Prophase
chromosomes condense
Where the Cell Cycle Fits in Unit 4
The previous guide, Positive Feedback, finished the feedback-loop part of Unit 4. This page begins the cell-cycle part of the unit by showing how cells move through growth, DNA replication, mitosis, and cytokinesis. After this page, study Cell Cycle Checkpoints to understand how cells decide whether it is safe to continue dividing.
Current concept
Cell Cycle
Growth, DNA copying, and division.
- 1 Unit 4 Hub
- 2 Cell Communication
- 3 Ligands and Receptors
- 4 Reception, Transduction, Response
- 5 Cell Signaling Pathways
- 6 Feedback Mechanisms
- 7 Negative Feedback
- 8 Positive Feedback
- 9 Cell Cycle You are here
- 10 Cell Cycle Checkpoints
- 11 Cyclins and CDKs
- 12 Signal Amplification
- 13 Second Messengers
- 14 Phosphorylation Cascade
- 15 Cancer and Cell Cycle Regulation
- 16 Apoptosis
- 17 Unit 4 Practice Questions
- 18 Unit 4 FRQ
What is the cell cycle in AP Biology?
The cell cycle is the ordered process a cell follows to grow, copy its DNA, and divide. It includes interphase, made of G1, S phase, and G2, followed by M phase and cytokinesis. AP Biology tests whether students can connect cell-cycle phases to checkpoints, regulation, DNA replication, mitosis, and cancer.
Say it fast
The cell cycle grows cells, copies DNA, and divides cells.
Cell Cycle Explorer
Cell cycle explorer — tap each phase
G1 is the first growth phase. The cell grows, performs normal functions, and checks whether conditions are right for DNA replication.
S phase is when DNA is replicated. Each chromosome is copied so sister chromatids can later separate during mitosis.
G2 is the final preparation phase before mitosis. The cell checks DNA replication and prepares division machinery.
M phase includes mitosis, where the nucleus divides. Chromosomes are separated into two nuclei.
Cytokinesis divides the cytoplasm and produces two daughter cells. It usually follows mitosis.
Checkpoints are control points that stop or delay the cell cycle if DNA damage, incomplete replication, or spindle problems are detected.
Interphase: The Preparation Stage
Interphase is the longest part of the cell cycle for many cells. During interphase, the cell grows, copies DNA, and prepares for mitosis. AP Biology often tests interphase by asking which phase includes DNA replication or cell growth.
For a phase-by-phase breakdown of G1, S, G2, M phase, and cytokinesis, use the Cell Cycle Phases guide.
Signaling from cell communication and feedback mechanisms can influence whether a cell commits to division during interphase.
Interphase is not resting; it prepares the cell for division.
G1 Phase: Growth and Decision-Making
G1 is the first growth phase after cell division. The cell increases in size, performs normal functions, and receives signals that influence whether it should continue toward DNA replication. The G1 checkpoint is important because it helps prevent damaged or unready cells from entering S phase.
AP callout: If a question asks whether a cell should commit to division, think about G1 regulation.
S Phase: DNA Replication
S phase is when DNA replication occurs. Each chromosome is copied to form sister chromatids. This matters because mitosis later separates those sister chromatids into daughter cells.
S phase copies DNA before mitosis.
G2 Phase: Final Preparation Before Mitosis
G2 is the second growth phase and final preparation stage before mitosis. The cell checks whether DNA replication is complete and whether DNA damage is present. If problems are found, the cell cycle can pause for repair before entering M phase.
M Phase: Mitosis Divides the Nucleus
M phase includes mitosis, the process that divides the nucleus. During mitosis, chromosomes condense, line up, separate, and form two nuclei. AP Biology often asks students to connect mitosis with chromosome movement and daughter-cell formation.
For a deeper look at chromosome separation during M phase, review Mitosis in the Cell Cycle.
