeine. The dependent variable is what is impacted. Here, that is the bees’ memory.

The control group is exposed to no caffeine, while the treatment group is exposed to caffeine in the nectar. The control group serves as a baseline to compare the treatment group to. If we hypothesize that caffeine has a negative impact on memory, then the probability of revisiting the nectar source should be higher for the treatment compared to the control.

The null hypothesis states that there is no significant difference in memory between the control and treatment groups. Any difference observed would be due to chance. To support the scientific hypothesis, scientists need the data to reject the null hypothesis.

The data here should be represented by a bar graph. There will be two bars, one for control and one for treatment. There should also be error bars because the standard errors are included in the data. The graph would look something like this:

2019 FRQ - #2 Ecological Relationship Between Two Protists

This question is about an experiment that investigates the ecological relationship between two protists. Are they competing for the same food? Does one predate on the other? Or do they live together in harmony and use different resources? That is what the scientists want to know.

The data collected in the experiment is given in the question, shown below.

Let’s answer the same list of questions again to really understand the experiment.

The scientific question being tested is: what kind of ecological relationship do protist species A and B have?

The independent variable is the treatment, which is the two species living together. The dependent variable is the population size of each species over time.

The control group is the species grown separately. The test group is the species grown together.

The null hypothesis states that there is no significant difference in population size between the control and treatment groups at each time point. Any difference observed would be due to chance.

The data here should be represented by a line graph, since we have time as a factor. Time should be on the x-axis -- this is almost always the case. There will be two lines, one for control and one for treatment. The graph would look something like this:

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Must Know About AP Biology Labs

23 min read • december 22, 2021

Mark Little

The AP labs and variations of these labs will be discussed with curriculum content and science practice connections. It is recommended that lab work take at least 25% of your class time. Many AP exam questions will provide you lab data in the prompt and ask you questions about the data set. 

Also, virtual labs and simulations will be referenced for extra practice and content review if needed. You may consider using the virtual labs or simulations for additional review and practice over topics you may have not done in your class.

There are several key ideas from all the lab investigations and simulations that you did in your class.

Be able to make observations, collect data, make the proper graph and evaluate the data collected.

Review and understand the conclusions from each lab.

Answer “what if” questions that may test your understanding and answer questions like “Based on these results, what would your follow up question be?” Or “Predict what would happen if…” Or “Propose what your next investigation would be…”

Perform calculations from the lab results or data sets.

Be able to write a prediction or hypothesis; identify the independent and dependent variables for the experiments. 

Be able to identify and justify appropriate controls for the experiment.

Unit 1: Chemistry of Life

There are no College Board recommended labs for this unit. Knowledge of basic chemistry such as elements, molecules, compounds are essential to learn. Properties of water are an important concept of this unit. 

Knowing basic information about the four types of macromolecules or biological molecules is important too. These molecules are the: 1) Carbohydrates 2) Proteins 3) Lipids 4) Nucleic Acids. 

Often at the start of the year, you were taught about a lab write format including how to write a proper question to investigate; how to write a hypothesis or prediction; how to properly draw and label a graph; and how to properly analyze the data collected. Also, you may have been taught about CER in AP Biology and improving your skills in argumentation.

An activity to review macromolecules is Pattern Matching.

An activity to review water properties is Water Molecule - Model Building

A tutorial for Biomolecules: Biomolecules

Unit 2: Cell Structure and Function

This unit teaches about the cell, the basic unit of life. Cells are a key component to life’s organization and provide and provide conditions for our cell organelles to function. Cell organelles provide what is often referred to as “ compartmentalization”, and this concept helps with cellular organization. 

Cells have membranes that separate the internal and external environment and these membranes control what enters and exits the cell and helps maintain a condition called homeostasis . 

The major lab for Unit 2 is about Diffusion and Osmosis. The lab has three parts. In the first procedure, you will be asked to study the relationship between surface area and volume by varying the size of artificial cells. This lab is often done with agar. Your teacher makes agar cubes with an indicator of different sizes with and you place the cubes into a solution of vinegar (an acid) for about 10 minutes and the indicator in the cube will change color. Surface area and volume related calculations are done. These calculations are done to determine what happens to the surface area to volume ratio as a cell gets larger. You should see as the surface area to volume ratio decreases, the cell will get larger.

In the second procedure you will use a model of a living cell, dialysis tubing, and study osmosis and diffusion. As a student you fill your dialysis bags with different solutions and weigh before and after to measure the change in mass. (usually leave overnight) The dialysis tubing acts like a selectively permeable membrane. A selectively  permeable membrane allows certain molecules or ions to pass through actively or passively. 

An example might be you placed in the dialysis tubing different “concentrations of sugar” and place them in beakers of distilled water overnight. You should see water flow from areas of high water potential to areas of low water potential. In this case, the tubing with the highest concentration of sugar will gain the most mass. Water moves down the concentration gradient, meaning from areas of high water concentration “least amount of sugar” to areas of lower water “solutions with the most sugar.”

In the third procedure, you will observe osmosis in living cells. You cut out potato cores for example and weigh them and place them in different concentrations of a solution, (usually salt or sugar) and weigh them before and after to determine the percent change in mass. Percent change in mass shows a direct relationship between the current mass and previous mass. Cores are left overnight. 

You should notice some cores will gain mass and others will lose mass. In potatoes water will move from areas of high water concentration to areas of lower water concentration through a semipermeable membrane the "potato cells.” The movement of water from higher concentration to lower concentration is determined by the starch and water concentration in the potato.

This lab looks at the movement of materials through the cell membrane and throughout the cytoplasm of the cell. The movement in and out of the cell is limited by the cell membranes and organelle membranes because they are selectively permeable. Water moves through membranes by a special type of diffusion called osmosis . 

Water moves through special protein channels called aquaporins in the cell membrane. These special protein channels allow water to move more quickly than it would through osmosis. Most other substances like (Na+ or K+) ions also move through different protein channels. Carbohydrates and other larger molecules will require movement through transport proteins. 

Water moves from areas of high water concentration ( high water potential ) to areas of low water concentration ( low water potential ). Water has high water potential when it has low solute (like salt) dissolved in it and therefore has a high free water concentration. Water has low water potential when it has high solute concentration and therefore a low free water concentration.

A hypertonic solution has a higher solute concentration and a lower water potential. A hypotonic solution has a lower solute concentration and a higher water potential. Isotonic solutions have equal water potential. 

Water moves from high water potential to low water potential or hypotonic solution to a hypertonic solution. In terms of free energy and water potential, water moves from high free energy and areas of low free energy. 

Some tutorials and virtual labs include:

Biomembranes I Tutorial: Membrane Structure and Transport

Osmosis and Diffusion Lab: Osmosis and Diffusion

The following are simulations by Jon Darkow. These are excellent reviews of content and help you work on your science practice skills. Worksheets are provided. You can now run these simulations on your phone.

