Cell Cycle — AP Biology Study Guide

For: AP Biology candidates sitting AP Biology.

Covers: Phases of interphase (G1, S, G2, G0), mitotic phase, cell cycle checkpoints, regulation by cyclins and cyclin-dependent kinases (CDKs), MPF function, and cell cycle dysregulation leading to cancer.

You should already know: Basic eukaryotic cell structure including the nucleus and cytoskeleton. Enzyme-substrate binding and activation. Fundamentals of DNA replication.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Biology style for educational use. They are not reproductions of past College Board / Cambridge / IB papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official mark schemes for grading conventions.

1. What Is Cell Cycle?

The cell cycle is the ordered, regulated sequence of growth, DNA replication, and nuclear and cytoplasmic division that all eukaryotic somatic cells undergo to produce genetically identical daughter cells. It is one of the core topics in AP Biology Unit 4 (Cell Communication and Cell Cycle), which accounts for 10-15% of the total AP exam score, with cell cycle content making up roughly a third of that unit weight. This topic appears regularly in both multiple-choice (MCQ) and free-response (FRQ) sections, and is often paired with concepts from signal transduction, genetics, and evolution to create multi-concept questions. Unlike prokaryotic cell division (binary fission), the eukaryotic cell cycle includes distinct checkpoints and regulatory mechanisms that ensure accurate DNA replication and segregation. Errors in this regulation are the direct cause of cancer, one of the most common context-based FRQ topics for this unit.

2. Phases of the Cell Cycle

The cell cycle is divided into two broad stages: interphase, the long period of growth and DNA replication between cell divisions, and the mitotic (M) phase, the short period of active division. Interphase accounts for ~90% of the total cell cycle duration in most somatic cells, and is further split into three subphases:

1. G1 (Gap 1): The cell grows in size, produces organelles and proteins required for DNA replication, and passes the first key regulatory checkpoint. If the cell does not receive a signal to divide, it exits the cycle into G0, a quiescent non-dividing state that can be temporary (for cells like liver cells that can re-enter the cycle) or permanent (for fully differentiated cells like neurons or skeletal muscle).
2. S (Synthesis): The cell replicates all of its nuclear DNA. A key point of confusion here is the difference between DNA content and chromosome number: after replication, each chromosome consists of two identical sister chromatids connected at a single centromere, so the total mass of DNA doubles, but chromosome number (counted by number of centromeres) remains the same. For example, a human somatic cell is 2n=46 in G1, and remains 2n=46 after S phase – it just has twice the amount of DNA.
3. G2 (Gap 2): The cell continues to grow, produces proteins required for mitosis, and passes the second regulatory checkpoint that confirms complete, error-free DNA replication.

The mitotic phase includes mitosis (nuclear chromosome segregation) and cytokinesis (cytoplasmic division to produce two daughter cells).

Worked Example

A researcher measures the total DNA content of a diploid goat somatic cell in early G1, and records a value of 6.8 picograms (pg). What is the expected DNA content of a goat cell in late G2, and what is the expected DNA content of a goat daughter cell immediately after cytokinesis of mitosis?

1. Early G1 occurs before DNA replication in S phase, so 6.8 pg is the baseline DNA content for a diploid goat somatic cell.
2. All DNA is replicated during S phase, so total DNA doubles by the end of S, and remains doubled through G2 before division.
3. Calculate late G2 DNA content: $6.8\text{ pg}\times 2=13.6\text{ pg}$.
4. After mitosis and cytokinesis, each daughter cell receives half the G2 DNA content, equal to the original G1 content of the parent cell: $13.6\text{ pg}÷2=6.8\text{ pg}$.

Final answer: 13.6 pg (late G2), 6.8 pg (post-cytokinesis daughter cell).

Exam tip: If an exam question asks for chromosome number instead of DNA content, remember that the number of centromeres (not chromatids) determines chromosome number, so chromosome number does not change until anaphase of mitosis.