Metaphase
chromosomes line up
Anaphase
sister chromatids separate
Telophase
nuclei reform
Cytokinesis: Splitting the Cell
Cytokinesis is the division of the cytoplasm after mitosis. In animal cells, the membrane pinches inward. In plant cells, a cell plate forms. Cytokinesis produces two daughter cells after nuclear division is complete.
Cell Cycle Checkpoints Control Progress
Checkpoints are control points that help determine whether the cell cycle should continue. The G1 checkpoint checks growth conditions and DNA damage, the G2 checkpoint checks DNA replication, and the M checkpoint checks spindle attachment. These checkpoints reduce the chance that damaged cells divide.
For a deeper look at each control point, see the cell cycle checkpoints guide and how cyclins and CDKs regulate progression.
How the Cell Cycle Connects to Cancer
Cancer can result when cell-cycle regulation fails. If growth signals stay active, checkpoints are bypassed, tumor suppressor genes fail, or damaged cells avoid apoptosis, cells may divide uncontrollably. AP Biology questions often ask students to explain cancer as failed regulation, not just fast cell division.
Study cancer and cell cycle regulation for mutation scenarios, and apoptosis for how programmed cell death removes damaged cells.
How AP Biology Tests the Cell Cycle
“DNA is replicated”
S phase is being tested.
“Cell grows before DNA copying”
G1 phase is likely.
“Final preparation before mitosis”
G2 phase is likely.
“Sister chromatids separate”
Anaphase of mitosis is likely.
“Spindle attachment checked”
M checkpoint is being tested.
“Damaged DNA bypasses checkpoint”
Cancer or failed regulation may be involved.
How to Answer Cell Cycle FRQs
Identify the cell-cycle phase
Name where the event occurs.
State the key event
Describe what normally happens there.
Identify checkpoint or signal
Explain what controls progression.
Predict the consequence
Describe what changes if regulation fails.
AP FRQ writing frame
During ___ phase, the cell ___. The checkpoint matters because ___. If this control fails, the result may be ___.
Common AP Bio Cell Cycle Mistakes
Calling interphase a resting phase
Fix: Interphase includes growth, DNA replication, and preparation.
Saying DNA replicates during mitosis
Fix: DNA replication happens during S phase before mitosis.
Confusing mitosis and cytokinesis
Fix: Mitosis divides the nucleus; cytokinesis divides the cell.
Forgetting checkpoints
Fix: Cell-cycle progression is regulated, not automatic.
Saying cancer is only fast division
Fix: Cancer involves failed regulation of growth, checkpoints, apoptosis, or signaling.
Mixing meiosis with mitosis
Fix: Mitosis makes body cells; meiosis makes gametes.
Cell Cycle Clue Lab
Revealed: 0 of 4 scenarios
A cell copies all of its DNA before division.
Answer: This happens during S phase.
A cell checks whether DNA is damaged before copying it.
Answer: This is connected to G1 checkpoint control.
Sister chromatids move to opposite sides of the cell.
Answer: This happens during anaphase of mitosis.
A checkpoint fails and a cell with damaged DNA keeps dividing.
Answer: This can contribute to cancer because regulation has failed.
Cell Cycle MCQ Practice
Answer all eight questions. Choices shuffle on reload—focus on phase logic, not letter memorization.
Question 1 of 8
Start
Correct: 0
Answered: 0
Accuracy: 0%
More drills: Unit 4 practice questions, practice by topic, or daily AP Biology practice.
Cell Cycle FRQ Practice
Open each card, draft your response, then reveal the rubric and sample. For more free-response practice, open the Unit 4 FRQ guide.
0 of 2 FRQs opened
Prompt
A cell receives signals to divide and enters the cell cycle. Before mitosis, it must grow, copy DNA, and check whether DNA replication is complete.
- A. Identify the phase in which DNA replication occurs.
- B. Explain why the G2 checkpoint matters before mitosis.
- C. Predict what could happen if DNA damage is not repaired before division.
Scoring rubric
- 1 pt — Identifies S phase as DNA replication phase.
- 1 pt — Explains G2 checkpoint verifies replication completion or DNA integrity.