1. Diffusion and Surface area/volume:  Diffusion and SA/V

2. Water Potential and Osmosis: Water Potential

Unit 3: Cellular Energetics

This unit tends to be one of the harder units for students because it covers a lot of complex topics. This unit focuses on cellular energetics, showing how energy is captured and used by organisms. You will learn about enzymes and how they lower the activation energy for chemical reactions and how the environment can influence the effectiveness of enzyme activity. Finally, you study “cellular respiration and photosynthesis” and how energy is used to drive these processes. 

There are three labs associated often done with this unit. The first lab is about enzymes. Enzymes are organic catalysts (usually proteins) that control many of the reactions that occur in living organisms. Enzymes are used in all metabolic reactions to control the rate of reactions and decrease the amount of activation energy necessary for the reaction to take place. Enzymes are specific for each reaction and are reusable. 

The chemical an enzyme works on is called the substrate . An example of a substrate is hydrogen peroxide. An example of an enzyme is catalase. Many biological enzymes end in “ase.” Enzymes have an area called the active site to which a specific substrate will bond temporarily while the reaction is taking place. Many scientists call this temporary bond the induced fit model .

This lab looks at how abiotic (nonliving) or biotic (living) factors can change or influence enzymatic reactions. There are many forms of this lab using many different types of equipment. One version uses hydrogen peroxide as the substrate that is broken down by the enzyme catalase. A couple of the variables chosen to test are varying the amount of substrate, varying the amount of enzyme, and varying the temperature or pH. A control is set up for each lab. One way is to substitute water for the enzyme.

An enzyme virtual lab review:

Enzyme Catalysis Lab:  Enzyme Catalysis

Enzyme Diversity: Enzymes

Lactase Enzyme:  Lactase

The second lab is about photosynthesis . In eukaryotes, photosynthesis takes place in the chloroplast . There are two general processes of photosynthesis:  1) Light-dependent reaction (often called the light reaction) and this takes place in the thylakoid membranes 2) Light Independent reaction (often called the dark reaction or Calvin cycle) and this takes place in the stroma or fluid area of the chloroplast. 

The process of photosynthesis occurs in a series of steps regulated by enzymes. During photosynthesis, light energy is captured to build carbohydrates that are full of energy stored in the chemical bonds. Autotrophs are organisms that capture free energy from the sun through photosynthesis and chemosynthesis and heterotrophs from the carbon products (like carbohydrates) of other organisms. 

Living systems require free energy and matter to grow and to reproduce. 

The general summary equation for photosynthesis is

H2O + CO2 + light → carbohydrate (CH2O) + O2 + H2O

There are multiple versions of this lab. A common version is called the floating disc lab. In your classroom, you may have used ivy or baby spinach leaves and punched out discs and used a syringe to sink the discs and timed how long it took for the discs to float again. You may have run this experiment by varying the amount of baking soda in water, “providing carbon dioxide”, varying the distance from a light source, or the color of light. 

Photosynthesis tutorial and virtual lab review:

Photosynthesis tutorial:  Photosynthesis

Photosynthesis lab: Photosynthesis

The following is a simulation by Jon Darkow. This is an excellent review of content and helps you work on your science practice skills. Worksheets are provided. You can now run these simulations on your phone.

Photosynthesis Simulation: Photosynthesis

The third lab is about cellular respiration. Living systems require energy to grow and reproduce. If organisms are lacking or deficient in energy, this deficiency can cause harm to the organism or to the population or even the ecosystem level. Different organisms use different strategies to store and preserve energy, often called free energy. Autotrophs are organisms that capture free energy from the sun through photosynthesis and chemosynthesis and heterotrophs obtain energy from the carbon products (like carbohydrates) of other organisms. In the process of cellular respiration,  

At the cellular level, the intake of oxygen gas and the output of carbon dioxide gas is associated with the production of ATP and this process is called cellular respiration . ATP is generated by mitochondria within the cell. During cellular respiration, the energy stored within macromolecules such as glucose is released and utilized to phosphorylate (add a phosphate to) ADP, producing ATP. 

In the presence of oxygen, glucose can release large amounts of energy. The process of cellular respiration also produces water and carbon dioxide gas as waste products. 

C6H12O6 + 6 O2 -> 6H2O + 6CO2(g) + energy

Organisms that utilize oxygen for the breakdown of glucose are called aerobic organisms . Plants and animals are both examples of aerobic organisms.

Cell respiration takes place in the mitochondria in eukaryotic organisms and has three general steps: 1) Glycolysis (anaerobic -- no oxygen required) in the cytoplasm 2) Krebs or Citric Acid Cycle (aerobic -- requires oxygen) in the mitochondrial matrix and 3) Electron Transport Chain that is aerobic as well.

There are many versions of this lab. One version uses respirometers you can make out of a syringe or other materials and place germinating, non-germinating seeds and beads as a control in them. You measure the respiration rate by measuring oxygen consumption. 

A cell respiration tutorial or virtual lab review:

Cell Respiration tutorial:  Cell Respiration

The following are simulations by Jon Darkow. These are an excellent review of content and help you work on your science practice skills. Worksheets are provided. You can now run these simulations on your phone.

1. Cell Respiration Accounting:  Cell Respiration

2. Cell Respiration Multiple Dose Model: Cell Respiration Dose

Unit 4: Cell Communication and Cell Cycle

You have seen cell communication before. In Unit 2 the various types of carrier proteins involved in facilitated diffusion are an example of cell signaling. Also in unit 3,  light is used to trigger a response to start the process of photosynthesis (light-dependent reaction).

You will learn how cells use energy, transfer information and replicate by using cellular communication. One basic pathway for this process is called the signal transduction pathway. There are three steps. 1) Reception -- a protein at the surface that detects a chemical signal 2) Transduction -- a conformational change at the protein surface causes change and starts a “transduction” of the signal through a series of steps and 3) Response -- the signal triggers a specific response. 

The signal transduction process allows cells to maintain homeostasis by being able to respond to changes to the environment. Also, signal transduction plays a major role in regulating the cell cycle that is important for life.

Blood sugar regulation (production of insulin and glucagon), neurotransmission, the immune system with helper and killer T cells are examples of cell communication you may know.

There is one major recommended lab. It is about cell division: mitosis and meiosis . For the mitosis lab, a common version is to use onion root tips and by using different chemicals (lectin or caffeine are the common ones) to see if you can inhibit or stimulate onion root tip growth. One way is to cut the tips and stain them and count to see at what stage of mitosis the individual cells are or to have the root tips cut to the same length, measure them and see if there is a change in length over time.

Most teachers do the meiosis portion of the lab with Unit 5.

Cell Communication and Mitosis tutorials and lab review:

Mitosis tutorial: Mitosis

Biomembranes II and Cell Communication Tutorial: Cell Communication

The following are simulations by Jon Darkow. These are excellent reviews of content and help you work on your science practice skills. Worksheets are provided. You can now run these simulations on your phone. Please note: Only two examples are provided here. 

