Cambridge International AS & A Level Biology 9700: The mitotic cell cycle – 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 – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover topic 5, The mitotic cell cycle, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027: sub-topics 5.1 (Replication and division of nuclei and cells) and 5.2 (Chromosome behaviour in mitosis). It is all AS Level content, tested on Paper 1 and Paper 2 and assumed for Paper 4. Calculators are allowed in all 9700 papers. Questions 7, 11 and 12 use data-handling and microscope skills that also appear in practical work.

Revise first with the study guide and the revision notes. The 9700 course hub and the printable checklist show where this topic sits, and the free 9700 AS diagnostic checks your AS topics together.

Questions

1. A replicated chromosome is seen in a cell at the start of prophase.

(a) Name the proteins around which the DNA in the chromosome is wound. [1] (b) Name the region that holds the two sister chromatids together. [1]

2. Outline what happens in each of the three phases of interphase. [3]

3. A lizard regrows part of its tail after losing it. Explain why the new tail cells are genetically identical to the cells of the rest of the lizard’s body. [3]

4. Outline the role of telomeres during DNA replication. [3]

5. Red blood cells in humans survive for only a few months. Outline how stem cells in the bone marrow keep the number of red blood cells steady. [3]

6. Some chemicals in tobacco smoke can cause mutations. Explain how a mutation in a single lung cell could lead to the formation of a tumour. [4]

7. A student counted cells in each stage of the cell cycle in a squash of a root tip. The results were: interphase 680, prophase 64, metaphase 24, anaphase 16, telophase 16. One complete cell cycle in this root tip takes 20 hours.

(a) Calculate the percentage of cells counted that were in mitosis. [2] (b) Calculate how long, in minutes, the cells spend in metaphase. [2] (c) State one assumption you made in (b). [1]

8. Four cells, A to D, are seen in a stained root tip slide. Identify the stage of the mitotic cell cycle shown by each cell.

• A: chromosomes arranged in a line across the middle of the cell, with spindle fibres attached at the centromeres.
• B: a clear nucleus with a nucleolus; no separate chromosomes visible.
• C: two groups of chromosomes at opposite ends of the cell, with a nuclear envelope forming around each group.
• D: thread-like chromosomes visible inside the nucleus; no nucleolus visible.

[4]

9. State three ways in which mitosis and cytokinesis in a plant cell differ from those in an animal cell. [3]

10. The mass of DNA in one cell of a fungus was measured through one complete mitotic cell cycle. In G1 the cell contained 6.6 pg of DNA.

(a) State the mass of DNA in the cell during G2. [1] (b) Describe and explain the changes in the mass of DNA in the cell from the start of G1 until the end of cytokinesis. [4]

11. A drug stops spindle fibres forming. Root tips were grown in water (control) or in a solution of the drug for 6 hours. 1000 cells from each were then classified.

Stage	Control	Drug
Interphase	900	830
Prophase	45	45
Metaphase	25	125
Anaphase	15	0
Telophase	15	0

(a) Calculate the percentage increase in the number of cells in metaphase caused by the drug. [2] (b) Explain why no cells treated with the drug were found in anaphase or telophase. [3] (c) Drugs like this can be used to slow the growth of tumours. Suggest why they slow tumour growth, and why they can also affect healthy tissues such as the bone marrow. [3]

12. A photomicrograph shows an animal cell in which two groups of V-shaped chromatids are moving towards opposite poles of the cell. The species has 12 chromosomes in each body cell.

(a) Identify the stage of mitosis shown. [1] (b) State how many chromatids the cell contained at metaphase. [1] (c) State how many chromosomes each daughter cell will contain. [1] (d) The cell is 54 mm long on the photomicrograph, which has a magnification of × 1500. Calculate the actual length of the cell in µm. [2] (e) Describe how cytokinesis would take place in this cell. [2]

Answers

1. (a) Histones (histone proteins) [1]. (b) Centromere [1]. Examiner insight: One-word “name” answers are marked on spelling close enough to be unambiguous; “centriole” or “centrosome” for centromere is a different structure and gets no credit.

2. G1: the cell grows and makes proteins and organelles [1]. S: DNA is replicated, so each chromosome becomes two chromatids [1]. G2: further growth / preparation for division (for example, more organelles and energy stores) [1]. Examiner insight: Each mark is tied to a named phase; a list of events with no phase labels, or “DNA replicates in G2”, loses the matching mark.

3. The new cells are made by mitosis [1]. Before division, DNA is replicated exactly in S phase, so sister chromatids are identical [1]. In anaphase, each daughter nucleus receives one chromatid of every chromosome, so it has the same genes as the parent cell [1]. Examiner insight: “Because it is mitosis” alone earns one mark at most; the explanation marks need replication and the separation of identical chromatids.

4. Telomeres are repeated, non-coding base sequences at the ends of chromosomes [1]. At each DNA replication, the end of the DNA molecule cannot be fully copied, so a short length is lost [1]. The length lost is telomere rather than a gene, so genes near the ends are not lost [1]. Examiner insight: Answers that only say telomeres “protect the chromosome” are too vague; the mark scheme needs the link to shortening at replication and loss of genes.

