IB DP Biology: Cell and nuclear division: mitosis, meiosis and the cell cycle (DP Biology) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – the IB holds copyright in its own papers. Use these alongside the official past papers available through your school or the IB store.

These questions practise sub-topic D2.1, Cell and nuclear division, from the IB Biology guide, first assessment 2025. Questions 1 to 8 cover D2.1.1 to D2.1.11 (SL and HL), except that the mitotic index in question 4(a) belongs to D2.1.17 and is HL only. Questions 9 to 12 cover D2.1.12 to D2.1.17 and are HL only. For explanations use the study guide or the revision notes. Course hub: IB DP Biology. Printable list: checklist.

Questions

1. Describe how cytokinesis differs in an animal cell and in a plant cell. [2]

2. Cytokinesis is not always equal.

(a) State two examples of unequal cytokinesis. [2] (b) Explain why each daughter cell must still receive at least one mitochondrion. [2]

3. The table shows the DNA content of one nucleus at four stages in an organism with 2n = 6 (data supplied).

Stage	DNA per nucleus (au)
W: before replication	6.0
X: after replication	12.0
Y: after meiosis I	6.0
Z: after meiosis II	3.0

(a) Explain the change in DNA content between W and X. [2] (b) State the number of chromatids in a nucleus at stage X. [1] (c) Identify which stage could be a gamete nucleus and give a reason. [1]

4. A student examined a root-tip squash and counted 300 cells: 262 in interphase, 21 in prophase, 6 in metaphase, 3 in anaphase and 8 in telophase.

(a) (HL only) Calculate the mitotic index as a percentage to 3 significant figures. [2] (b) Describe how you would recognise a cell in anaphase in a micrograph. [2]

5. Explain how histones and microtubule motors contribute to chromosome behaviour in mitosis and meiosis. [3]

6. A diploid cell has four chromosomes (2n = 4). Outline what happens to the chromosomes from replication until four nuclei have formed. [5]

7. A child has Down syndrome.

(a) State the number of chromosomes in a body cell of the child and the reason. [2] (b) Explain how non-disjunction in meiosis II could have produced the gamete involved. [2]

8. A plant has 2n = 10.

(a) Calculate the number of chromosome combinations possible from random orientation of bivalents, ignoring crossing over. [2] (b) Explain how crossing over adds to the variation in the gametes. [3]

9. (HL only) (a) Give one example of cell proliferation for growth in a plant and one in an animal embryo. [2] (b) Explain how cell proliferation helps heal a skin wound. [2]

10. (HL only) A cell cycle lasts 22 hours (data supplied): G1 9 h, S 8 h, G2 4 h, and mitosis plus cytokinesis 1 h.

(a) Calculate the percentage of the cycle spent in interphase, to 3 significant figures. [2] (b) Explain why DNA replication in S phase must be completed before mitosis. [2] (c) Describe two events in interphase that show it is not a resting stage. [2]

11. (HL only) The table shows the concentrations of two cyclins (invented data, in au) in a cell. Cyclin P must reach 40 au for the cell to pass the G1 checkpoint. Cyclin R must reach 50 au for the cell to pass the checkpoint before mitosis.

Time (h)	Cyclin P	Cyclin R
0	8	5
4	25	5
8	42	12
12	30	38
16	12	55
20	6	20

(a) Identify the first time shown at which the cell could pass the G1 checkpoint. [1] (b) Describe the pattern shown by cyclin P. [2] (c) Deduce, with a reason, the earliest time shown at which the cell could pass the checkpoint before mitosis. [2] (d) Suggest why the fall in cyclin concentrations is important. [1]

12. (HL only) Tissue samples were examined, with 200 cells counted in each. Tumour P had 38 cells in mitosis, tumour Q had 9 and normal tissue had 6.

(a) Calculate the mitotic index of tumours P and Q, as percentages. [2] (b) Explain how mutation of a tumour suppressor gene can lead to a tumour. [2] (c) Tumour P is malignant and tumour Q is benign. Evaluate how well the data and other features support this. [4]

Answers

1. In an animal cell, a ring of actin and myosin contracts and pinches the membrane inward [1]. In a plant cell, vesicles build new membrane and new wall in sections across the middle of the cell [1]. [2] Examiner insight: A “describe” answer on this topic usually gains credit only when both cell types are covered and the structures are named; “the cell splits in two” repeats the question.

2. (a) Oogenesis in humans [1]; budding in yeast [1]. [2] (b) Mitochondria cannot be made from scratch and arise only by division of existing ones [1]. A daughter with none could not make ATP, or could not regenerate them [1]. [2] Examiner insight: Strong answers give the reason (mitochondria come from pre-existing mitochondria) and not just “they need energy”.

3. (a) DNA doubled from 6.0 to 12.0 au [1] because DNA replication occurred in S phase [1]. [2] (b) 12 chromatids (6 chromosomes, 2 each) [1]. [1] (c) Z, because it is haploid with half the original DNA (3.0 au) [1]. [1] Examiner insight: Quote the figures from the table when explaining a change, and do not call Y a gamete just because its DNA equals W.