Cell Cycle Simulation: Cell Cycle

Insulin Secretion and Membrane Transport:  Insulin

The following is a click and learn simulation from HHMI bio-interactive. One does a great job reviewing Cell Cycle Regulation and the other is about p53 genes and cancer.

HHMI Cell Cycle Regulation: Cell Cycle

HHMI The p53 Gene and Cancer:  p53

A unique form of Cell Communication is called Quorum Sensing. It is used by bacteria. 

HHMI Quorum Sensing: Quorum Sensing 

Another extension your class may have done is to read the book(or sections of the book) The Immortal Life of Henrietta Lacks by Rebecca Skloot , do an activity and discuss the HeLa cells. This book is a fantastic read and is relevant today because she had some of her cervical cancer tissue removed with her knowledge.

If interested in the Henrietta Lacks story: Henrietta Lacks

Unit 5: Heredity

You will learn about the concepts and processes associated with heredity. These include the passing on or transmission of genetic information using chromosomes from one generation to the next using the process of meiosis. Meiosis is a process that makes certain genetic diversity which is important for speciation to occur.

Meiosis is a process where a single cell divides two times to form four non-identical cells with half the original amount of genetic information. These particular cells are our sex cells; egg and sperm. They are haploid (n) which means they have half the number of chromosomes as the parent cell. The parent cell is called diploid (2n).

You study Mendelian and Non-Mendelian genetics and learn about patterns of inheritance. Meiosis is studied and its importance to our genetic variation. Lastly, you learn about how the environmental factors (like lack of oxygen, alcohol, etc.) and nondisjunction (failure of one or more homologous chromosomes or sister chromatids to separate)

Mendelian genetics is often used to try to explain inheritance and biological diversity based on studying pea plants. He found out that genes come in pairs and are inherited in genes, one from each parent. Mendelian genetics studies three main ideas or laws. 

1) Law of Segregation : the two alleles or forms of a trait will separate randomly, in which there is a 50% chance for either allele to show up in either gamete. 2) Law of Independent Assortment : One allele of one gene separates of an allele of a different gene. (This deals with two genes) 3) Law of dominance, which means one character or trait is dominant and the other is recessive, the dominant form of the trait will be expressed

Non-Mendelian genetics simply put do not follow Mendelian genetics laws. Listed below are a few examples:

Incomplete dominance: For example, Red and White flowers are crossed and form pink flowers.

Codominance:   Both genes expressed equally. For example blood typing:  AB blood typing is an example. (Types AA (Ai), BB (Bi), AB, ii or type O)

Polygenic Inheritance:   Human height is controlled by many genes!

Sex-linked: Usually X linked which means the trait usually shows up in males. An example of this is red-green color blindness.

A virtual lab that reviews various Mendelian and Non-mendelian cross. This will provide you data sets to evaluate and provide Chi-Square practice if needed. Included is a very short tutorial and questions so you can practice the types of crosses you need to review.

Virtual Lab site: Virtual Fly Lab

Virtual Fly Tutorial: Virtual Fly Video tutorial

Virtual Fly Practice Problems: Virtual Fly Practice Problems

Drosophila-Apterous vs. Wild-type simulation:  Apterous

Drosophilia-White-Eyed vs Wild-type simulation: White-Eyed

Gene Linkage and Recombination: Gene Linkage

Unit 6: Gene Expression and Regulation

Gene expression examines the role of DNA and RNA (Nucleic Acids) in gene expression. You will study the differences in structure between DNA and RNA. Understanding protein synthesis (transcription and translation) is key to understanding how an individual genotype leads to phenotype gene expression. The regulation of gene expression is very important for survival which transitions into Unit 7 Natural Selection (Evolution). 

The last topic discussed in the unit is biotechnology. You will study about all the advances in biotechnology and its importance to understanding biotechnology. Topics such as genetic engineering, GMO, gene cloning and transgenic organisms will be discussed.

DNA and RNA are the main sources of inherited information. Information is generally passed on from DNA-mRNA-Protein-(Expression). This process is often referred to as the Central Dogma . This information is stored in chromosomes and in eukaryotes the chromosomes are linear and in prokaryotes they are circular. Prokaryotes also contain small circular chromosomes called plasmids.

DNA is made (replication) in a 5’ to 3’ direction and is often called semiconservative, which means each strand is half new DNA and has the old strand. When making proteins DNA is copied into mRNA and the mRNA leaves the nucleus and goes to a ribosome. The mRNA attaches to the ribosome and tRNA brings amino acids to the mRNA and they make an amino acid chain through the process of transcription.

The mRNA can undergo modifications. A couple of things that can happen are the addition of a poly-A tail on one end of the addition of a GTP cap. Also, a phrase you may have been taught is “Exons are always expressed.”  Introns are spliced out of the transcript and the exons are transcribed. This process of splicing or cutting and the keeping of exons can result in different forms of the mRNA sometimes called alternative splicing.

Transcription, Translation and Gene Regulation Tutorials and Lab Reviews

DNA Replication Tutorial:  DNA Replication

Transcription Tutorial:  Transcription

Translation Tutorial:  Translation

The Lac Operon in Ecoli Tutorial: Lac Operon

Restriction Digest Tutorial:  Restriction Digest

Molecular Biology Labs Tutorial (Gel Electrophoresis and Transformation)

The following are simulations by Jon Darkow. These are excellent reviews of content and help you work on your science practice skills. Worksheets are provided. You can now run these simulations on your phone. 

Genotyping with Electrophoresis Simulation: Genotyping

Lac Operon with Diauxic Simulation: Lac Operon

The following is a click and learn simulation from HHMI biointeractive. These do a great job reviewing difficult topics in AP Biology. There are several. I will suggest two that may be the most helpful focusing on eukaryotic gene regulation that is difficult to explain. There is a worksheet provided to guide you for each.

Regulation of the Lactase Gene Click and Learn: Lactase Gene

RNA Interference:  RNAi

The following is a nice overview of protein synthesis and its relationship to gene expression and RNAi. Petunias and RNAi . (The first half of it.)

The specific labs done with this unit are related to gene expression and biotechnology and there are numerous variations of these labs. They are also very expensive and therefore your school may not have been able to afford them. One usually focuses on genetic transformation and you insert a gene of interest into a bacteria and make it change color.  

A sample procedure is you have a set up with 3-5 plates. Sometimes you can divide a plate in half to get more lab sets. You make two tubes with bacteria, usually labeled + for the bacteria with the plasmid with the gene of interest to insert into bacteria and one without the plasmid labeled - . The First of the 3-5 plates is LB, which has good bacteria food on it. It is a control plate to test if your bacteria are alive or not. You should expect growth on both plates. 

The second plate is LBA. This plate has Ampicillin on it, an antibiotic.  The + bacteria with the plasmid have antibiotic resistance will grow on the plate and the bacteria with - will not grow on the plate because they do not have the resistance to ampicillin.  

