These are original Marlbridge practice questions, not past exam questions. The mark points are a suggested Marlbridge rubric to help you check your work; they are not an official scoring guideline. No constants are needed. Assume chromosomes separate correctly and ignore crossing over unless a question says otherwise. All data sets are fictional.
Question 1 (multiple choice · foundation)
Which event happens during meiosis I but does not happen during mitosis?
- (A) Spindle fibres attach to the kinetochores of chromosomes.
- (B) Homologous chromosomes pair up along their length (synapsis).
- (C) Sister chromatids separate and move to opposite poles.
- (D) The nuclear envelope breaks down.
Answer and explanation
Answer: (B). Synapsis of homologous chromosomes happens only in prophase I. It is what allows the homologues to be lined up as pairs and then separated in anaphase I.
- (A) happens in mitosis and in both meiotic divisions. All of them use a spindle.
- (C) happens in mitosis (anaphase) and in meiosis II (anaphase II), but not in meiosis I, where sister chromatids stay joined.
- (D) happens at the start of mitosis and of meiosis I, so it is not unique to meiosis.
Question 2 (multiple choice · core)
A species of fern has 2n = 20. How many chromosomes and how many chromatids are in each cell at the end of telophase I, after cytokinesis?
- (A) 20 chromosomes, 40 chromatids
- (B) 10 chromosomes, 20 chromatids
- (C) 10 chromosomes, 10 chromatids
- (D) 20 chromosomes, 20 chromatids
Answer and explanation
Answer: (B). Meiosis I sends one member of each homologous pair to each cell, so each cell has 20 ÷ 2 = 10 chromosomes. Sister chromatids have not yet separated, so each chromosome still has 2 chromatids: 10 × 2 = 20 chromatids.
- (A) is a cell in prophase I or metaphase I, before the homologues separate.
- (C) is a cell at the end of meiosis II, after the sister chromatids have separated.
- (D) is a diploid G1 cell, or a cell made by mitosis.
Question 3 (multiple choice · core)
Which statement correctly describes homologous chromosomes?
- (A) They are the two identical copies of a chromosome made during S phase, joined at a centromere.
- (B) They are two chromosomes, one from each parent, that carry the same genes in the same order but may carry different alleles.
- (C) They are any two chromosomes in a cell that have the same length.
- (D) They are the two chromosomes that move to opposite poles in anaphase of mitosis.
Answer and explanation
Answer: (B). Homologues are a maternal and a paternal version of the same chromosome. They match in genes, gene order and centromere position, but they can differ in alleles.
- (A) describes sister chromatids, not homologues.
- (C) is not enough: two different chromosomes can be about the same length but carry different genes.
- (D) describes separated sister chromatids. In mitosis, homologues do not pair or move apart as a pair.
Question 4 (diagram · core)
The diagram shows three cells in anaphase from an animal with 2n = 4. Long and short chromosomes belong to different homologous pairs.
(a) Identify the type of division (mitosis, meiosis I or meiosis II) shown by each cell. Justify each answer. (b) State the number of chromatids in cell Q and in cell R. (c) State how many chromosomes will be in each daughter cell produced by cell P, and whether each chromosome will have one or two chromatids.
Worked solution and suggested Marlbridge rubric
(a)
- Cell P: anaphase II (meiosis II). Single chromatids are moving apart, so sister chromatids have separated. Only 2 chromosomes go to each pole, which is n, so the cell is already haploid.
- Cell Q: anaphase of mitosis. Single chromatids are separating, and 4 go to each pole, which is 2n. Each pole receives one long and one short chromosome of each parental type, so both new cells will be diploid.
- Cell R: anaphase I (meiosis I). The chromosomes moving to each pole still have two chromatids, so sister chromatids have not separated. The long solid and long open homologues are going to opposite poles, so homologues are separating.
(b) Cell Q: 8 single chromatids (4 at each pole). Cell R: 4 duplicated chromosomes × 2 = 8 chromatids.
(c) Each daughter cell of P will have 2 chromosomes, each with one chromatid.
| Point | What earns it |
|---|---|
| 1 | P identified as meiosis II with reasoning about single chromatids and n chromosomes per pole |
| 1 | Q identified as mitosis with reasoning about 2n (or both homologues) at each pole |
| 1 | R identified as meiosis I with reasoning about duplicated chromosomes or homologues separating |
| 1 | (b) 8 and 8, and (c) 2 chromosomes with one chromatid each |
Do not award a point for an identification without a reason.
Question 5 (data analysis · core)
A researcher measures the DNA content of 800 nuclei from a sample of testis tissue of a fictional mammal, where cells are dividing by both mitosis and meiosis. In this species a diploid nucleus in G1 contains 3.2 pg of DNA. The fictional results:
| DNA per nucleus / pg | 1.6 | 3.2 | 6.4 |
|---|---|---|---|
| Number of nuclei | 412 | 316 | 72 |
(a) Calculate the percentage of nuclei in each class. (b) Identify the class that contains cells that have completed meiosis II. Justify your answer. (c) The 3.2 pg class may contain two different kinds of cell. Name them, and describe how a microscope could tell them apart. (d) Explain why no nuclei with more than 6.4 pg are expected.