3. Cell Cycle Checkpoints and Regulation

Cell cycle checkpoints are regulatory control points where the cell halts progression until conditions are favorable to continue. The three main checkpoints are:

1. G1 Checkpoint (Restriction Point): Located at the end of G1, before entry into S phase. Checks for adequate cell size, sufficient nutrients, growth factor signals, and undamaged DNA.
2. G2 Checkpoint: Located at the end of G2, before entry into M phase. Checks that all DNA is fully and accurately replicated.
3. Spindle (M) Checkpoint: Located at the end of metaphase, before anaphase. Checks that all chromosome kinetochores are correctly attached to spindle microtubules from opposite poles.

Regulation of checkpoints relies on two key protein groups: cyclins (regulatory proteins whose concentration oscillates, or cycles, throughout the cell cycle) and cyclin-dependent kinases (CDKs) (protein kinases that are always present in the cell in an inactive form). CDKs only become active when bound to a specific cyclin, and active cyclin-CDK complexes phosphorylate target proteins to push the cell through the checkpoint. The most well-studied complex is MPF (M-phase Promoting Factor), which is made of cyclin B bound to CDK. Cyclin B concentration gradually increases through G1 and S, peaks at metaphase, then is degraded at the end of M phase, inactivating MPF to allow exit from M phase.

Worked Example

Researchers measure cyclin B concentration and MPF activity in asynchronous (unsynchronized) dividing hamster cells. They record the following data across the cell cycle:

Explain the relationship between cyclin B concentration and MPF activity, and why this relationship exists.

1. First, compare the trends: as cyclin B concentration increases from G1 through G2, MPF activity increases proportionally, and when cyclin B concentration drops in M phase, MPF activity also drops.
2. MPF is a complex of cyclin B (the regulatory subunit) and CDK (the catalytic subunit). CDK is present at a constant concentration in all phases of the cell cycle.
3. CDK can only be active when bound to cyclin B, so higher cyclin B concentration means more active MPF complexes, and lower cyclin B concentration means fewer active complexes.
4. At the end of M phase, cyclin B is targeted for degradation by the anaphase-promoting complex, which removes the cyclin, inactivates MPF, and allows the cell to exit M phase and return to G1 for the next cycle.

Exam tip: AP FRQs almost always ask you to interpret a graph of cyclin concentration vs MPF activity – always explicitly state that CDK concentration is constant, only cyclin concentration cycles, so activity depends on cyclin binding.

4. Cell Cycle Dysregulation and Cancer

Cancer is defined as uncontrolled cell division caused by mutations that disrupt cell cycle regulation. For cancer to develop, mutations typically disrupt two classes of cell cycle regulatory genes:

1. Proto-oncogenes: Normal genes that code for proteins that stimulate cell division (e.g., growth factor receptors, cyclins). A gain-of-function mutation in a proto-oncogene converts it to an oncogene, which causes constant, excessive stimulation of cell division even when growth signals are absent. Only one mutated copy is needed for this effect, because the mutation causes a new, hyperactive function.
2. Tumor suppressor genes: Normal genes that code for proteins that inhibit cell division, repair DNA errors, or trigger apoptosis (programmed cell death) for damaged cells (e.g., p53, BRCA1). Loss-of-function mutations that inactivate both copies of the gene remove the "brake" on cell division, allowing damaged cells to continue dividing and accumulate more mutations. Normal cells also follow density-dependent inhibition (stop dividing when they form a single layer) and anchorage dependence (must attach to a substrate to divide), while cancer cells ignore both rules.

Worked Example

The retinoblastoma (Rb) gene is a tumor suppressor gene that normally prevents excessive progression through the G1 checkpoint. A child inherits one mutated, non-functional copy of the Rb gene, and one normal functional copy. Explain why this child has a >90% lifetime risk of developing retinoblastoma (a retinal cancer), while a child born with two functional Rb copies has a <1% risk.

