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Histology and Cell Biology

Cell cycle and apoptosis Practice Questions

19 free Cell cycle and apoptosis practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.

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Question 1 of 19 Medium

Which phase of the cell cycle is characterized by DNA synthesis (duplication of the genome)?

  1. A G1 phase
  2. B S phase
  3. C G2 phase
  4. D M phase

Correct answer: S phase

The S phase is when DNA replication occurs, producing sister chromatids for each chromosome. This ensures that each daughter cell will receive a full complement of genetic material.

Question 2 of 19 Medium

What is the primary role of cyclin-dependent kinases (CDKs) in the eukaryotic cell cycle?

  1. A They directly replicate DNA molecules during S phase
  2. B They degrade cyclins in order to allow cell cycle progression
  3. C They phosphorylate target proteins to trigger phase transitions
  4. D They act as transcription factors for cell-cycle genes

Correct answer: They phosphorylate target proteins to trigger phase transitions

CDKs, when bound to their cyclin partners, phosphorylate target proteins that promote progression from one phase to the next (e.g., G1/S or G2/M transitions), thereby regulating the orderly cell cycle.

Question 3 of 19 Medium

Which checkpoint prevents a cell with DNA damage from entering S phase, allowing time for repair or triggering cell death if damage is irreparable?

  1. A G2/M checkpoint
  2. B Spindle (M) checkpoint
  3. C G1/S checkpoint
  4. D Cytokinesis checkpoint

Correct answer: G1/S checkpoint

The G1/S checkpoint (or restriction point) assesses cell size, DNA integrity, and external signals; if DNA damage is detected, the cell can arrest or activate repair mechanisms, preventing entry into S phase.

Question 4 of 19 Medium

What is the significance of the G0 phase in the cell cycle?

  1. A It is the phase where genomic DNA synthesis takes place
  2. B A quiescent, non-dividing resting state outside the cycle
  3. C It marks the very onset of mitosis
  4. D It is the phase during which cytokinesis occurs

Correct answer: A quiescent, non-dividing resting state outside the cycle

G0 is a quiescent phase; cells that are terminally differentiated or not actively dividing enter G0 instead of re-entering G1, thus pausing the cell division cycle.

Question 5 of 19 Medium

Which of the following proteins is a key tumor suppressor that regulates both cell-cycle checkpoints and apoptosis in response to DNA damage?

  1. A Cyclin B
  2. B CDK2
  3. C p53
  4. D Cyclin A

Correct answer: p53

p53 acts as a tumor suppressor, inducing cell cycle arrest at checkpoints (e.g., G1/S or G2/M) to allow DNA repair; if damage is irreparable, p53 can also trigger apoptosis to prevent propagation of mutated cells.

Question 6 of 19 Medium

Which phase immediately follows completion of DNA replication in the cell cycle and precedes mitosis?

  1. A G1 phase
  2. B S phase
  3. C G2 phase
  4. D Anaphase

Correct answer: G2 phase

After S phase (DNA synthesis), the cell enters G2 phase to synthesize proteins and organelles needed for mitosis, and to perform final checks before entering M phase.

Question 7 of 19 Medium

What is the main outcome of the metaphase (spindle) checkpoint during mitosis?

  1. A Ensure DNA replication is complete before division begins
  2. B Prevent premature entry into S phase before G1 completes
  3. C Confirm all chromatids are attached to spindle fibers before anaphase
  4. D Ensure cytokinesis proceeds after the chromosomes separate

Correct answer: Confirm all chromatids are attached to spindle fibers before anaphase

The metaphase (M-phase) or spindle checkpoint verifies correct attachment of all kinetochores to spindle fibers; if any are unattached, anaphase is delayed to prevent mis-segregation of chromosomes.

Question 8 of 19 Medium

Which of the following best describes apoptosis?

  1. A Uncontrolled cell death resulting from acute injury
  2. B Programmed cell death with chromatin condensation and DNA fragmentation
  3. C Cell division occurring without cytokinesis or separation
  4. D Necrosis driven by inflammation and tissue swelling

Correct answer: Programmed cell death with chromatin condensation and DNA fragmentation

Apoptosis is a regulated process of programmed cell death featuring chromatin condensation, nuclear fragmentation, membrane blebbing, and DNA fragmentation — distinct from necrosis which involves uncontrolled cell death and inflammation.

Question 9 of 19 Medium

Which apoptosis pathway is initiated by extracellular ligands binding to specialized receptors on the target cell membrane?

  1. A Intrinsic (mitochondrial) pathway
  2. B Autophagy pathway
  3. C Extrinsic (death receptor) pathway
  4. D Necroptosis pathway

Correct answer: Extrinsic (death receptor) pathway

The extrinsic pathway of apoptosis is activated when death ligands (e.g., Fas ligand) bind to death receptors on the cell membrane, triggering a cascade that activates caspases and leads to programmed cell death.

Question 10 of 19 Medium

Which of the following is a hallmark biochemical event during the execution phase of apoptosis?

  1. A Activation of cyclin-dependent kinases
  2. B Phosphorylation of the Rb tumor-suppressor protein
  3. C Caspase activation cleaving proteins such as lamins and PARP
  4. D A marked increase in the rate of DNA replication

Correct answer: Caspase activation cleaving proteins such as lamins and PARP

Caspases, a family of cysteine proteases, are activated during apoptosis, cleaving key structural and regulatory proteins (such as nuclear lamins and PARP), resulting in the morphological and biochemical changes characteristic of apoptotic cell death.

Question 11 of 19 Medium

A cell with irreparable DNA damage fails the G2/M checkpoint. What is the most likely cellular fate?