4. (a) (HL only) Cells in mitosis: 21 + 6 + 3 + 8 = 38 [1]. 38 ÷ 300 × 100 = 12.7% [1]. [2] (b) Chromatids (now chromosomes) are separated into two groups [1] moving towards opposite poles, with no nuclear envelope around them [1]. [2] Examiner insight: Give the final answer to the requested number of significant figures and show the sum of mitotic cells; including the interphase cells in the numerator is a common slip.

5. DNA is wound around histones [1] and supercoiled, so chromosomes become short and compact, which stops tangling when moved [1]. Microtubule motors move chromosomes along spindle microtubules [1]. [3] Examiner insight: Tie each structure to its role: histones go with condensation, microtubules and motors go with movement.

6. DNA replicates, so each of the 4 chromosomes has two chromatids [1]. In meiosis I homologous chromosomes pair as bivalents [1] and then separate, giving two cells with 2 chromosomes each [1]. In meiosis II the chromatids separate [1]. Four haploid nuclei form, each with 2 chromosomes [1]. [5] Examiner insight: Say what separates in each division and give chromosome numbers; answers that only list division names, without the pairing and separation, tend to gain little.

7. (a) 47 [1], because there are three copies of chromosome 21 [1]. [2] (b) The two chromatids of chromosome 21 failed to separate in meiosis II [1], so one gamete received both (24 chromosomes) and fused with a normal gamete (23) [1]. [2] Examiner insight: Name the division and what failed to separate; “chromosomes did not separate” is not specific about meiosis II.

8. (a) n = 5 [1]; 2⁵ = 32 [1]. [2] (b) In prophase I, non-sister chromatids of homologues exchange sections [1] at chiasmata [1], giving new combinations of alleles on one chromosome, so gametes differ from one another and from the parent chromosomes [1]. [3] Examiner insight: Place crossing over in prophase I and say it is between homologues; random orientation in metaphase I is a different source of variation.

9. (HL only) (a) Plant meristem [1]; early animal embryo (cleavage divisions) [1]. [2] (b) Cells at the wound edge divide [1] to replace lost cells and close the gap [1]. [2] Examiner insight: Name the type of tissue (meristem, embryo, skin) and not simply “growth”.

10. (HL only) (a) Interphase = 9 + 8 + 4 = 21 h [1]; 21 ÷ 22 × 100 = 95.5% [1]. [2] (b) Each chromosome needs two chromatids [1] so that each daughter cell gets a full copy of the DNA [1]. [2] (c) Any two: synthesis of proteins; DNA replication; growth of the cell; increase in mitochondria or chloroplasts [1] [1]. [2] Examiner insight: Interphase includes G1, S and G2, so check you added all three before dividing.

11. (HL only) (a) 8 h [1]. [1] (b) Cyclin P rises from 8 au to a peak of 42 au at 8 h [1] and then falls to 6 au by 20 h [1]. [2] (c) 16 h [1], because that is the first time cyclin R is above its 50 au threshold (55 au) [1]. [2] (d) It lets the cell be controlled again in the next cycle, since each checkpoint needs a rise to the threshold [1]. [1] Examiner insight: Use values and times from the table; mention both the rise and the fall when asked for a pattern.

12. (HL only) (a) P: 38 ÷ 200 × 100 = 19.0% [1]. Q: 9 ÷ 200 × 100 = 4.5% [1]. [2] (b) A tumour suppressor gene normally restrains cell division [1]. A mutation stops it working, so division is no longer restrained and cells divide in an uncontrolled way [1]. [2] (c) Support: P has a mitotic index (19.0%) much higher than normal tissue (3.0%), consistent with fast division in a malignant tumour [1]. Q is close to normal (4.5%), consistent with slow growth in a benign tumour [1]. Limitation: mitotic index measures division rate only [1], while malignancy also means invading neighbouring tissue and forming secondary tumours, so more evidence is needed [1]. [4] Examiner insight: In an “evaluate” question, give both what the data support and what they cannot show; a verdict with no limitation tends to lose credit.

Where marks are usually lost

• Stating “cells split” for cytokinesis without naming actin and myosin or vesicles and wall.
• Leaving out the reason a daughter cell needs a mitochondrion.
• Calling a chromatid a chromosome, or forgetting that replication gives two chromatids.
• Counting interphase cells as dividing cells in a mitotic index.
• Not giving a mitotic index to the number of significant figures requested.
• Saying DNA halves in meiosis I.
• Explaining non-disjunction without saying which division failed.
• Placing crossing over in the wrong phase or confusing it with random orientation.
• Treating interphase as a resting stage.
• Calling every tumour cancer.

Next steps

• Review the revision notes and the study guide.
• Go to the IB DP Biology hub and the checklist.
• Take one of the free 10-minute diagnostics.
• Book a free trial class.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization (sub-topic D2.1).