The last plate is LBAA and you only put the + bacteria on it.  This plate contains ampicillin and a sugar arabinose.  Arabinose is needed for the gene to be turned on to change the color of the bacteria from white to green.  You should notice that if the plasmid was taken up by the bacteria the bacteria will be a green or another bright color depending on the plasmid. See the image of the results.

The other usually focuses on gel electrophoresis, the use of restriction enzymes to cut DNA at specific spots while running the DNA through a gel. The reference to the DNA Learning Center website explores several other topics including Gel Electrophoresis (restriction analysis). Gel electrophoresis is often used by forensic scientists to evaluate DNA evidence in real life and on TV shows like CSI. 

Restriction enzymes in simple terms are DNA scissors. They cut DNA at specific places by looking for a specific DNA sequence. It is a way to separate sequences of DNA by size. You load DNA into wells in a gel and run electricity through it to help separate the DNA by size. The smaller pieces will travel farther than larger ones.  

DNA is negatively charged and DNA will travel towards the positive electrode. Here is a sample of what a gel looks like.

1. Virtual Gel Electrophoresis:  Gel Electrophoresis

2. DNA Learning Center Transformation

Unit 7: Natural Selection (Evolution)

First, the unit 7 title is natural selection , which is the process by which organisms that are better adapted to the environment will tend to survive better. Charles Darwin thoroughly studies and wrote about natural selection and this process became the driving force of evolution .

Natural selection is often discussed throughout your AP Biology course. Natural selection states that populations that are better to adapt to their environment tend to survive and are able to reproduce. During the unit, you will study about the different mechanisms of evolutionary change. 

Also, the topic of artificial selection (like breeding racehorses) and how that affects variation in species is learned about. Another topic of study is Hardy-Weinberg equilibrium as a way to predict changes in allele frequencies for populations that are not evolving. This involves analyzing data sets and working on some mathematical skills to help justify a conclusion. 

One other topic will be building and interpretation of cladograms or phylogenetic trees. You will look at the relationships and common ancestry of species. Finally, you will look at different hypotheses about the formation of the Earth. 

For more practice and understanding about topics associated with natural selection and evolution. Here are nice tutorials:

Evolution tutorial: Evolution 101

A lab tutorial for Hardy-Weinberg:

Hardy-Weinberg Lab Tutorial:  Hardy-Weinberg

A simulation for Hardy-Weinberg Lab Simulation

Hardy Weinberg Simulation

Here is a worksheet to guide you on how to use it with some suggested practice simulations. Radford Worksheet.

Jon Darkow Simulations: These are excellent reviews of content and help you work on your science practice skills. Worksheets are provided. You can now run these simulations on your phone. 

Evolution of Populations Simulations: Evolution of Populations 

Guppy Evolution Simulation: Guppy Evolution

Ground Finch Evolution: Ground Finch

HHMI Click and learn. These are quick reviews of common in-class labs.

Sorting Finches Click and Learn (Speciation):  Finches

Creating Phylogenetic Trees Click and Learn (Evolutionary Relationships): Trees

Sorting Shells Click and Learn: Shells  

The three labs associated with natural selection or evolution are an artificial selection lab, a Hardy-Weinberg lab and a BLAST lab comparing DNA sequences.

The Hardy-Weinberg lab is commonly done in class. In general, Hardy-Weinberg law states that large, random mating populations will not be affected by evolutionary processes such as mutations or types of selection. The allele frequencies will not change from one generation to the next. Examples are mentioned earlier in this section.

The artificial selection lab is not done by students that often due to time. You choose a trait to measure (such as plant height or a number of trichomes or hairs on leaves) and plant a couple of strains of fast-growing plants such as a Wisconsin Fast Plant and grow them. 

You artificially select which plants will grow until they flower after about two weeks (you remove the weaker, not as tall plants or plants with the most hairs or trichomes on the leaves) and allow more room for the ones you select to grow and allow them better access to the nutrients. You leave about 2 or 3 plants). You cross-pollinate the plants when they flower and when they grow to plant the seeds produced and compare the trait-like root hair number between the initial generation and second generation. These data are used on the AP Exam. Such as this in past free-response questions from 2014 . 

A BLAST lab is used to study DNA and study genes of interest. The lab uses Bioinformatics methodology to study similarities and differences in genomes. You learn to construct a cladogram or phylogenetic tree that is used to study the evolutionary relatedness of species. Cladograms ALWAYS so upon the AP Biology exam so it is an essential skill to learn how to make one and interpret a cladogram. 

There are many versions of the lab. The key is not understanding how to use BLAST but they understand the evolutionary relationships and the making of a cladogram.

HHMI Click and Learn Creating Phylogenetic Trees is a great practice.

If you want to physically make one and check your answers. Go here. Phylogenetic Trees . 

Unit 8: Ecology  

Unit 8 Ecology is the last unit of your AP Biology course. It is the topic that brings the course together and helps you make connections to the rest of the course.

You look at how systems interact and how these systems respond to changes in the environment. The flow of energy through a system is a key topic of discussion and study as well as studying the interactions of species and what happens to species when the system is altered significantly.

Major topics in the unit include response to environmental changes, flow of energy, population ecology, energy/food pyramids, community ecology and disruptions to the systems (such as invasive species (zebra mussels), disease (smallpox, dutch elm disease), human activity (logging, urbanization (building of larger cities, markets, etc.)

There is lots of math in this unit with the Simpson Diversity Index, Population Growth Rates, Exponential Growth, Population Growth. 

There are three labs with this unit. One is not done very often and the other two there are many forms of. The first lab is Energy Dynamics . It looks at the flow of energy. 

One version looks at a cabbage white butterfly life cycle and Wisconsin Fast Plants you study how to determine the net primary productivity. You end up learning to estimate the energy flow between the producers the Wisconsin Fast Plants and the White Cabbage Butterfly. There is a lot of math with this lab.

This is a virtual lab that reviews primary productivity. Attached also is a guide for it.

A simulation for Energy Dynamic Lab Tutorial:  Energy Dynamics

Here is a worksheet that may help guide you through the tutorial: Worksheet

The second lab is about Transpiration . Transpiration is the process by which water is carried through plants to stomata , small pores located on the underside of a leaf. It is often described as evaporation from plant leaves. It helps create a negative pressure gradient that helps draw up water and minerals through plants from the roots.

This is a common lab done in AP Biology classes. There are many versions.

One of the most common ways is called the whole plant. You set up plants under different conditions (100% light, humidity (plant under a bag with water sprayed in it to simulate humidity), dark environment and wind for example). You weigh them initially and weigh them every day to see how much water they lost over a week’s time and determine which plant condition had the greatest % of water loss.

This lab is sometimes done with unit 1: Chemistry of Life, because of water and its properties as a focus.

There are a couple of virtual labs that review transpiration. It uses photometers, which is a different version but covers the same concepts.

