AP Biology

## Learning Outcomes

- Describe how cell cycle checkpoints regulate progression through G1, S, G2, and M phase.
- Explain how cyclins and cyclin-dependent kinases control entry into mitosis.
- Distinguish between proto-oncogenes, oncogenes, and tumor suppressor genes.
- Explain why mutations in cell cycle control genes can lead to cancer.
- Connect cancer cell behavior to loss of density-dependent inhibition and anchorage dependence.
- Describe how apoptosis prevents damaged cells from continuing through the cell cycle.
- Predict the effect of altered cyclin, CDK, or checkpoint activity on mitosis using experimental data.

## AP Biology Syllabus

In AP Biology Unit 4, regulation of the cell cycle is tested as part of cell communication and cell cycle control. You should be able to explain how internal signals, checkpoint pathways, cyclins, cyclin-dependent kinases, and apoptosis help maintain normal growth, and how failure of these controls can lead to cancer. Questions often ask you to interpret graphs, experimental treatments, mutant phenotypes, or data on numbers of cells in different cell cycle phases.

- Cell cycle checkpoints determine whether a cell is ready to proceed to the next phase.
- Cyclins and cyclin-dependent kinases form active complexes that promote cell cycle progression.
- Mitosis-promoting factor helps trigger entry into mitosis.
- DNA damage can stop the cell cycle while repair occurs.
- If damage cannot be repaired, apoptosis can remove the damaged cell.
- Cancer can result when cells override checkpoints and divide without normal control.
- Oncogenes promote excessive cell division, while tumor suppressor gene loss removes inhibitory control.
- Cancer cells often ignore density-dependent inhibition and anchorage dependence.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why can a mutation in a cell cycle control protein cause uncontrolled cell division?
2. How do cyclins and cyclin-dependent kinases work together to promote progression through the cell cycle?
3. What is the difference between a proto-oncogene and an oncogene?
4. Why does loss of a tumor suppressor gene increase the chance of cancer?
5. How do apoptosis, density-dependent inhibition, and anchorage dependence normally help prevent abnormal growth?

## Introduction

The cell cycle is the sequence of events by which a cell grows, replicates its DNA, and divides. In multicellular organisms, this process must be controlled carefully. Cells need to divide for growth, tissue repair, and replacement of dead cells, but they should not divide when their DNA is damaged, when they lack the needed resources, or when the tissue already has enough cells.

Cancer develops when normal controls over the cell cycle fail. A cancer cell is not simply a cell that divides quickly; it is a cell that divides inappropriately, ignores signals that normally stop division, avoids programmed cell death, and may spread into nearby tissues or other parts of the body. For AP Biology, the key idea is that cancer is linked to disruptions in regulation: checkpoints fail, growth-promoting genes become overactive, growth-inhibiting genes lose function, and apoptosis may be avoided.

> **Key Term: cell cycle**
> The ordered sequence of events in a cell’s life, including interphase, mitosis, and cytokinesis.
>
> **Key Term: cancer**
> A disease state in which cells grow and divide abnormally and may invade surrounding tissues or spread to other parts of the body.

### Checkpoints prevent damaged cells from dividing

Most of a cell’s life is spent in interphase, which includes G1, S, and G2. During G1, the cell grows and prepares for DNA replication. During S phase, DNA is replicated, so each chromosome is copied into two identical sister chromatids held together at a centromere. During G2, the cell continues to grow and produces proteins needed for mitosis, such as proteins involved in spindle formation. Mitosis then distributes the replicated genetic material into daughter nuclei, and cytokinesis divides the cytoplasm.

Because DNA replication and chromosome separation must be accurate, eukaryotic cells use checkpoints. A checkpoint is a control point at which the cell evaluates whether conditions are suitable for continuing. If something is wrong, cell cycle progression can stop.

> **Key Term: cell cycle checkpoint**
> A control mechanism that monitors whether a cell is ready to proceed to the next stage of the cell cycle.