1. Tumor suppressor genes follow the "two-hit hypothesis": both copies of the gene must be inactivated to eliminate all functional protein.
2. The child already has one heritable non-functional copy (the first "hit") in all retinal cells.
3. Only one additional somatic mutation in the remaining functional copy (the second "hit") in a single retinal cell is needed to completely eliminate functional Rb protein.
4. Without functional Rb, the G1 checkpoint does not halt cell division in cells with damaged DNA, leading to uncontrolled growth and cancer. A child with two functional copies needs two independent mutations in the same cell to eliminate Rb function, which is extremely rare, hence the low risk.

Exam tip: Always remember the difference between mutation types: gain-of-function for proto-oncogenes/oncogenes (one mutation is enough) vs loss-of-function for tumor suppressors (two hits required). AP exam writers love to test this distinction in MCQ and FRQ.

5. Common Pitfalls (and how to avoid them)

• Wrong move: Stating that chromosome number doubles after S phase because DNA replication produces two sister chromatids per chromosome. Why: Students confuse DNA content (mass) with chromosome number, which is defined by the number of centromeres. Correct move: Always count centromeres to get chromosome number – two sister chromatids connected at one centromere count as one chromosome, so chromosome number does not change after S phase.
• Wrong move: Claiming CDKs are the proteins whose concentration oscillates during the cell cycle. Why: Students mix up the names and roles of the two regulatory complex components. Correct move: Remember "cyclIN cycles" – cyclin concentration cycles up and down, CDK concentration is always constant; only CDK activity changes based on cyclin binding.
• Wrong move: Stating that a gain-of-function mutation in a tumor suppressor gene causes cancer. Why: Students mix up the roles of proto-oncogenes and tumor suppressor genes. Correct move: Always associate proto-oncogenes with gain-of-function mutations that produce oncogenes (excess cell division) and tumor suppressors with loss-of-function mutations that remove cell division brakes.
• Wrong move: Claiming G0 is always a temporary phase that all cells eventually exit to re-enter the cell cycle. Why: Textbooks often describe G0 as a "resting phase", leading students to assume it is only a temporary pause. Correct move: Recognize that G0 can be permanent for fully differentiated cells like neurons or skeletal muscle cells, which never divide again in adulthood.
• Wrong move: Stating that the spindle checkpoint occurs at the end of prophase, before metaphase. Why: Students misremember the order of M phase events and checkpoint function. Correct move: Recall the spindle checkpoint checks for attachment of all kinetochores to microtubules after chromosomes align at the metaphase plate, so it occurs at the end of metaphase, before anaphase begins.

6. Practice Questions (AP Biology Style)

Question 1 (Multiple Choice)

A researcher treats dividing mammalian cells with a chemical inhibitor that prevents cyclin B degradation at the end of metaphase. What effect will this treatment most likely have on the cell cycle? A. The cell will immediately exit the cell cycle into G0. B. The cell will be unable to enter anaphase and will arrest in metaphase. C. MPF will remain active, and the cell will be unable to exit M phase after mitosis. D. Cyclin B will bind to CDK to trigger entry into G2 phase from S phase.

Worked Solution: Cyclin B is the regulatory subunit of MPF, the cyclin-CDK complex that drives progression through M phase. Normally, cyclin B is degraded at the end of metaphase, which inactivates MPF and allows the cell to progress through telophase, cytokinesis, and exit M phase back to G1. If cyclin B cannot be degraded, MPF remains continuously active, so the cell cannot exit M phase. Option A is incorrect because there is no link between stable cyclin B and G0 entry. Option B is incorrect because anaphase onset is triggered by degradation of securin, not cyclin B. Option D is incorrect because MPF triggers entry into M phase from G2, not entry into G2 from S. The correct answer is C.