A simulation of Transpiration Lab:  Transpiration

A different simulation using photometers and explains the procedure in a little more detail:  Photometers

The last lab is about Animal Behavior. This lab is one of the more common labs done in AP Biology and often done at the beginning of the year. There are many versions. 

In most cases, students set up two choices in choice chambers and you place certain critters in to test (fruit flies, pillbugs, mealworms…) You place different foods (sweet like sugar or sour) or environments (light or dark) for the critters to choose from and see if the critter tested show a preference. It is possible to have more than two choices of environments for the critters to choose from.

You study to see if the critters are attracted to the environment or food or do they go away from it or is it random, no pattern. Kinesis is a movement with no pattern while taxis can be positive (towards it) or negative (away from it). There are different types of taxis (Phototaxis-towards or away from light for example).

Here is a simulation of the Animal Behavior Lab. This simulation covers the main concepts of the lab to review:

Animal Behavior Simulation: Animal Behavior

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Null hypothesis

Null hypothesis n., plural: null hypotheses [nʌl haɪˈpɒθɪsɪs] Definition: a hypothesis that is valid or presumed true until invalidated by a statistical test

Table of Contents

Null Hypothesis Definition

Null hypothesis is defined as “the commonly accepted fact (such as the sky is blue) and researcher aim to reject or nullify this fact”.

More formally, we can define a null hypothesis as “a statistical theory suggesting that no statistical relationship exists between given observed variables” .

In biology , the null hypothesis is used to nullify or reject a common belief. The researcher carries out the research which is aimed at rejecting the commonly accepted belief.

What Is a Null Hypothesis?

A hypothesis is defined as a theory or an assumption that is based on inadequate evidence. It needs and requires more experiments and testing for confirmation. There are two possibilities that by doing more experiments and testing, a hypothesis can be false or true. It means it can either prove wrong or true (Blackwelder, 1982).

For example, Susie assumes that mineral water helps in the better growth and nourishment of plants over distilled water. To prove this hypothesis, she performs this experiment for almost a month. She watered some plants with mineral water and some with distilled water.

In a hypothesis when there are no statistically significant relationships among the two variables, the hypothesis is said to be a null hypothesis. The investigator is trying to disprove such a hypothesis. In the above example of plants, the null hypothesis is:

There are no statistical relationships among the forms of water that are given to plants for growth and nourishment.

Usually, an investigator tries to prove the null hypothesis wrong and tries to explain a relation and association between the two variables.

An opposite and reverse of the null hypothesis are known as the alternate hypothesis . In the example of plants the alternate hypothesis is:

There are statistical relationships among the forms of water that are given to plants for growth and nourishment.

The example below shows the difference between null vs alternative hypotheses:

Alternate Hypothesis: The world is round Null Hypothesis: The world is not round.

Copernicus and many other scientists try to prove the null hypothesis wrong and false. By their experiments and testing, they make people believe that alternate hypotheses are correct and true. If they do not prove the null hypothesis experimentally wrong then people will not believe them and never consider the alternative hypothesis true and correct.

The alternative and null hypothesis for Susie’s assumption is:

• Null Hypothesis: If one plant is watered with distilled water and the other with mineral water, then there is no difference in the growth and nourishment of these two plants.
• Alternative Hypothesis:  If one plant is watered with distilled water and the other with mineral water, then the plant with mineral water shows better growth and nourishment.

The null hypothesis suggests that there is no significant or statistical relationship. The relation can either be in a single set of variables or among two sets of variables.

Most people consider the null hypothesis true and correct. Scientists work and perform different experiments and do a variety of research so that they can prove the null hypothesis wrong or nullify it. For this purpose, they design an alternate hypothesis that they think is correct or true. The null hypothesis symbol is H 0 (it is read as H null or H zero ).

Why is it named the “Null”?

The name null is given to this hypothesis to clarify and explain that the scientists are working to prove it false i.e. to nullify the hypothesis. Sometimes it confuses the readers; they might misunderstand it and think that statement has nothing. It is blank but, actually, it is not. It is more appropriate and suitable to call it a nullifiable hypothesis instead of the null hypothesis.

Why do we need to assess it? Why not just verify an alternate one?

In science, the scientific method is used. It involves a series of different steps. Scientists perform these steps so that a hypothesis can be proved false or true. Scientists do this to confirm that there will be any limitation or inadequacy in the new hypothesis. Experiments are done by considering both alternative and null hypotheses, which makes the research safe. It gives a negative as well as a bad impact on research if a null hypothesis is not included or a part of the study. It seems like you are not taking your research seriously and not concerned about it and just want to impose your results as correct and true if the null hypothesis is not a part of the study.

Development of the Null

In statistics, firstly it is necessary to design alternate and null hypotheses from the given problem. Splitting the problem into small steps makes the pathway towards the solution easier and less challenging. how to write a null hypothesis?

Writing a null hypothesis consists of two steps:

• Firstly, initiate by asking a question.
• Secondly, restate the question in such a way that it seems there are no relationships among the variables.

In other words, assume in such a way that the treatment does not have any effect.

The usual recovery duration after knee surgery is considered almost 8 weeks.

A researcher thinks that the recovery period may get elongated if patients go to a physiotherapist for rehabilitation twice per week, instead of thrice per week, i.e. recovery duration reduces if the patient goes three times for rehabilitation instead of two times.

Step 1: Look for the problem in the hypothesis. The hypothesis either be a word or can be a statement. In the above example the hypothesis is:

“The expected recovery period in knee rehabilitation is more than 8 weeks”

Step 2: Make a mathematical statement from the hypothesis. Averages can also be represented as μ, thus the null hypothesis formula will be.

In the above equation, the hypothesis is equivalent to H1, the average is denoted by μ and > that the average is greater than eight.

Step 3: Explain what will come up if the hypothesis does not come right i.e., the rehabilitation period may not proceed more than 08 weeks.

There are two options: either the recovery will be less than or equal to 8 weeks.

H 0 : μ ≤ 8

In the above equation, the null hypothesis is equivalent to H 0 , the average is denoted by μ and ≤ represents that the average is less than or equal to eight.

What will happen if the scientist does not have any knowledge about the outcome?

Problem: An investigator investigates the post-operative impact and influence of radical exercise on patients who have operative procedures of the knee. The chances are either the exercise will improve the recovery or will make it worse. The usual time for recovery is 8 weeks.

Step 1: Make a null hypothesis i.e. the exercise does not show any effect and the recovery time remains almost 8 weeks.

H 0 : μ = 8

In the above equation, the null hypothesis is equivalent to H 0 , the average is denoted by μ, and the equal sign (=) shows that the average is equal to eight.

Step 2: Make the alternate hypothesis which is the reverse of the null hypothesis. Particularly what will happen if treatment (exercise) makes an impact?

In the above equation, the alternate hypothesis is equivalent to H1, the average is denoted by μ and not equal sign (≠) represents that the average is not equal to eight.

