AP Biology: Cell Cycle Checkpoints Practice Questions
Test yourself on AP Biology: Cell Cycle Checkpoints with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. A dividing eukaryotic cell follows the sequence G1, S, G2, and M. During which phase is its nuclear DNA replicated before mitosis?
Explanation. Nuclear DNA replication occurs during S phase. G1 precedes replication, and G2 follows it. During mitotic anaphase, sister chromatids that were produced by replication separate; the cell does not normally replicate its genome again at this stage.
2. DNA damage activates a checkpoint in a cell that has not yet entered S phase. The cell temporarily delays entry into S phase. What is the most direct benefit of this delay?
Explanation. Delaying entry into S phase provides an opportunity to repair DNA before replication copies the damaged template. Checkpoint arrest is a regulatory response, not a repair enzyme or a guarantee of successful recovery. Sister chromatids normally separate later, during mitosis.
3. A cell has started DNA replication but still contains unreplicated regions. Its checkpoint signaling prevents entry into mitosis. Which transition is being restrained most directly?
Explanation. Control at the G2-to-M transition helps prevent mitotic entry when DNA replication is incomplete or DNA remains damaged. The cell has already begun S phase, so preventing the original G1-to-S transition is no longer the relevant action. The spindle checkpoint acts after mitosis has begun.
4. A cell in mitosis has one chromosome with an unattached kinetochore. Its spindle assembly checkpoint remains active. Which event should be delayed?
Explanation. An unattached kinetochore can maintain spindle checkpoint signaling and delay anaphase onset. This gives spindle attachments time to form before sister chromatids separate, reducing the chance of unequal chromosome distribution. It does not reverse the earlier S phase or universally stop protein synthesis.
5. Researchers prevent cyclin degradation in a model cell while leaving its associated cyclin-dependent kinase functional. This cyclin normally helps maintain mitotic kinase activity and is degraded during mitotic exit. What outcome is most directly predicted?
Explanation. Cyclin abundance helps control the activity of associated cyclin-dependent kinases. If a mitotic cyclin is not degraded, the corresponding kinase activity can persist and interfere with normal exit from mitosis. This prediction concerns the stated cyclin-kinase pair, not every kinase in the cell.
6. An undamaged diploid cell in G1 has a nuclear DNA content labeled 2C. It completes one S phase and reaches G2 without dividing. What is its expected nuclear DNA content?
Explanation. One complete S phase doubles nuclear DNA content from 2C to 4C. Each chromosome now contains two sister chromatids, but replication alone does not double the number of homologous chromosome sets. A normal diploid cell in G2 is still diploid despite having twice its G1 DNA content.
7. A DNA-content assay groups intact single cells into 2C, intermediate, and 4C populations. Both G2 cells and cells in early mitosis can have 4C DNA. What additional measurement would best distinguish G2 from early mitosis?
Explanation. DNA content alone cannot distinguish G2 from early mitosis because both can contain 4C DNA. Chromosome condensation and organization of a mitotic spindle provide information about mitotic state. Repeating the same DNA-content classification does not resolve that ambiguity by itself.
8. A checkpoint inhibitor is dissolved in a solvent before being added to cultured cells. Investigators want to compare checkpoint responses with and without the inhibitor. Which control best isolates the effect of the inhibitor from that of the solvent?
Explanation. A vehicle control receives the same solvent concentration without the active inhibitor. Matching the cell background, culture conditions, timing, and solvent exposure makes the inhibitor the intended difference. Changing species, temperature, or solvent concentration introduces alternative explanations.
9. Two otherwise matched cell lines receive the same DNA-damaging treatment in G1. After treatment, 8% of checkpoint-intact cells and 52% of checkpoint-deficient cells enter S phase within the observation window. Untreated controls of both lines show similar S-phase entry. Which conclusion is best supported?
Explanation. The larger fraction entering S phase in the deficient line, together with similar untreated controls, supports a role for the checkpoint in delaying entry after damage. These observations do not directly measure repair speed, mutation frequency, or daughter-cell viability, so those outcomes cannot be assumed.
10. A researcher scores 240 intact cells from a culture snapshot and identifies 36 as mitotic by chromosome morphology. What is the mitotic index for this sample?
Explanation. The mitotic index is the fraction of scored cells in mitosis: 36 divided by 240 equals 0.15, or 15%. This is a snapshot proportion. By itself, it does not establish how quickly the population completes division, because cells can accumulate in mitosis when progression is delayed.
11. A treatment increases the fraction of cells observed in mitosis, but time-lapse imaging shows that many treated cells remain in metaphase much longer than controls. Which interpretation best fits both observations?
Explanation. A longer residence time in metaphase can increase the proportion of cells seen in mitosis without increasing completed divisions. The time-lapse observation supports delayed progression. A high mitotic index alone does not prove rapid proliferation, and these data do not establish loss of every checkpoint.
12. Matched cells are exposed to a reversible treatment that disrupts spindle attachment. Checkpoint-intact cells delay anaphase, while cells lacking the spindle checkpoint begin anaphase with some kinetochores still unattached. Which outcome is more likely in the checkpoint-deficient group?
Explanation. Beginning anaphase before spindle attachments are properly established increases the risk of chromosome segregation errors. Daughter cells may receive unequal chromosome numbers. The checkpoint reduces this risk by delaying separation; its loss does not guarantee a specific error in every cell.
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FAQ
How do the major cell-cycle checkpoints differ?
G1 control can delay DNA replication when conditions are unsuitable or DNA is damaged. G2-to-M control helps prevent mitotic entry with incomplete replication or damaged DNA. The spindle assembly checkpoint delays anaphase when kinetochore attachment requirements are not met.
Does twice the DNA content mean twice the ploidy?
No. A normal diploid cell doubles its DNA content during S phase as sister chromatids form, but it remains diploid in G2. DNA content and the number of homologous chromosome sets describe different features of the cell.
Does a high mitotic index prove that cells divide faster?
No. More cells may be entering mitosis, or cells may be spending longer there because progression is delayed. Time-lapse measurements of mitotic duration and completed divisions help distinguish these explanations.