Question 2 (Free Response)

The p53 protein halts the cell cycle at the G1 checkpoint if damaged DNA is detected. If damage cannot be repaired, p53 triggers apoptosis. (a) Identify the classification of the p53 gene, and explain why loss-of-function mutations in p53 promote cancer development. (b) A researcher exposes two groups of cells to ionizing radiation that causes DNA double-strand breaks. Group 1 has functional p53, Group 2 has non-functional p53. Predict the difference in the outcome of the cell cycle between the two groups 24 hours after exposure. Justify your prediction. (c) Many chemotherapy drugs target rapidly dividing cancer cells by damaging DNA. Explain why p53 status of a tumor affects how well it responds to this type of chemotherapy.

Worked Solution: (a) The p53 gene is a tumor suppressor gene. Tumor suppressor genes produce proteins that inhibit cell cycle progression and eliminate damaged cells to prevent replication of mutated DNA. When p53 loses function, the cell cycle does not halt after DNA damage, so damaged cells continue to divide and accumulate additional mutations that can lead to uncontrolled growth and cancer. (b) Prediction: Group 1 (functional p53) will arrest the cell cycle at G1, and most damaged cells will undergo apoptosis, resulting in few viable cells. Group 2 (non-functional p53) will not arrest the cell cycle, so damaged cells will continue dividing, resulting in many viable mutated cells. Justification: Functional p53 activates DNA repair pathways or triggers apoptosis for irreparable damage, while non-functional p53 cannot initiate these responses, so damaged cells progress through the cell cycle regardless of DNA damage. (c) Chemotherapy that damages DNA works by triggering apoptosis of rapidly dividing cancer cells. Tumors with functional p53 will detect chemotherapy-induced DNA damage and trigger apoptosis, so they respond well to treatment. Tumors with non-functional p53 do not trigger apoptosis after DNA damage, so they continue to divide even after damage, leading to treatment resistance.

Question 3 (Application / Real-World Style)

Flow cytometry is a technique that measures the DNA content of individual cells in an asynchronous population of dividing cells. A researcher analyzes 10,000 rapidly dividing mouse fibroblast cells, with a total cell cycle length of 22 hours. They count 5,800 cells with 6 pg DNA, 2,700 cells with 12 pg DNA, and 1,500 cells with between 6 and 12 pg DNA. Calculate the length of S phase in these cells, and interpret your result.

Worked Solution:

1. First, assign DNA content to phases: 6 pg = G1 (pre-replication), 12 pg = G2 and M phase (post-replication, pre-cytokinesis), 6-12 pg = actively replicating DNA in S phase.
2. Calculate the percentage of cells in S phase: $\frac{1500}{10000}=0.15$, or 15% of the population.
3. For an asynchronous population, the percentage of cells in a phase equals the percentage of total cell cycle time spent in that phase.
4. Calculate S phase length: $0.15\times 22\text{ hours}=3.3\text{ hours}$.

Interpretation: In this population of mouse fibroblasts, S phase (the phase where DNA is replicated) lasts approximately 3.3 hours, which is consistent with observed S phase durations in mammalian somatic cells.

7. Quick Reference Cheatsheet

8. What's Next

Mastery of the cell cycle is a prerequisite for the next core topics in Unit 4: mitosis and meiosis, where you will dive into the specific steps of chromosome segregation and gamete formation. This chapter also connects directly to earlier topics in Unit 4: cell communication and signal transduction, because extracellular signals like growth factors act to trigger passage through the G1 restriction point, so understanding cell cycle regulation is required to connect signaling pathways to cell division outcomes. Beyond Unit 4, cell cycle dysregulation is a core topic in Mendelian and molecular genetics, where you will study inherited cancer predisposition and mutation accumulation. Without a solid grasp of cell cycle phases, checkpoints, and regulation, you will struggle to answer multi-concept FRQs that connect these topics across units.

• Mitosis
• Meiosis
• Signal Transduction Pathways
• Inheritance of Genetic Disorders