Significance Tests

To get a reasonable and probable clarification of statistics (data), a significance test is performed. The null hypothesis does not have data. It is a piece of information or statement which contains numerical figures about the population. The data can be in different forms like in means or proportions. It can either be the difference of proportions and means or any odd ratio.

The following table will explain the symbols:

P-value is the chief statistical final result of the significance test of the null hypothesis.

• P-value = Pr(data or data more extreme | H 0 true)
• | = “given”
• Pr = probability
• H 0 = the null hypothesis

The first stage of Null Hypothesis Significance Testing (NHST) is to form an alternate and null hypothesis. By this, the research question can be briefly explained.

Null Hypothesis = no effect of treatment, no difference, no association Alternative Hypothesis = effective treatment, difference, association

When to reject the null hypothesis?

Researchers will reject the null hypothesis if it is proven wrong after experimentation. Researchers accept null hypothesis to be true and correct until it is proven wrong or false. On the other hand, the researchers try to strengthen the alternate hypothesis. The binomial test is performed on a sample and after that, a series of tests were performed (Frick, 1995).

Step 1: Evaluate and read the research question carefully and consciously and make a null hypothesis. Verify the sample that supports the binomial proportion. If there is no difference then find out the value of the binomial parameter.

Show the null hypothesis as:

H 0 :p= the value of p if H 0 is true

To find out how much it varies from the proposed data and the value of the null hypothesis, calculate the sample proportion.

Step 2: In test statistics, find the binomial test that comes under the null hypothesis. The test must be based on precise and thorough probabilities. Also make a list of pmf that apply, when the null hypothesis proves true and correct.

When H 0 is true, X~b(n, p)

N = size of the sample

P = assume value if H 0 proves true.

Step 3: Find out the value of P. P-value is the probability of data that is under observation.

Rise or increase in the P value = Pr(X ≥ x)

X = observed number of successes

P value = Pr(X ≤ x).

Step 4: Demonstrate the findings or outcomes in a descriptive detailed way.

• Sample proportion
• The direction of difference (either increases or decreases)

Perceived Problems With the Null Hypothesis

Variable or model selection and less information in some cases are the chief important issues that affect the testing of the null hypothesis. Statistical tests of the null hypothesis are reasonably not strong. There is randomization about significance. (Gill, 1999) The main issue with the testing of the null hypothesis is that they all are wrong or false on a ground basis.

There is another problem with the a-level . This is an ignored but also a well-known problem. The value of a-level is without a theoretical basis and thus there is randomization in conventional values, most commonly 0.q, 0.5, or 0.01. If a fixed value of a is used, it will result in the formation of two categories (significant and non-significant) The issue of a randomized rejection or non-rejection is also present when there is a practical matter which is the strong point of the evidence related to a scientific matter.

The P-value has the foremost importance in the testing of null hypothesis but as an inferential tool and for interpretation, it has a problem. The P-value is the probability of getting a test statistic at least as extreme as the observed one.

The main point about the definition is: Observed results are not based on a-value

Moreover, the evidence against the null hypothesis was overstated due to unobserved results. A-value has importance more than just being a statement. It is a precise statement about the evidence from the observed results or data. Similarly, researchers found that P-values are objectionable. They do not prefer null hypotheses in testing. It is also clear that the P-value is strictly dependent on the null hypothesis. It is computer-based statistics. In some precise experiments, the null hypothesis statistics and actual sampling distribution are closely related but this does not become possible in observational studies.

Some researchers pointed out that the P-value is depending on the sample size. If the true and exact difference is small, a null hypothesis even of a large sample may get rejected. This shows the difference between biological importance and statistical significance. (Killeen, 2005)

Another issue is the fix a-level, i.e., 0.1. On the basis, if a-level a null hypothesis of a large sample may get accepted or rejected. If the size of simple is infinity and the null hypothesis is proved true there are still chances of Type I error. That is the reason this approach or method is not considered consistent and reliable. There is also another problem that the exact information about the precision and size of the estimated effect cannot be known. The only solution is to state the size of the effect and its precision.

Null Hypothesis Examples

Here are some examples:

Example 1: Hypotheses with One Sample of One Categorical Variable

Among all the population of humans, almost 10% of people prefer to do their task with their left hand i.e. left-handed. Let suppose, a researcher in the Penn States says that the population of students at the College of Arts and Architecture is mostly left-handed as compared to the general population of humans in general public society. In this case, there is only a sample and there is a comparison among the known population values to the population proportion of sample value.

• Research Question: Do artists more expected to be left-handed as compared to the common population persons in society?
• Response Variable: Sorting the student into two categories. One category has left-handed persons and the other category have right-handed persons.
• Form Null Hypothesis: Arts and Architecture college students are no more predicted to be lefty as compared to the common population persons in society (Lefty students of Arts and Architecture college population is 10% or p= 0.10)

Example 2: Hypotheses with One Sample of One Measurement Variable

A generic brand of antihistamine Diphenhydramine making medicine in the form of a capsule, having a 50mg dose. The maker of the medicines is concerned that the machine has come out of calibration and is not making more capsules with the suitable and appropriate dose.

• Research Question: Does the statistical data recommended about the mean and average dosage of the population differ from 50mg?
• Response Variable: Chemical assay used to find the appropriate dosage of the active ingredient.
• Null Hypothesis: Usually, the 50mg dosage of capsules of this trade name (population average and means dosage =50 mg).

Example 3: Hypotheses with Two Samples of One Categorical Variable

Several people choose vegetarian meals on a daily basis. Typically, the researcher thought that females like vegetarian meals more than males.

• Research Question: Does the data recommend that females (women) prefer vegetarian meals more than males (men) regularly?
• Response Variable: Cataloguing the persons into vegetarian and non-vegetarian categories. Grouping Variable: Gender
• Null Hypothesis: Gender is not linked to those who like vegetarian meals. (Population percent of women who eat vegetarian meals regularly = population percent of men who eat vegetarian meals regularly or p women = p men).

Example 4: Hypotheses with Two Samples of One Measurement Variable

Nowadays obesity and being overweight is one of the major and dangerous health issues. Research is performed to confirm that a low carbohydrates diet leads to faster weight loss than a low-fat diet.

• Research Question: Does the given data recommend that usually, a low-carbohydrate diet helps in losing weight faster as compared to a low-fat diet?
• Response Variable: Weight loss (pounds)
• Explanatory Variable: Form of diet either low carbohydrate or low fat
• Null Hypothesis: There is no significant difference when comparing the mean loss of weight of people using a low carbohydrate diet to people using a diet having low fat. (population means loss of weight on a low carbohydrate diet = population means loss of weight on a diet containing low fat).

Example 5: Hypotheses about the relationship between Two Categorical Variables

A case-control study was performed. The study contains nonsmokers, stroke patients, and controls. The subjects are of the same occupation and age and the question was asked if someone at their home or close surrounding smokes?