Worked solution and suggested Marlbridge rubric
(a) Total = 412 + 316 + 72 = 800. 1.6 pg: 412 ÷ 800 × 100 = 51.5%. 3.2 pg: 316 ÷ 800 × 100 = 39.5%. 6.4 pg: 72 ÷ 800 × 100 = 9.0%. (Check: 51.5 + 39.5 + 9.0 = 100.0.)
(b) The 1.6 pg class. Cells that have completed meiosis II have half the DNA of a G1 diploid cell (3.2 ÷ 2 = 1.6 pg), because DNA was copied once but the cell divided twice. No cell made by mitosis can have less than 3.2 pg.
(c) (1) Diploid cells in G1 (not yet replicated), and (2) haploid cells that have finished meiosis I but not meiosis II (n chromosomes, each still with two chromatids). Under a microscope, count the chromosomes when the cells divide: the diploid cells show 2n chromosomes (and, in meiosis I, homologous pairs), while the haploid cells show only n chromosomes lined up singly, as in metaphase II.
(d) DNA is replicated only once before a division (3.2 → 6.4 pg). The highest DNA content is in cells after S phase, before the first division of mitosis or meiosis. There is no second S phase, so 6.4 pg is the maximum.
| Point | What earns it |
|---|---|
| 1 | All three percentages correct |
| 1 | 1.6 pg chosen with the reason that DNA is halved twice from 6.4 pg, or halved from the G1 value |
| 1 | Both cell types in the 3.2 pg class named, with a valid way to tell them apart |
| 1 | Only one replication before division, so no higher class |
Accept “secondary spermatocytes” for the haploid cells in (c), but the name is not required.
Question 6 (constructed response · core)
A student writes: “Meiosis is just mitosis done twice in a row.”
(a) Give two ways in which this statement is partly correct. (b) Give three differences that show the statement is wrong.
Model answer and suggested Marlbridge rubric
(a) Any two: both processes use a spindle apparatus to move chromosomes; both have prophase, metaphase, anaphase and telophase; both end with cytokinesis (cleavage furrow or cell plate); meiosis II separates sister chromatids in the same way that mitosis does.
(b) Any three:
- DNA is replicated once before meiosis, not before each division. “Mitosis twice” would need two S phases.
- In prophase I, homologues pair up (synapsis). This never happens in mitosis.
- In metaphase I, pairs of homologues line up, not single chromosomes.
- In anaphase I, homologues separate while sister chromatids stay joined. Mitosis always separates sister chromatids.
- Meiosis produces four haploid cells; mitosis produces two diploid cells.
- Cells from meiosis are genetically different from each other and from the parent; mitosis gives identical cells.
| Point | What earns it |
|---|---|
| 1 | Two valid similarities |
| 1 | First valid difference, clearly stated |
| 1 | Second valid difference |
| 1 | Third valid difference |
Do not award two points for two versions of the same difference (for example “four cells” and “more cells”).
Question 7 (prediction · stretch)
A fictional drug, compound K, stops the proteins that hold sister chromatids together at the centromere from breaking down. It has no other effect.
(a) Compound K is added to cells at the start of meiosis II. Predict the effect on anaphase II and on the products of meiosis. (b) Compound K is added to a different set of cells at metaphase I. A student predicts that the homologues will be unable to separate in anaphase I. Evaluate this prediction. (c) Predict the effect of compound K on cells dividing by mitosis, and explain your answer.
Worked solution
(a) Sister chromatids cannot separate, so anaphase II cannot happen normally. The cells cannot produce four haploid cells with single-chromatid chromosomes; meiosis stops at (or fails after) metaphase II.
(b) The prediction is not supported. In anaphase I, the homologues separate while the sister chromatids remain attached at their centromeres. Keeping the centromere proteins intact is exactly what normally happens in anaphase I, so compound K should not stop the homologues moving apart. (The block would appear later, in anaphase II, as in part (a).)
(c) Mitosis would also be blocked at anaphase. In mitosis, anaphase is the separation of sister chromatids, which needs the centromere proteins to break down.
Suggested mark points (3): 1 for (a) linking intact centromere proteins to failure of sister chromatid separation in anaphase II; 1 for (b) rejecting the prediction because sister chromatids stay joined during anaphase I anyway; 1 for (c) mitotic anaphase blocked, with the reason.
Common error: treating anaphase I and anaphase II as the same event. Only anaphase II (and mitotic anaphase) separates sister chromatids.
How did you do?
- Q1 or Q3 wrong: re-read “The vocabulary you need first” and “Meiosis I” in the study guide.
- Q2 or Q5 counts wrong: rework Worked example 1 and Figure 2; count chromosomes by centromeres and DNA by chromatids.
- Q4 wrong: rework Worked example 2: ask whether homologues are paired, how the count compares with 2n, and whether chromosomes are duplicated.
- Q6 or Q7 incomplete: re-read “Mitosis and meiosis compared” and the anaphase I and anaphase II bullet points.
Then tick off the topic checklist.