- 1 pt — Connects G2 delay to repair before M phase.
- 1 pt — Predicts damaged DNA may pass to daughter cells if unrepaired.
- 1 pt — Links failed repair to mutation or cancer risk.
- 1 pt — Clear phase-checkpoint-outcome reasoning.
Sample response
DNA replication occurs during S phase. The G2 checkpoint matters because it checks whether DNA replication is complete and whether damage is present before mitosis begins. If DNA damage is not repaired before division, daughter cells may inherit mutations, which can contribute to failed regulation and cancer over time.
Self-check
Status: Draft your answer first—then open the rubric or sample.
Prompt
A mutation causes a cell-cycle checkpoint protein to stop functioning. Cells with damaged DNA continue dividing.
- A. Describe the normal role of a cell-cycle checkpoint.
- B. Explain how checkpoint failure can affect daughter cells.
- C. Connect checkpoint failure to cancer development.
Scoring rubric
- 1 pt — Describes checkpoint as control point that pauses cycle when problems detected.
- 1 pt — Names relevant checkpoint (G1, G2, or M) or general pause function.
- 1 pt — Explains daughter cells may receive damaged DNA or wrong chromosome number.
- 1 pt — Connects repeated faulty division to accumulation of mutations.
- 1 pt — Links failed regulation to cancer (not just fast division).
- 1 pt — Clear cause-effect chain from mutation to disease outcome.
Sample response
A cell-cycle checkpoint normally pauses the cycle when DNA is damaged, replication is incomplete, or spindle attachment is incorrect. If a checkpoint fails, cells with damaged DNA may still divide, passing errors to daughter cells. Over time, accumulated mutations and loss of control can lead to unregulated division characteristic of cancer.
Self-check
Status: Draft your answer first—then open the rubric or sample.
Cell Cycle FAQs
What is the cell cycle in AP Biology?
The cell cycle is the ordered sequence of events that lets a cell grow, copy DNA, and divide. It includes interphase, mitosis, and cytokinesis. AP Biology focuses on how the cycle is regulated by checkpoints and signals.
What are the main phases of the cell cycle?
The main phases are G1, S phase, G2, M phase, and cytokinesis. G1, S, and G2 make up interphase. M phase includes mitosis, and cytokinesis divides the cytoplasm.
What happens during interphase?
Interphase is when the cell grows, copies DNA, and prepares for division. It includes G1, S phase, and G2. It is not a resting phase because major preparation events happen there.
What happens during S phase?
S phase is when DNA replication occurs. Each chromosome is copied to form sister chromatids. This prepares chromosomes to separate properly during mitosis.
What is the difference between mitosis and cytokinesis?
Mitosis divides the nucleus and separates chromosomes into two nuclei. Cytokinesis divides the cytoplasm and produces two daughter cells. They are related but not the same process.
Why are cell cycle checkpoints important?
Checkpoints help the cell decide whether it is safe to continue the cycle. They can pause the cycle if DNA is damaged, replication is incomplete, or spindle attachment is incorrect. This helps prevent damaged cells from dividing.
How does the cell cycle connect to cancer?
Cancer can happen when cell-cycle regulation fails. Cells may divide even when DNA is damaged or growth signals are abnormal. AP Biology answers should explain the failed control mechanism, not just say cells divide quickly.
What is the G1 checkpoint?
The G1 checkpoint checks whether the cell is ready to copy DNA. It responds to cell size, nutrients, signals, and DNA damage. If conditions are not safe, the cell may pause instead of entering S phase.
What is the M checkpoint?
The M checkpoint checks whether chromosomes are correctly attached to spindle fibers. This helps ensure sister chromatids separate properly. If the checkpoint fails, daughter cells may receive abnormal chromosome numbers.
How should I answer cell cycle FRQs?
Start by naming the phase or checkpoint involved. Then explain what normally happens at that step and predict what changes if regulation fails. Strong answers connect checkpoints to DNA damage, chromosome separation, or cancer risk.