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Cell Biology

Cell Cycle and Checkpoints Practice Questions

39 free Cell Cycle and Checkpoints practice questions for the Zoology. 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 39 Medium

Which phase of the cell cycle is primarily responsible for DNA replication?

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

Correct answer: S phase

The S (synthesis) phase is the period during which the entire genome is replicated. This ensures that each daughter cell receives an identical set of DNA after cell division.

Question 2 of 39 Medium

During which phase does the cell mainly grow and prepare the machinery required for DNA synthesis?

  1. A G1 phase
  2. B S synthesis phase
  3. C G2 preparatory phase
  4. D Mitotic prophase

Correct answer: G1 phase

G1 phase is characterized by active cell growth and synthesis of proteins and organelles. These preparations are essential before the cell enters the S phase.

Question 3 of 39 Medium

Which checkpoint ensures that DNA replication is complete and undamaged before mitosis begins?

  1. A G1 checkpoint
  2. B S checkpoint
  3. C G2 checkpoint
  4. D Spindle checkpoint

Correct answer: G2 checkpoint

The G2 checkpoint verifies that DNA replication has been completed successfully and checks for DNA damage. Only cells that pass this checkpoint proceed to mitosis.

Question 4 of 39 Medium

Cyclins primarily function by:

  1. A Directly repairing DNA damage
  2. B Forming complexes with CDKs
  3. C Binding to spindle fibers
  4. D Inhibiting chromosome condensation

Correct answer: Forming complexes with CDKs

Cyclins regulate the cell cycle by forming cyclin-CDK complexes. These complexes phosphorylate target proteins to drive the cell through specific cell cycle stages.

Question 5 of 39 Medium

Which cyclin-CDK complex is most important for the transition from G1 to S phase?

  1. A Cyclin A paired with CDK2
  2. B Cyclin B paired with CDK1
  3. C Cyclin D–CDK4/6
  4. D Cyclin E paired with CDK1

Correct answer: Cyclin D–CDK4/6

Cyclin D–CDK4/6 initiates progression through G1 and prepares the cell for S phase entry. It plays a critical role in responding to growth signals.

Question 6 of 39 Medium

The spindle assembly checkpoint primarily monitors:

  1. A DNA damage during G1 phase
  2. B Completion of DNA replication
  3. C Attachment of chromosomes to spindles
  4. D Completion of cytokinesis and division

Correct answer: Attachment of chromosomes to spindles

The spindle assembly checkpoint ensures that all chromosomes are properly attached to spindle microtubules. This prevents unequal chromosome segregation during anaphase.

Question 7 of 39 Medium

Which event is characteristic of the G2 phase of the cell cycle?

  1. A DNA replication begins
  2. B Chromosome condensation starts
  3. C Protein synthesis for mitosis
  4. D Separation of sister chromatids

Correct answer: Protein synthesis for mitosis

During G2 phase, the cell synthesizes proteins required for mitosis, such as tubulin. This phase acts as a final preparation stage before cell division.

Question 8 of 39 Medium

What happens to cyclin levels during the cell cycle?

  1. A They remain entirely constant
  2. B They increase continuously
  3. C They fluctuate and are degraded in stages
  4. D They are present only during active mitosis

Correct answer: They fluctuate and are degraded in stages

Cyclin concentrations rise and fall in a cyclical manner. Their regulated degradation ensures proper timing and direction of cell cycle progression.

Question 9 of 39 Medium

Which phase occupies the longest duration in a typical animal cell cycle?

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

Correct answer: G1 phase

G1 phase is usually the longest phase, as the cell spends considerable time growing and responding to external signals. Its length can vary depending on cell type and conditions.

Question 10 of 39 Medium

Which protein acts as a major inhibitor of cyclin-CDK complexes during DNA damage?

  1. A p53
  2. B Tubulin
  3. C Cohesin
  4. D Actin

Correct answer: p53

p53 is a tumor suppressor protein that becomes activated in response to DNA damage. It induces CDK inhibitors, halting the cell cycle to allow repair.

Question 11 of 39 Medium

The restriction point in G1 phase determines whether a cell:

  1. A Immediately begins mitotic division
  2. B Enters quiescence or proceeds to S phase
  3. C Completes cytokinesis and separates fully
  4. D Activates spindle fibers for chromosome capture

Correct answer: Enters quiescence or proceeds to S phase

At the restriction point, the cell commits to DNA replication and division or exits into a quiescent state (G0). This decision depends on growth signals and nutrient availability.

Question 12 of 39 Medium

Which cyclin is primarily associated with mitosis?

