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MCAT Cell Cycle, Mitosis and Meiosis Practice Questions

By StudyStore · Published 2026-10-29 · 9 original questions · about 14 min

Nine free MCAT-style practice questions on cell cycle phases, cyclins and checkpoints, mitosis, cytokinesis, cancer genes and gametogenesis, with full explanations.

Quick concept primer

A dividing somatic cell moves through G1, S, G2 and M. DNA is copied only in S phase, so a diploid cell that holds an amount of DNA called 2C in G1 holds 4C from the end of S until the chromatids split. Progress is driven by cyclins that bind and switch on cyclin-dependent kinases. Checkpoints act as brakes: at G1/S, DNA damage stabilizes p53, which induces p21 to block CDK activity; at the metaphase to anaphase transition, the spindle checkpoint holds the anaphase-promoting complex off until every kinetochore is attached.

Mitosis keeps the chromosome number, while meiosis halves it. In meiosis I homologous pairs separate, so the daughter cells are haploid even though each chromosome still has two chromatids; meiosis II separates the chromatids. Spermatogenesis yields four sperm per primary spermatocyte, while oogenesis yields one egg plus polar bodies, with arrests in prophase I and metaphase II. Cancer usually needs gain of function in proto-oncogenes plus loss of both copies of tumor suppressors. The Cell Division pack adds an Anki deck and question set on this unit, or try the free sampler.

Traps: chromosome number is counted by centromeres, not chromatids, and cytokinesis is a separate step from nuclear division.

9 practice questions

Try each question before opening the explanation. Click "Show answer and explanation" to check yourself. All questions are original and written for study practice.

Question 1

In a mammalian species, a diploid cell in G1 contains 6 pg of nuclear DNA. Assuming no DNA is lost, what DNA content is expected in a primary spermatocyte at metaphase I, a secondary spermatocyte at metaphase II, and a spermatid, in that order?

  1. 6 pg, 3 pg, 1.5 pg
  2. 12 pg, 12 pg, 6 pg
  3. 12 pg, 6 pg, 3 pg
  4. 6 pg, 6 pg, 3 pg
Show answer and explanation

Answer: C. 12 pg, 6 pg, 3 pg

After S phase the primary spermatocyte has doubled its DNA to 12 pg. Meiosis I splits homologs between two cells, so each secondary spermatocyte holds 6 pg, and meiosis II splits chromatids, leaving 3 pg in each spermatid, half the G1 amount. Choices A and D forget the S phase doubling, and B forgets that meiosis I already halves the DNA.

Question 2

A researcher measures DNA content in 2000 cells from an unsynchronized culture with a 20 hour cycle. Assume the time spent in each phase is proportional to the fraction of cells found in it.

DNA contentCells
2C1200
Between 2C and 4C300
4C500

About how long do these cells spend in G1?

  1. 12 hours
  2. 3 hours
  3. 5 hours
  4. 15 hours
Show answer and explanation

Answer: A. 12 hours

Cells with 2C DNA are in G1, and 1200 of 2000 is 0.60, so G1 takes about 0.60 times 20, or 12 hours. Cells between 2C and 4C are in S phase (0.15, about 3 hours), and 4C cells are in G2 or M (0.25, about 5 hours). Choice D wrongly adds the S phase cells to G1.

Question 3

Fibroblasts are exposed to radiation that breaks DNA during G1. Which change most directly keeps these cells from entering S phase?

  1. Degradation of p53 by the proteasome
  2. Activation of separase
  3. Hyperphosphorylation of the Rb protein
  4. Induction of p21, which inhibits cyclin-CDK complexes
Show answer and explanation

Answer: D. Induction of p21, which inhibits cyclin-CDK complexes

DNA damage stabilizes p53, which acts as a transcription factor for p21, a CDK inhibitor. With G1/S cyclin-CDK activity blocked, Rb stays underphosphorylated and keeps E2F bound, so S phase genes stay off. Degrading p53 or hyperphosphorylating Rb would push the cell forward, and separase acts at anaphase, not at G1/S.

