Question 1

A researcher is studying the cell cycle in a population of rapidly dividing cells. She calculates the mitotic index using the following data: Out of 1,000 cells observed, 80 are in mitosis. What is the mitotic index, and what does it indicate about the cell population?

Accepted Answer

The mitotic index is $$0.08$$, indicating that $$8\% of $$the cells are in mitosis at any given time.

Question 2

Which of the following best describes what happens to a cell that enters the $$G_0$$ phase and undergoes terminal differentiation?

Accepted Answer

The cell permanently exits the cell cycle and specializes for a specific function.

Question 3

CENP-A is a specialized histone variant found at centromeres. Which of the following statements best distinguishes the inner kinetochore (CCAN) from the outer kinetochore (KMN network)?

Accepted Answer

The inner kinetochore (CCAN) directly binds centromeric chromatin containing CENP-A, while the outer kinetochore (KMN network) interacts with microtubules.

Question 4

A cell is able to assemble a functional mitotic spindle even after its centrosomes are destroyed. Which molecular mechanism enables this process?

Accepted Answer

A localized Ran-GTP gradient near chromosomes promotes microtubule nucleation and spindle assembly independently of centrosomes.

Question 5

During mitosis, Anaphase A and Anaphase B are powered by different motor proteins and involve distinct movements. Which of the following correctly matches the movement and the primary motor protein for each phase?

Accepted Answer

Anaphase A: Chromosome movement toward poles via depolymerization at kinetochores (dynein); Anaphase B: Spindle pole separation via kinesin-5 motors.

Question 6

Which of the following best explains the structural and directional differences between cytokinesis in plant and animal cells?

Accepted Answer

Plant cells form a cell plate via the phragmoplast from the center outward, while animal cells use a contractile ring to pinch the membrane inward.

Question 7

During the metaphase-to-anaphase transition, which sequence of molecular events ensures proper sister chromatid separation?

Accepted Answer

Securin is degraded, releasing separase, which cleaves cohesin complexes holding sister chromatids together.

Question 8

The Retinoblastoma protein ($$Rb) $$regulates the $$G_1 to$$ $$S$$ phase transition. Which of the following best describes its mechanism of action?

Accepted Answer

$$Rb$$ binds and inhibits E2F transcription factors; phosphorylation of $$Rb$$ releases E2F, allowing $$S$$ phase gene expression.

Question 9

How is DNA re-replication prevented within a single cell cycle?

Accepted Answer

Replication licensing factors (MCM helicases) are loaded onto DNA only during $$G_1$$; after initiation, geminin inhibits re-loading until the next cycle.

Question 10

A cell with unattached kinetochores fails to progress through mitosis. Which checkpoint proteins are responsible for this arrest, and how do they function?

Accepted Answer

Mad and Bub proteins inhibit the anaphase-promoting complex (APC/C) at unattached kinetochores, enforcing the Spindle Assembly Checkpoint (SAC).

Question 11

In a flow cytometry experiment, which cell cycle phases can be distinguished based on DNA content, and what are their relative DNA amounts?

Accepted Answer

$$G_1$$ cells have $$1	imes$$ DNA content, $$S$$ phase cells have intermediate DNA content, and $$G_2/M$ cells have 2$$	imes$$ DNA content.

Question 12

Activation of Mitotic Cdk (MPF) requires a series of phosphorylation and dephosphorylation steps. Which sequence correctly describes these steps?

Accepted Answer

Cdk binds cyclin, is phosphorylated at an activating site, phosphorylated at an inhibitory site, then dephosphorylated at the inhibitory site.

Question 13

A mutation in a cell prevents the activation of the Ras protein. Which step in the mitogenic growth factor signaling pathway leading to the $$G_1$$ \rightarrow S$$ transition would be directly affected?

Accepted Answer

The activation of the MAPK cascade downstream of RTK would be blocked, preventing $$G_1$$ \rightarrow S$$ progression.

Question 14

Which of the following best describes the difference between the extrinsic and intrinsic apoptosis pathways up to Caspase-3 activation?

Accepted Answer

The extrinsic pathway is initiated by death receptor signaling at the cell surface, while the intrinsic pathway is triggered by mitochondrial cytochrome c release.

Question 15

A patient’s tumor cells show a high mitotic index and a large proportion of cells in $$G_2/M$ as measured by flow cytometry. Which of the following is the most likely explanation?

Accepted Answer

The tumor cells are rapidly dividing, with many cells preparing for or undergoing mitosis.

Question 16

A scientist treats animal cells with a drug that inhibits actin polymerization. What is the most likely effect on cytokinesis?

Accepted Answer

Cytokinesis will fail because the contractile ring cannot form to pinch the cell membrane.

Question 17

If geminin is nonfunctional in a cell, what is the most likely consequence during the cell cycle?

Accepted Answer

DNA may be re-replicated within a single cell cycle, leading to genomic instability.

Question 18

A cell is exposed to a mitogenic growth factor, leading to activation of the RTK-Ras-MAPK pathway. Which downstream effect is most directly responsible for entry into $$S$$ phase?

Accepted Answer

Phosphorylation of $$Rb$$ protein, releasing E2F to activate $$S$$ phase gene transcription.

Question 19

A cell with a mutation in the Mad2 protein proceeds through anaphase despite having unattached kinetochores. What is the most likely cellular consequence?

Accepted Answer

Chromosome missegregation and aneuploidy due to failure of the spindle assembly checkpoint.

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