• Research Question: Did second-hand smoke enhance the chances of stroke?
• Variables: There are 02 diverse categories of variables. (Controls and stroke patients) (whether the smoker lives in the same house). The chances of having a stroke will be increased if a person is living with a smoker.
• Null Hypothesis: There is no significant relationship between a passive smoker and stroke or brain attack. (odds ratio between stroke and the passive smoker is equal to 1).

Example 6: Hypotheses about the relationship between Two Measurement Variables

A financial expert observes that there is somehow a positive and effective relationship between the variation in stock rate price and the quantity of stock bought by non-management employees

• Response variable- Regular alteration in price
• Explanatory Variable- Stock bought by non-management employees
• Null Hypothesis: The association and relationship between the regular stock price alteration ($) and the daily stock-buying by non-management employees ($) = 0.

Example 7: Hypotheses about comparing the relationship between Two Measurement Variables in Two Samples

• Research Question: Is the relation between the bill paid in a restaurant and the tip given to the waiter, is linear? Is this relation different for dining and family restaurants?
• Explanatory Variable- total bill amount
• Response Variable- the amount of tip
• Null Hypothesis: The relationship and association between the total bill quantity at a family or dining restaurant and the tip, is the same.

Try to answer the quiz below to check what you have learned so far about the null hypothesis.

Choose the best answer. 

Send Your Results (Optional)

• Blackwelder, W. C. (1982). “Proving the null hypothesis” in clinical trials. Controlled Clinical Trials , 3(4), 345–353.
• Frick, R. W. (1995). Accepting the null hypothesis. Memory & Cognition, 23(1), 132–138.
• Gill, J. (1999). The insignificance of null hypothesis significance testing. Political Research Quarterly , 52(3), 647–674.
• Killeen, P. R. (2005). An alternative to null-hypothesis significance tests. Psychological Science, 16(5), 345–353.

©BiologyOnline.com. Content provided and moderated by Biology Online Editors.

Last updated on June 16th, 2022

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BIOLOGY JUNCTION

Test And Quizzes for Biology, Pre-AP, Or AP Biology For Teachers And Students

Lab & Ap Sample 2 Mitosis & Meiosis

Introduction Cells come from preexisting cells. New cells are formed during cell division which involves both replication of the cell’s nucleus, karyokinesis, and division of the cytoplasm, cytokinesis. The two kinds of cellular division are mitosis and meiosis. Mitosis usually makes body cells, somatic cells. Making an adult organism from an egg, asexual reproduction, regeneration, and the maintenance and repair of body parts are performed during mitotic cell division. This process called meiosis makes gametes, in animals, and spores, in plants. Gamete or spore cells have half the chromosomes that the parent cell has.

In plants mitosis takes place in the meristems which are normally found at the tips of stems or roots. However, in animal cells cell division takes place every where as new cells are formed and old ones are replaced. Studying mitosis can be accomplished by looking at tissues where there are many cells in a process of meiosis. Two examples are an onion root tip, or developing embryos, in animals such as whitefish blastula. A blastula is formed after an egg is fertilized and the egg begins to divide. There are several phases of the mitotic cell cycle. A precursor to mitosis is interphase. The actual steps of the mitotic cell cycle are prophase, metaphase, anaphase, and telophase. Interphase is a stage in the cell cycle in which the cell is not dividing. The nucleus contains a nucleolus and also contains chromatin. During interphase DNA replication occurs. The first phase of mitotic cell division is prophase. During prophase the chromatin begins to thicken until noticeable chromosomes are formed. Each chromosome has two chromatids that are joined at the centromere. During the later part of prophase, the nuclear envelope and nucleolus disappear. Mitotic spindle fibers, composed of microtubules, also become apparent. Following prophase is metaphase. By the time the cell has reached metaphase the chromosomes have moved to the center of the mitotic spindle. The centromere of the chromosome attaches to the spindle. The centromeres of each chromosome line up on an area called the metaphase plate. Metaphase is followed by anaphase. In the beginning of anaphase, the centromeres of each pair of chromatids separate and moved by the spindle fibers to the opposite ends of the cell. When the daughter chromosomes reach the ends of the cell the form a clump at each spindle pole. The final phase of mitosis is telophase. Telophase is identified by a recognizable condensation of the chromosomes, which is followed by the formation of a new nuclear envelope. The chromosomes slowly uncoil into chromatin once again and the nucleoli and nuclear envelope reform. It is then possible for cytokinesis, the division of the cytoplasm into two cells, to occur. In an animal cells a cleavage furrow forms and the cell pinches off into two new daughter cells.

The process of meiosis involves two nuclear divisions that result in the formation of four haploid cells. Meiosis I, a reduction division, is the first division to reduce the chromosome number from diploid to haploid and separates the homologous pairs. Meiosis II separates the sister chromatids resulting in four haploid gametes. Unlike mitosis meiosis increases genetic variation. In meiosis I each pair of homologous chromosomes come together which is known as a synapse. Chromatids of homologous chromosomes may exchange parts which is called crossing over. The distance between two genes on a chromosome may be estimated by calculating the percentage of crossing over that takes place between them. Meiosis I is preceded by interphase. During interphase DNA synthesis occurs and each chromosome is made of two chromatids joined at the centromeres. The first step of meiosis I is prophase I. During prophase I homologous chromosomes come together and synapse. A tetrad consisting of four chromatids is also formed. Prophase I is followed by metaphase I. In metaphase I the crossed over tetrads line up in the center of the cell. In anaphase I the homologous chromosomes separate and are moved to opposite ends of the cell. The final phase of meiosis I is telophase I. During telophase I centriole duplication is completed. Most of the time cytokinesis and formation of the nuclear envelope occur in order two make to cells. Meiosis II a second mitotic cell division then takes place in order to separate the chromatids in the two daughter cells made in meiosis I. This reduces the amount of DNA to one strand per chromosome. This is the only difference between meiosis I and II. Before meiosis II there is period called interphase or interkenesis. DNA replication does not take place in interphase II. Interphase II is followed by prophase II, No DNA replication occurs in prophase II and replicated centrioles separate and move to opposite sides of the chromosome groups. During metaphase II the chromosomes are centered in the middle of each daughter cell. During anaphase II the centromere regions of the chromatids are separate. The last stage of meiosis II is telophase II. In telophase II the chromosomes are at opposite ends of the cell and a nuclear envelope forms, and sometimes the cytoplasm divides.