  1. A Cyclin D, active in early G1
  2. B Cyclin E, active at G1/S
  3. C Cyclin A, active in S phase
  4. D Cyclin B

Correct answer: Cyclin B

Cyclin B associates with CDK1 to trigger entry into mitosis. Its degradation is essential for exit from mitosis.

Question 13 of 39 Medium

Which phase of mitosis involves alignment of chromosomes at the equatorial plate?

  1. A Prophase, when chromosomes condense
  2. B Metaphase
  3. C Anaphase, when chromatids separate
  4. D Telophase, when nuclei reform

Correct answer: Metaphase

In metaphase, chromosomes align at the metaphase plate. This alignment ensures accurate separation of sister chromatids.

Question 14 of 39 Medium

What is the main function of CDKs in the cell cycle?

  1. A Binding DNA directly at promoters
  2. B Degrading cyclins for cycle exit
  3. C Phosphorylating target proteins
  4. D Forming and organizing spindle fibers

Correct answer: Phosphorylating target proteins

Cyclin-dependent kinases (CDKs) phosphorylate specific substrates to regulate cell cycle transitions. Their activity depends on association with cyclins.

Question 15 of 39 Medium

Which checkpoint prevents cells with damaged DNA from entering S phase?

  1. A G1 checkpoint
  2. B G2 checkpoint
  3. C Spindle checkpoint
  4. D M checkpoint

Correct answer: G1 checkpoint

The G1 checkpoint assesses DNA integrity before replication begins. Cells with damage are prevented from entering S phase until repair occurs.

Question 16 of 39 Medium

In which phase does cytokinesis usually begin?

  1. A Prophase
  2. B Metaphase
  3. C Anaphase
  4. D Telophase

Correct answer: Anaphase

Cytokinesis often begins during late anaphase. It continues through telophase until the cytoplasm is fully divided.

Question 17 of 39 Medium

Which cell cycle phase is absent in rapidly dividing embryonic cells?

  1. A G1 phase
  2. B S phase, the DNA synthesis period
  3. C G2 phase, the pre-mitotic gap
  4. D M phase, the mitotic division

Correct answer: G1 phase

Rapidly dividing embryonic cells often lack a G1 phase. This allows faster progression through the cell cycle with minimal growth periods.

Question 18 of 39 Medium

What triggers the transition from metaphase to anaphase?

  1. A Cyclin D degradation
  2. B Activation of spindle assembly checkpoint
  3. C Degradation of securin
  4. D DNA replication completion

Correct answer: Degradation of securin

Degradation of securin releases separase, which cleaves cohesin proteins. This allows sister chromatids to separate during anaphase.

Question 19 of 39 Medium

Which molecule ensures sister chromatids remain attached until anaphase?

  1. A Tubulin, a spindle subunit
  2. B Cohesin
  3. C Kinesin, a motor protein
  4. D Dynein, a motor protein

Correct answer: Cohesin

Cohesin proteins hold sister chromatids together after DNA replication. Their cleavage is necessary for chromosome segregation.

Question 20 of 39 Medium

Entry into G0 phase is best described as:

  1. A A permanent arrest of the cell cycle
  2. B A temporary, non-dividing state
  3. C A phase of rapid DNA synthesis
  4. D A stage of chromosome separation

Correct answer: A temporary, non-dividing state

G0 is a quiescent state where cells are metabolically active but not dividing. Many cells can re-enter the cell cycle from G0 when stimulated.

Question 21 of 39 Medium

Which phase of the cell cycle is primarily involved in cell growth before DNA replication?

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

Correct answer: G1 phase

The G1 phase is the first gap phase where the cell grows and prepares for DNA synthesis. It involves protein synthesis and organelle duplication.

Question 22 of 39 Medium

During which phase does DNA replication occur?

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

Correct answer: S phase

DNA replication takes place during the S phase. Each chromosome is duplicated to form sister chromatids.

Question 23 of 39 Medium

Which phase prepares the cell for mitosis by producing proteins and organelles?

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

Correct answer: G2 phase

The G2 phase follows DNA replication and prepares the cell for mitosis. It includes synthesis of proteins needed for chromosome segregation.

Question 24 of 39 Medium

Which phase includes both mitosis and cytokinesis?

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

Correct answer: M phase

The M phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). It results in two daughter cells.

Question 25 of 39 Medium

Which checkpoint ensures that DNA is undamaged before replication begins?

  1. A G1 checkpoint
  2. B G2 checkpoint
  3. C Metaphase checkpoint
  4. D Spindle checkpoint

Correct answer: G1 checkpoint

The G1 checkpoint checks for DNA damage before the cell enters S phase. It prevents replication of damaged DNA.

Question 26 of 39 Medium

Which checkpoint ensures DNA replication is complete before mitosis?