Question 4

Each of the following normally happens during prophase or prometaphase of mitosis EXCEPT:

  1. Chromatin condenses into visible chromosomes
  2. Separase cleaves the centromeric cohesin joining sister chromatids
  3. The nuclear envelope breaks apart
  4. Spindle microtubules attach to kinetochores
Show answer and explanation

Answer: B. Separase cleaves the centromeric cohesin joining sister chromatids

Cohesin is cleaved by separase at the start of anaphase, which lets sister chromatids move to opposite poles. Condensation, nuclear envelope breakdown and kinetochore capture by microtubules all occur before the chromosomes line up at metaphase, so choices A, C and D describe early mitotic events.

Question 5

A drug that blocks myosin II motor activity is added to animal cells just after anaphase ends. What is the most likely result after telophase?

  1. A single cell with two nuclei
  2. Two daughter cells, each with half the normal chromosome number
  3. Chromatids that fail to separate
  4. Formation of a cell plate between the new nuclei
Show answer and explanation

Answer: A. A single cell with two nuclei

Animal cytokinesis depends on a contractile ring of actin and myosin II that pinches the membrane inward. Nuclear division is already complete, so blocking the ring leaves one binucleate cell. Chromatid separation has already happened, so C is wrong, and a cell plate built from vesicles is how plant cells divide, not animal cells.

Question 6

Which mutation is most likely to promote uncontrolled growth when present in only one of a cell's two copies of the gene?

  1. A nonsense mutation in one copy of the Rb gene
  2. A deletion of one copy of the p53 gene
  3. A point mutation that locks a Ras protein in its GTP-bound active state
  4. A mutation that inactivates one copy of a DNA mismatch repair gene
Show answer and explanation

Answer: C. A point mutation that locks a Ras protein in its GTP-bound active state

Ras is a proto-oncogene, and a mutation that keeps it bound to GTP sends a constant growth signal even when the other copy is normal, so it acts dominantly at the cell level. Rb, p53 and mismatch repair genes are tumor suppressors or caretakers, and losing one copy usually leaves enough function until the second copy is also lost.

Question 7

At what stage is a human oocyte arrested from fetal life until the months before ovulation, and at what stage is it arrested when it is released at ovulation?

  1. Metaphase I, then prophase II
  2. G1, then metaphase I
  3. Prophase II, then metaphase II
  4. Prophase I, then metaphase II
Show answer and explanation

Answer: D. Prophase I, then metaphase II

Primary oocytes enter meiosis before birth and stop in prophase I until hormonal cycles after puberty resume meiosis in a maturing follicle. Meiosis I finishes shortly before ovulation, and the secondary oocyte released at ovulation is held in metaphase II, finishing meiosis II only if fertilized. The other choices misplace one or both arrests.

Question 8

A secondary oocyte from an organism with a diploid number of 2n = 46 is held in metaphase II. How many chromosomes and how many chromatids does it contain?

  1. 46 chromosomes and 92 chromatids
  2. 23 chromosomes and 46 chromatids
  3. 23 chromosomes and 23 chromatids
  4. 46 chromosomes and 46 chromatids
Show answer and explanation

Answer: B. 23 chromosomes and 46 chromatids

Meiosis I separated homologs, so the secondary oocyte is haploid with 23 chromosomes, but each chromosome still has two sister chromatids, giving 46 chromatids. Choice A describes a diploid cell after S phase, choice C describes a cell after meiosis II, and choice D counts chromatids as if they were chromosomes.

Question 9

In a dividing cell, one kinetochore remains unattached to the spindle while all other chromosomes sit at the metaphase plate. What keeps the cell from entering anaphase?

  1. The anaphase-promoting complex is held inactive, so securin is not degraded
  2. Cyclin D levels rise and restart G1
  3. Topoisomerase stops relaxing supercoiled DNA
  4. The nuclear envelope reforms around the chromosomes
Show answer and explanation

Answer: A. The anaphase-promoting complex is held inactive, so securin is not degraded

An unattached kinetochore generates a wait signal that inhibits the anaphase-promoting complex. Without that complex, securin is not tagged for destruction, so separase stays inhibited and cohesin keeps the chromatids together. Cyclin D acts in G1, topoisomerase is not the checkpoint target, and the nuclear envelope reforms only in telophase.

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Written by StudyStore from standard scientific facts as a study aid. These are not official exam questions and are not affiliated with or endorsed by any testing organization. Always check values against the reference provided with your exam or course.

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