Sordaria fimicola is fungus that may be used to show the results of crossing over during meiosis. Sordaria throughout most of its life is haploid, but becomes diploid after the fusion of two different types of nuclei, which forms a diploid nucleus. In Sordaria meiosis results in the making of eight haploid ascospores found in a sac called an ascus. Most asci are found in a perithecium. The life cycle of Sordaria fimicola is as follows: a spore is discharged through an ascus. The ascospore then undergoes mitosis, which forms a filament. The filament then undergoes mitosis, which forms a mycelium. Mycelial fusion and fertilization then takes place. This forms a diploid zygote. The zygote undergoes mitosis to form four haploid nuclei. The nuclei also undergo mitosis and form eight haploid nuclei, which then form eight ascospores. When mycelia of a mutant strain of Sordaria and a wild type of Sordaria undergo meiosis four black and four tan ascospores form. The arrangement of the ascospores reflects whether crossing over has occurred or not. Gametes, egg and sperm, are made during meiosis. Each egg and sperm cell contains half the total chromosomes a normal cell of that species would have. When the egg and sperm unite during fertilization the total chromosome number is restored.

Exercise 3A.1 –  Hypothesis While looking at prepared slides of onion root tip cells and whitefish blastula cells under a microscope I will be able to identify and draw the stages of mitosis in these cells.

Materials The materials used in this experiment were a light microscope and prepared slides of onion root tip cells and whitefish blastula cells.

Methods Using the microscope examine the slides of onion root tip cells and whitefish blastula cells. Begin by locating the merismatic region of the onion or the blastula using the 10 X objective. Then use the 40 X objective to study individual cells. Identify one cell that clearly represents each phase. Sketch and label the cell on a separate piece of paper.

Exercise 3A.2  –  Hypothesis When undergoing mitosis most of the cells in an onion root tip will be in interphase. More cells will be in the stage of prophase than metaphase. More cells will be in metaphase than anaphase and more cells will be in anaphase than telophase.

Materials The materials used in this experiment were a prepared slide of an onion root tip and a light microscope.

Methods Obtain a prepared slide of an onion root tip and observe every cell in one high power field of view and determine which phase of the cell cycle it is in. Make sure to do this in pairs so one person can observe the cells and the other person can record which phase the cell is in. Make sure to count three full fields of view and at least 200 cells. Then, record your data in table 3.1. Next, calculate the percentage of cells in each phase by using the equation; percentage of cells in stage X 1,440 minutes =_________ minutes of cell cycle spent in stage.

Exercise 3B.1  –  Hypothesis Using beads it will be possible to show the stages of meiosis I and meiosis II.

Materials The materials used in this exercise were chromosome simulation kits containing four strands of beads. Two strands will be one color and the other two strands should be another color.

Methods To show the process of interphase place one strand of each color near the center of your work area. Next, simulate DNA replication by bringing the magnetic centromere region of one strand in contact with the centromere region of the other of the same color. Do the same with its homolog. Next, to show crossing over in prophase I pop the beads apart on one chromatid at the fifth bead. Do the same with the other chromatid. Then reconnect the beads to those of the other color. Proceed through prophase I of meiosis and note how crossing over results in recombination of genetic information. Then to show metaphase I place the chromosomes near the middle of the cell. During anaphase I, the homologous chromosomes separate and are “pulled” to opposite ends of the cell. Next, to show telophase I place each chromosome at opposite sides of the cell.

In prophase II of meiosis II replicated centrioles separate and move to opposite sides of the chromosome groups. Next, to show metaphase II arrange the chromosomes so they are centered in the middle of each daughter cell. Then, separate the chromatids of the chromosomes and pull the daughter chromosomes toward the opposite sides of the daughter cell in order to show anaphase II. Finally in order to show telophase II, place the chromosomes at opposite sides of the dividing cell.

Exercise 3B.2  –  Hypothesis There will be more asci that maintain a 4:4 relationship of not crossing over than asci that do cross over.

Materials The materials used in this exercise were a prepared slide of Sordaria fimicola and a light microscope.

Methods Begin by obtaining a prepared slide that contain asci of Sordaria fimicola. Then, using the 10 X objective, view the slide and locate a group of hybrid asci. Make sure to count at least 50 hybrid asci and enter your data in table 3.3.

Results Exercise 3A.1

Exercise 3A.2

Exercise 3B.2

Questions: Exercise 3A.1

1.Why is it more accurate to call mitosis “nuclear replication” than “cellular division”?

In mitosis two new nuclei are being formed. Also cytokinesis is actually a part of mitosis.

2. Explain why the whitefish blastula and onion root tip are selected for a study of mitosis?

A blastula is a hollow ball of cells that forms when an egg divides quickly; a large amount of mitosis is taking place here. The onion root tip is the place where growth occurs in the onion so a large amount of mitosis is taking place here.

Exercise 3A.1

1. If your observations had not been restricted to the area of the root tip that is actively dividing, how would your results have been different?

The tips are located in the meristem. Cells that are not in the meristem do not divide as quickly but they are elongating and differentiating. None of the phases would have been visible.

2. Based on the data in table 3.1 what can you infer about the relative length of time an onion root-tip cell spends in each stage of cell division?

Prophase is the longest stage and telophase is the shortest.

Exercise 3B.1

1. List three major differences between the events of mitosis and meiosis?

Mitosis has one nuclear division and meiosis has two nuclear divisions. Mitosis makes two identical daughter cells. Meiosis makes four daughter cells that half the number of chromosomes that their parent cells had. Crossing over and the exchange of genes occurs in meiosis but not in mitosis.

2.Compare mitosis and meiosis with respect to the following:

3. How are meiosis I and meiosis II different?

Meiosis I starts with a tetrad and separates the homologs. In meiosis the strands separate into 4.

4. How do oogenesis and spermatogenesis differ?

In oogenesis an egg is formed and three cells called polar bodies die. In spermatogenesis sperm are formed.

5. Why is meiosis important for sexual reproduction?

In meiosis the chromosome number is reduced by half. When fertlization occurs chromosome number is restored. Gene exchange causes variation.

6.Using your data in table 3.3 determine the distance between the gene for spore color and the centromere. Record your results in table 3.3.

7. Draw a pair of chromosomes in MI and MII and show how you would get a 2:4:2 arrangement of ascospres by crossing over.

Error Analysis In exercise 3A.2 inaccurate results were received. There should have been fewer cells in telophase than any of the other phases and cells should have spent less time in telophase than in any of the other phases. The results received are inaccurate because the number of cells that were in telophase were improperly counted. We received results that more cells were in telophase and spent more time in telophase than anaphase. In exercise 3B.2 improperly identifying some of the asci as crossover or non-crossover might have caused the results that were received to be inaccurate.

Conclusions:

From these experiments one can conclude that it is possible to look at mitotic stages of onion root tip cells and whitefish blastula through a microscope and draw them. Also, from these experiments one can conclude that most of the cell cycle in an onion root tip is spent in interphase. Prophase is after interphase in time spent in each cycle. Metaphase is after prophase. Anaphase is after metaphase. The least amount of time is spent in telophase. Also, a person can simulate the chromosomes in meiosis I and meiosis II using a chromosome simulation kit. Finally, one can conclude form the results of the experiments that more asci do not cross over in Sordaria fimicola than the number of asci that do cross over.