Checkpoint pathways are a form of intracellular signaling. They respond to information about the cell’s internal condition. For example, if DNA damage is detected, the cell should not proceed into division. Passing damaged DNA to daughter cells could produce cells with mutations in genes that control growth, metabolism, signaling, or survival. A checkpoint can pause the cell cycle, giving the cell time to repair its DNA.

The major checkpoint idea for AP Biology is not memorizing every molecular detail. Instead, you should understand the logic:

- If DNA is damaged, the cell cycle should pause.
- If chromosomes have not replicated correctly, the cell should not enter mitosis.
- If chromosomes are not attached properly to spindle fibers, sister chromatids should not separate.
- If the damage is too severe to repair, the cell may undergo apoptosis.

> **Key Term: apoptosis**
> Programmed cell death, a regulated process that removes cells that are damaged, unneeded, or potentially harmful.

Apoptosis is a protective process. For example, during embryonic development, apoptosis removes tissue between developing fingers. In the context of cancer prevention, apoptosis can eliminate cells with serious DNA damage before they divide and pass mutations to daughter cells. Once apoptosis begins, it proceeds in a controlled, irreversible way.

> **Test Tip:**
> If an AP Biology question says a mutation causes cells with DNA damage to continue dividing, connect your answer to checkpoint failure and possible cancer. A high-scoring explanation should state that damaged DNA can be passed to daughter cells, increasing mutation accumulation.

Cancer cells often override checkpoints. They may continue through the cell cycle even when DNA damage is present. Over time, these cells can accumulate more mutations, increasing abnormal behavior. This is why checkpoint control and apoptosis are closely linked to cancer.

### Cyclins, CDKs, and mitosis-promoting factor control cell cycle progression

Cell cycle checkpoints regulate cell cycle progression partly through proteins called cyclins and cyclin-dependent kinases. These proteins help determine when a cell moves from one phase to another.

> **Key Term: cyclin**
> A regulatory protein whose concentration changes during the cell cycle and helps activate cyclin-dependent kinases.
>
> **Key Term: cyclin-dependent kinase**
> An enzyme that adds phosphate groups to target proteins but is active only when bound to a cyclin.

Cyclin-dependent kinases, often shortened to CDKs, are present at relatively constant levels during the cell cycle. However, they are inactive unless bound to cyclins. Cyclin levels rise and fall as the cell cycle proceeds. When the correct cyclin binds to a CDK, the cyclin-CDK complex becomes active. This active complex can phosphorylate target proteins, causing changes that move the cell cycle forward.

A key example is mitosis-promoting factor, or MPF. MPF forms when a cyclin binds to a cyclin-dependent kinase. MPF triggers the beginning of mitosis. Cyclin concentration reaches a high level just before mitosis begins, allowing enough active MPF to form.

> **Key Term: mitosis-promoting factor**
> An active cyclin-CDK complex that triggers entry into mitosis.

Think of CDKs as machines that are present throughout the cell cycle but need cyclins as “on” switches. If cyclin levels are low, CDKs remain inactive and cell cycle progression is limited. If cyclin levels rise at the correct time, CDKs become active and push the cell toward the next stage.

This explains why mutations that alter cyclin or CDK activity can affect the cell cycle. If a mutation causes constant high production of a cyclin that promotes mitosis, MPF may remain active for too long or form too often. The cell may enter mitosis more frequently than normal. If a mutation prevents cyclin breakdown, the same problem can occur: signals that should decrease may remain active.

A mutation could also affect a CDK. If a CDK became active even without cyclin binding, cell cycle progression might occur when it should not. On the other hand, if a CDK could not bind cyclin, cells might fail to progress normally through the cell cycle.

> **Exam Warning:**
> Do not say cyclins and CDKs are the same thing. CDK levels are generally constant, while cyclin levels vary during the cell cycle. CDKs are enzymes; cyclins are regulatory proteins that activate them.

AP questions may show a graph with one molecule at a constant concentration and another rising before mitosis. The constant molecule is likely the CDK. The molecule that changes in concentration is likely the cyclin. Mitosis begins when enough active cyclin-CDK complex forms.