  1. A Proceed through mitosis anyway, risking mutated daughter cells
  2. B Enter G0 permanently without dividing, but remain metabolically active
  3. C Undergo apoptosis to eliminate the damaged cell
  4. D Differentiate into a nondividing cell type

Correct answer: Undergo apoptosis to eliminate the damaged cell

If DNA damage is detected at the G2/M checkpoint and cannot be repaired, the cell is often directed toward apoptosis to prevent propagation of genetic errors — a safeguard against potential malignancy.

Question 12 of 19 Medium

Which of the following best explains why mitotic and apoptotic cells share some morphological features such as chromatin condensation and membrane blebbing?

  1. A They both involve uncontrolled, unregulated cell death
  2. B Both remodel the cytoskeleton and activate proteases that alter the nuclear envelope
  3. C They both bypass the S phase of the cell cycle entirely
  4. D They both produce two viable daughter cells at completion

Correct answer: Both remodel the cytoskeleton and activate proteases that alter the nuclear envelope

Both mitosis and apoptosis involve remodeling of the cytoskeleton and modifications to the nuclear envelope; in apoptosis, caspases cleave structural proteins, leading to chromatin condensation and membrane blebbing similar in appearance to mitotic changes though with different outcomes.

Question 13 of 19 Medium

Which of the following describes a reason why the cell cycle is tightly regulated with checkpoints rather than proceeding continuously?

  1. A To accelerate tissue growth as quickly as biologically possible
  2. B To permit uncontrolled proliferation in response to injury
  3. C To verify DNA is accurately replicated and errors repaired before division
  4. D To force immediate entry into mitosis right after cytokinesis

Correct answer: To verify DNA is accurately replicated and errors repaired before division

Checkpoints monitor for DNA damage, proper replication, and accurate chromosome attachment to the spindle, thereby preventing propagation of genetic errors to daughter cells and maintaining genomic integrity.

Question 14 of 19 Medium

Which cyclin/CDK complex is primarily active during the G1-to-S phase transition allowing the cell to commit to DNA replication?

  1. A Cyclin B / CDK1
  2. B Cyclin D / CDK4 or CDK6 and Cyclin E / CDK2
  3. C Cyclin A / CDK1
  4. D Cyclin A / CDK2

Correct answer: Cyclin D / CDK4 or CDK6 and Cyclin E / CDK2

Cyclin D–CDK4/6 complexes in early G1 and later Cyclin E–CDK2 complexes near the G1/S boundary phosphorylate targets that allow progression into S phase, committing the cell to DNA replication.

Question 15 of 19 Medium

Which of the following is NOT a typical morphological feature of a cell undergoing apoptosis?

  1. A Cell shrinkage and membrane blebbing
  2. B Chromatin condensation and nuclear fragmentation
  3. C Inflammatory cell infiltration and swelling
  4. D Formation of apoptotic bodies

Correct answer: Inflammatory cell infiltration and swelling

Unlike necrosis, apoptosis does not provoke inflammation or cause cell swelling; instead, cells shrink, condense chromatin, form apoptotic bodies, and are phagocytosed without eliciting an inflammatory response.

Question 16 of 19 Medium

Which statement best describes the relationship between cell cycle regulation and apoptosis in tissue homeostasis?

  1. A Cell proliferation and apoptosis operate independently, sharing no regulators
  2. B Checkpoint and apoptosis pathways are coupled, so severe damage triggers death over division
  3. C Apoptosis occurs exclusively in cells that have permanently stopped dividing
  4. D Cells always complete the cycle regardless of any DNA damage present

Correct answer: Checkpoint and apoptosis pathways are coupled, so severe damage triggers death over division

Cell cycle regulation and apoptosis are linked: if damage is detected at checkpoints and cannot be repaired, apoptotic pathways are activated to eliminate the cell, balancing proliferation and death to maintain tissue homeostasis.

Question 17 of 19 Medium

Which proteins are the main effectors that execute the biochemical processes of apoptosis by cleaving cellular substrates?

  1. A Cyclins
  2. B Cyclin-dependent kinases (CDKs)
  3. C Caspases
  4. D Bcl-2 family only

Correct answer: Caspases

Caspases are a family of cysteine proteases activated during apoptosis; they cleave key cellular proteins such as nuclear lamins and PARP, leading to controlled cell disassembly.

Question 18 of 19 Medium

What happens if a cell bypasses the spindle checkpoint despite improper spindle attachment during mitosis?

  1. A The cell immediately undergoes apoptosis and dies
  2. B Daughter cells receive unequal chromosome numbers, causing aneuploidy
  3. C It arrests permanently at the G2 phase boundary
  4. D It differentiates into a nondividing specialized cell

Correct answer: Daughter cells receive unequal chromosome numbers, causing aneuploidy

Bypassing the spindle checkpoint can lead to mis-segregation of chromosomes during anaphase, producing daughter cells with abnormal chromosome numbers (aneuploidy), which can contribute to genomic instability.

Question 19 of 19 Medium

Which event typically precedes activation of apoptosis in a cell with extensive DNA damage that cannot be repaired?

  1. A Activation of cyclin-CDK complexes to drive cell division
  2. B Persistent cell-cycle checkpoint arrest with p53 activation
  3. C Immediate and unchecked entry into mitosis
  4. D Differentiation into a specialized nondividing cell

Correct answer: Persistent cell-cycle checkpoint arrest with p53 activation

When DNA damage is irreparable, persistent arrest at cell cycle checkpoints often triggers p53-mediated pathways that lead to apoptosis, preventing propagation of damaged DNA.

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