5. Stem cells are unspecialised and divide by mitosis [1]. Some daughter cells differentiate into red blood cells, replacing those that die [1]. Other daughter cells remain as stem cells, so the supply continues [1]. Examiner insight: Differentiation and mitosis are separate marking points; describing only “stem cells turn into blood cells” misses the mitosis mark.

6. The mutation is in a gene that controls cell division (the cell cycle) [1]. The cell no longer responds to controls that stop it dividing [1]. It divides by mitosis repeatedly and without control [1]. All daughter cells inherit the mutation, so they also divide uncontrollably, forming a mass of cells (the tumour) [1]. Examiner insight: A four-mark “explain” needs a sequence of linked points; stating “cells divide uncontrollably” once and then describing cancer symptoms scores only one mark.

7. (a) Cells in mitosis = 64 + 24 + 16 + 16 = 120 [1]; 120 / 800 × 100 = 15 % [1]. (b) 24 / 800 × 20 = 0.6 h [1]; 0.6 × 60 = 36 min [1]. (c) Cells are dividing at random times (not synchronised), so the proportion of cells in a stage equals the proportion of time spent in it [1]. Examiner insight: Allow error carried forward in (b) from a wrong total, but the answer must be in the unit asked for; leaving it as 0.6 h loses the final mark.

8. A: metaphase [1]. B: interphase [1]. C: telophase [1]. D: prophase [1]. Examiner insight: Give one stage name per cell; writing two stages (for example “prophase or metaphase”) is a contradiction and scores zero for that cell.

9. Plant cells have no centrioles; animal cells have centrioles at the poles [1]. Animal cells divide by the cell surface membrane pinching inwards [1]. In plants a cell plate forms from vesicles at the equator and a new cell wall is laid down between the daughter cells [1]. (Accept the new cell wall as a separate point in place of any one of these.) Examiner insight: Each difference must be comparative or clearly about one cell type; “plant cells have a cell wall” alone is a fact about structure, not about division, and scores nothing.

10. (a) 13.2 pg [1]. (b) The mass stays at 6.6 pg during G1 [1]. It doubles to 13.2 pg during S phase because DNA is replicated [1]. It stays at 13.2 pg through G2 and mitosis because the chromatids are still in one cell [1]. It halves to 6.6 pg at cytokinesis as the cytoplasm divides into two cells [1]. Examiner insight: Describe-and-explain questions need both the change and the reason; quoting the values 6.6 pg and 13.2 pg with units makes the description creditworthy.

11. (a) (125 − 25) / 25 × 100 [1] = 400 % [1]. (b) Spindle fibres attach to centromeres and shorten to pull sister chromatids apart [1]. Without a spindle, chromatids cannot separate / move to the poles [1], so cells stay at metaphase and cannot enter anaphase or telophase [1]. (c) Tumour cells divide by mitosis very often, so the drug stops many of them completing division and the tumour grows more slowly [1]. Bone marrow contains stem cells that divide frequently [1], so their mitosis is also blocked, reducing the supply of new blood cells [1]. Examiner insight: In (a), dividing by the new value (125) instead of the original (25) is a common method error and loses both marks; in (b), refer to chromatids, not chromosomes, being pulled apart.

12. (a) Anaphase [1]. (b) 24 chromatids [1]. (c) 12 chromosomes [1]. (d) 54 / 1500 = 0.036 mm [1]; × 1000 = 36 µm [1]. (e) The cell surface membrane pinches inwards around the middle of the cell [1] until the cytoplasm divides into two cells, each with one nucleus [1]. Examiner insight: In (d), one mark is usually for correct working and one for the final answer in the unit asked; an answer in mm when µm is requested loses the accuracy mark.

Where marks are usually lost

• Placing DNA replication in prophase or G2 instead of S phase.
• Writing “chromosomes are pulled to the poles” in anaphase instead of sister chromatids.
• Confusing centromere with centriole.
• Giving “it is mitosis” as the whole explanation of genetic identity.
• Leaving out “without control” or “repeatedly” when explaining tumour formation.
• Dividing by the wrong value in a percentage change.
• Forgetting to convert hours to minutes, or mm to µm.
• Stating a plant cell structure (cell wall) instead of a difference in the process of division.
• Describing a DNA mass graph without quoting values and units.

Next steps

• Go back over the mitotic cell cycle revision notes.
• Re-read any weak section in the mitotic cell cycle study guide.
• Revisit topic 4 with the cell membranes and transport study guide.
• See the 9700 course hub and the printable checklist.
• Take the free 10-minute 9700 AS diagnostic.
• Book a free trial class.

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus for 2025, 2026 and 2027 (Version 1), Cambridge International (Cambridge University Press & Assessment). Topic 5, The mitotic cell cycle, sub-topics 5.1 and 5.2.