A typical explanation might be:

> **Question:**
> A cancer cell has a mutation that causes constant high production of a cyclin involved in mitosis. Explain how mitosis might be affected.
>
> **Answer:**
> The high cyclin level would increase formation of active cyclin-CDK complexes such as MPF. This could cause the cell to enter mitosis more often or at inappropriate times, increasing cell division and contributing to uncontrolled growth.

Notice that the answer connects the molecule to the process. It does not merely say “more cyclin causes cancer.” It explains the mechanism: more cyclin leads to more active cyclin-CDK complex, which promotes mitosis.

### Oncogenes act like a stuck accelerator

Genes that regulate growth and division can be altered by mutation. Some genes normally help cells grow and divide when division is needed. These normal genes are called proto-oncogenes. When a proto-oncogene is mutated or overexpressed in a way that makes it excessively active, it may become an oncogene.

> **Key Term: proto-oncogene**
> A normal gene involved in promoting cell growth or cell cycle progression when properly regulated.
>
> **Key Term: oncogene**
> A mutated or abnormally regulated form of a growth-promoting gene that can help convert a normal cell into a cancer cell.

A useful AP Biology comparison is that an oncogene is like a stuck accelerator pedal. The cell receives too much “divide” signaling. The normal version of the gene may be needed for healthy growth, development, or repair, but the mutated version promotes division when it should not.

Oncogenes can arise in several general ways. You do not need to memorize a long list, but you should recognize common patterns:

- A mutation may produce a growth-promoting protein that is always active.
- A mutation may cause too much of a growth-promoting protein to be made.
- A gene may be placed under control of a stronger regulatory sequence, increasing expression.
- A signaling protein may send a continuous division signal even without the normal signal molecule.

For example, imagine a receptor pathway that normally responds to a growth factor. In a healthy cell, the growth factor binds the receptor only under certain conditions, activating a signal transduction pathway that promotes cell division. If a mutation makes the receptor active even without the growth factor, the cell may behave as if it is constantly receiving a growth signal.

This connects Unit 4 cell communication to cell cycle control. Cancer can result from altered communication pathways as well as altered cell cycle proteins. A cell that keeps receiving or producing growth signals may pass through checkpoints too often or at the wrong time.

Proto-oncogenes and oncogenes are often tested with cause-and-effect wording. A proto-oncogene is not “bad.” It is a normal gene. The problem occurs when it becomes overactive or is expressed inappropriately.

> **Test Tip:**
> If a question asks how an oncogene causes cancer, use “gain of function” reasoning. The gene product has increased activity, increased amount, or abnormal activation, causing excess cell division.

A single oncogene may increase cancer risk, but cancer usually involves multiple changes. Cells often need to gain growth-promoting changes and lose growth-inhibiting controls before becoming fully cancerous. That leads to the second major gene category: tumor suppressor genes.

### Tumor suppressor genes and apoptosis act as brakes

Tumor suppressor genes produce proteins that help prevent uncontrolled cell division. They may detect DNA damage, help stop the cell cycle, work with cyclin-CDK regulation, support DNA repair, or trigger apoptosis if damage cannot be repaired.

> **Key Term: tumor suppressor gene**
> A gene that produces a protein that helps prevent abnormal cell growth or cancer formation.

If oncogenes are like a stuck accelerator, tumor suppressor gene loss is like brake failure. The cell loses a control that normally slows or stops division. For AP Biology, the key contrast is:

- Oncogenes promote cancer by excessive growth-promoting activity.
- Tumor suppressor gene mutations promote cancer by loss of growth-inhibiting activity.

A tumor suppressor protein might normally stop the cell cycle at a checkpoint when DNA is damaged. If the gene for that protein is mutated so the protein no longer functions, the cell may continue into S phase or mitosis despite DNA damage. The daughter cells inherit the damaged DNA, and further mutations can accumulate.