  1. A G1 checkpoint
  2. B G2 checkpoint
  3. C Spindle checkpoint
  4. D Cytokinesis checkpoint

Correct answer: G2 checkpoint

The G2 checkpoint ensures that DNA replication is complete and error-free before mitosis begins. It prevents damaged DNA from being passed on.

Question 27 of 39 Medium

What is the main role of cyclins in the cell cycle?

  1. A DNA replication
  2. B Activate CDKs
  3. C Repair DNA
  4. D Form spindle fibers

Correct answer: Activate CDKs

Cyclins regulate the cell cycle by activating cyclin-dependent kinases (CDKs). These complexes control progression through different phases.

Question 28 of 39 Medium

Which protein complex promotes transition from G1 to S phase?

  1. A Cyclin A-CDK
  2. B Cyclin B-CDK
  3. C Cyclin D-CDK
  4. D Cyclin E-CDK

Correct answer: Cyclin E-CDK

Cyclin E-CDK complex is crucial for the transition from G1 to S phase. It initiates DNA replication.

Question 29 of 39 Medium

Which phase is the longest in a typical cell cycle?

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

Correct answer: G1 phase

The G1 phase is usually the longest phase. Cells spend most of their time growing and performing normal functions.

Question 30 of 39 Medium

Which checkpoint ensures that all chromosomes are correctly attached before the cell proceeds to anaphase?

  1. A G1 checkpoint
  2. B G2 checkpoint
  3. C Spindle checkpoint
  4. D G1 restriction point

Correct answer: Spindle checkpoint

The spindle checkpoint (spindle assembly checkpoint) ensures all chromosomes are properly attached before anaphase begins. This prevents unequal chromosome distribution.

Question 31 of 39 Medium

Which molecule is responsible for regulating CDK activity negatively?

  1. A Cyclins
  2. B CDKs
  3. C CDK inhibitors
  4. D DNA polymerase

Correct answer: CDK inhibitors

CDK inhibitors bind to cyclin-CDK complexes and inhibit their activity. This helps control cell cycle progression and prevent errors.

Question 32 of 39 Medium

Which phase involves alignment of chromosomes at the cell's equatorial plane?

  1. A Prophase
  2. B Metaphase
  3. C Anaphase
  4. D Telophase

Correct answer: Metaphase

During metaphase, chromosomes align at the equatorial plate. This ensures equal separation during anaphase.

Question 33 of 39 Medium

Which cyclin is associated with the regulation of mitosis?

  1. A Cyclin D
  2. B Cyclin E
  3. C Cyclin A
  4. D Cyclin B

Correct answer: Cyclin B

Cyclin B forms a complex with CDK1 to regulate entry into mitosis. It controls key events in the M phase.

Question 34 of 39 Medium

Which event occurs during the S phase?

  1. A Protein synthesis
  2. B DNA replication
  3. C Cell division
  4. D Chromosome alignment

Correct answer: DNA replication

The S phase is dedicated to DNA replication. Each chromosome is duplicated to ensure genetic continuity.

Question 35 of 39 Medium

Which point determines whether a cell will divide or enter G0 phase?

  1. A G2 checkpoint
  2. B Restriction point
  3. C Spindle checkpoint
  4. D Metaphase checkpoint

Correct answer: Restriction point

The restriction point in G1 determines whether the cell proceeds with division or enters a resting state (G0). It depends on internal and external signals.

Question 36 of 39 Medium

Which phase is characterized by chromosome condensation and nuclear envelope breakdown?

  1. A Prophase
  2. B Metaphase
  3. C Anaphase
  4. D Telophase

Correct answer: Prophase

During prophase, chromosomes condense and the nuclear envelope begins to break down. Spindle formation also starts in this phase.

Question 37 of 39 Medium

Which protein helps repair DNA damage before the cell proceeds in the cycle?

  1. A Cyclin B
  2. B p53
  3. C CDK1
  4. D Tau

Correct answer: p53

p53 is a tumor suppressor protein that halts the cell cycle in response to DNA damage. It allows time for repair or triggers apoptosis.

Question 38 of 39 Medium

Which phase includes the separation of sister chromatids?

  1. A Prophase
  2. B Metaphase
  3. C Anaphase
  4. D Telophase

Correct answer: Anaphase

During anaphase, sister chromatids are pulled apart toward opposite poles. This ensures equal genetic distribution.

Question 39 of 39 Medium

Which phase follows mitosis and completes cell division?

  1. A G1 phase
  2. B Synthesis phase
  3. C G2 phase
  4. D Cytokinesis

Correct answer: Cytokinesis

Cytokinesis divides the cytoplasm into two daughter cells. It usually follows mitosis and completes the cell cycle.

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