A mutation can alter a tumor suppressor gene by changing the amino acid sequence of its protein product. DNA is transcribed into mRNA, and mRNA is translated into a chain of amino acids. A change in the DNA sequence can change a codon, add a premature stop codon, or shift the reading frame. The resulting protein may fold incorrectly, fail to bind its target, or not be produced at all. Since protein shape is closely related to function, the tumor suppressor may no longer control the cell cycle.

Apoptosis is also part of this brake system. If a damaged cell cannot repair its DNA, apoptosis can remove it. A cancer cell must often avoid apoptosis to survive despite damage. This is one reason cancer cells can accumulate DNA changes over time.

> **Exam Warning:**
> Do not write that tumor suppressor genes “cause cancer” because they normally promote division. Their normal role is to inhibit abnormal growth. Cancer risk increases when their function is reduced or lost.

Cancer cells also differ from many normal somatic cells in how they respond to external growth conditions. Two key controls are density-dependent inhibition and anchorage dependence.

> **Key Term: density-dependent inhibition**
> A control in which many normal cells stop dividing when they become crowded.
>
> **Key Term: anchorage dependence**
> The requirement that many normal cells must be attached to a surface in order to divide.

Many somatic cells stop dividing when they contact neighboring cells and the tissue becomes crowded. This is density-dependent inhibition. It helps tissues maintain an appropriate size and structure. Many normal cells also require attachment to a surface or extracellular matrix before they divide. This is anchorage dependence.

Cancer cells often lose both controls. They may continue dividing even when crowded, forming a mass of cells called a tumor. They may also divide without normal attachment signals. This loss of response to normal environmental signals helps explain why cancer cells can grow under conditions that would stop normal somatic cells.

Not all tumors behave the same way, but AP Biology focuses on the general principle: cancer involves failure of regulation. Cells grow and divide without the normal checkpoint, signaling, contact, attachment, and death controls.

When you analyze experiments, look for evidence that a treatment affects regulation. If treated cells accumulate in mitosis, the treatment may stimulate entry into mitosis or block exit from mitosis. If cells accumulate in interphase, the treatment may prevent entry into mitosis or slow the cell cycle before M phase. If a spindle fiber inhibitor is used, cells may fail to separate chromatids correctly. If a mutation causes constant production of a cyclin, active cyclin-CDK complexes may form at abnormal times.

Strong AP free-response answers usually include three parts:

- Identify the altered molecule or process.
- Explain how it changes cell cycle progression.
- Connect the change to cell division, cancer, apoptosis, or checkpoint failure.

For example, if a mutation prevents apoptosis, a high-quality answer would not stop at “the cell does not die.” It would add that cells with DNA damage may survive and divide, passing mutations to daughter cells and increasing the chance of cancer.

## Key Point Checklist

This article has covered the following key knowledge points:

- The cell cycle includes interphase, mitosis, and cytokinesis.
- Interphase includes G1 growth, S phase DNA replication, and G2 preparation for mitosis.
- Cell cycle checkpoints stop progression if the cell is not ready to continue.
- DNA damage can activate checkpoints, allowing time for repair.
- If DNA damage is too severe, apoptosis can remove the damaged cell.
- CDKs are present at relatively constant levels but require cyclins for activation.
- Cyclin levels vary during the cell cycle and rise before mitosis.
- Mitosis-promoting factor is an active cyclin-CDK complex that helps trigger mitosis.
- Proto-oncogenes are normal growth-promoting genes that can become oncogenes.
- Oncogenes promote cancer through excessive growth-promoting activity.
- Tumor suppressor genes reduce cancer risk by inhibiting abnormal growth or promoting repair and apoptosis.
- Cancer cells often ignore density-dependent inhibition and anchorage dependence.

## Key Terms and Concepts

- cell cycle
- cancer
- cell cycle checkpoint
- apoptosis
- cyclin
- cyclin-dependent kinase
- mitosis-promoting factor
- proto-oncogene
- oncogene
- tumor suppressor gene
- density-dependent inhibition
- anchorage dependence


Regulation of cell cycle - Disruption leads to cancer