Cambridge International AS & A Level Biology 9700: The mitotic cell cycle – Revision Notes

These revision notes condense topic 5, The mitotic cell cycle, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. They cover 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 knowledge for Paper 4. For full explanations and worked examples, use the mitotic cell cycle study guide.

Then test yourself with the mitotic cell cycle practice questions. The 9700 course hub, the printable 9700 checklist and the free 10-minute 9700 AS diagnostic show where the topic sits.

5.1 Replication and division of nuclei and cells

Chromosome structure: the five syllabus terms

Term	One-line meaning
DNA	One long molecule per chromatid; carries the genes
Histone proteins	Proteins the DNA winds around; allow tight packing and condensing
Sister chromatids	Two identical copies of a chromosome made in S phase
Centromere	Region joining sister chromatids; spindle attachment point
Telomeres	Repeated non-coding sequences at the chromosome ends

Why genetically identical cells matter

Need	Example
Growth	More cells in a growing root tip or embryo
Replacement	New red blood cells; new skin surface cells
Repair	Cells dividing to close a wound
Asexual reproduction	Tubers, runners, bulbs; offspring identical to parent

Identical because: DNA copied exactly in S phase, then each daughter nucleus gets one chromatid of every chromosome.

The cycle in one table

Phase	Key event	DNA per cell
G1	Growth; protein and organelle synthesis	x
S	DNA replication	x → 2x
G2	Further growth; preparation for division	2x
Mitosis	Nuclear division	2x
Cytokinesis	Cytoplasm divides	x in each new cell

Telomeres in three steps

1. Each DNA replication cannot copy the very end of the molecule.
2. So a short piece is lost from each end every cycle.
3. The piece lost is telomere (non-coding), so genes are not lost.

Stem cells

• Definition: unspecialised cell that divides by mitosis and gives rise to cells that can differentiate.
• Role: replaces cells with a short lifespan (blood cells from bone marrow) and repairs damaged tissue (skin).
• One daughter can remain a stem cell, so the supply is not used up.

Tumours in a chain

Mutation in a gene controlling cell division → cell no longer stops dividing → repeated, uncontrolled mitosis → all daughter cells carry the mutation → mass of cells = tumour.

Worked reminder: reading a DNA mass graph

A graph shows DNA per cell of 8 pg, rising to 16 pg, then falling back to 8 pg.

• Flat at 8 pg: G1.
• Sloping rise from 8 pg to 16 pg: S phase, DNA replicating. The rise is gradual, not a vertical jump, because replication takes time.
• Flat at 16 pg: G2 and mitosis. DNA is still inside one cell, even after the chromatids separate in anaphase.
• Sudden drop to 8 pg: cytokinesis, when the cell splits.

Quote the values with units when you describe a graph like this.

5.2 Chromosome behaviour in mitosis

Stage checklist

Stage	Look for	Say in your answer
Prophase	Visible, condensing chromosomes	Chromosomes condense; two chromatids each; nucleolus disappears; spindle forms; nuclear envelope breaks down
Metaphase	Line across the middle	Chromosomes on the equator; spindle fibres attached at centromeres
Anaphase	V shapes moving apart	Centromeres divide; sister chromatids pulled to opposite poles, centromere first; spindle fibres shorten
Telophase	Two groups at poles	Chromosomes uncoil; nuclear envelopes re-form; spindle breaks down

Plant vs animal

	Animal	Plant
Centrioles	Yes	No
Spindle	Yes	Yes
Cytokinesis	Cell surface membrane pinches in	Cell plate from vesicles; new cell wall

Method in steps: “describe the behaviour of…” answers

The syllabus names four structures besides the chromosomes. Cover each one in order:

1. Chromosomes: condense, line up, chromatids separate, uncoil.
2. Nuclear envelope: breaks down at the end of prophase; re-forms in telophase.
3. Spindle: forms in prophase; attaches at centromeres in metaphase; fibres shorten in anaphase; breaks down in telophase.
4. Cell surface membrane: pinches in during cytokinesis in animal cells; in plant cells new membrane forms along the cell plate.

Name the stage for every event. An event without a stage rarely scores.

Method in steps: time spent in a stage

1. Count cells in each stage in one field of view (or several).
2. Proportion in stage = cells in stage / total cells counted.
3. Time in stage = proportion × length of the whole cycle.
4. Convert hours to minutes if the stage is short (× 60).
5. State the assumption: cells are dividing at random times (not in step).

Worked reminder

900 cells counted; 45 in prophase; the cycle lasts 16 h.

proportion = 45 / 900 = 0.05
time = 0.05 × 16 = 0.8 h = 48 min

Method in steps: chromosomes and chromatids

1. Chromosome number = number of centromeres.
2. After S phase (and up to anaphase), each chromosome = 2 chromatids.
3. In anaphase, once centromeres divide, each chromatid counts as a chromosome, so the cell briefly has double the chromosome number.
4. Each daughter cell ends with the same chromosome number as the parent.

Method in steps: actual size from a photomicrograph

1. Measure the image in mm with a ruler.
2. Actual size = image size / magnification.
3. Multiply mm by 1000 to give µm.

Command words for this topic

• State / Name: one word or phrase, such as “S phase” or “centromere”.
• Outline: the main points in order, with no detail beyond what is asked (for example, the cell cycle phases).
• Describe: what happens, stage by stage, with the names of the stages.
• Explain: give reasons, such as why daughter cells are identical or why a mutation leads to a tumour.
• Suggest: apply the ideas to an unfamiliar context, such as a drug that affects the spindle.
• Calculate: show the working and give the unit.

Must-know distinctions

• Chromosome vs chromatid: a chromatid is one of the two identical halves of a replicated chromosome.
• Mitosis vs cytokinesis: division of the nucleus vs division of the cytoplasm.
• Interphase vs mitosis: DNA is copied in interphase (S), not in mitosis.
• Metaphase vs anaphase: lined up at the equator vs chromatids moving apart.
• Centromere vs centriole: part of a chromosome vs an organelle that organises the spindle in animal cells.
• Telomere vs centromere: chromosome ends vs the joining point of sister chromatids.
• Stem cell vs specialised cell: can still divide and differentiate vs has already differentiated.

Quick self-test

1. Name the proteins that DNA is wound around in a chromosome.
2. In which phase of the cycle does DNA replicate?
3. Give two uses of mitosis in a multicellular organism.
4. A cell has 20 pg of DNA in G1. How much DNA does it have in G2?
5. A species has 24 chromosomes in each body cell. How many chromatids are there in a cell at metaphase?
6. How many chromosomes are in each daughter cell of the species in question 5?
7. State the stage in which sister chromatids separate.
8. Name one structure present in dividing animal cells but not in plant cells.
9. Explain how telomeres stop genes being lost.
10. 900 root tip cells are counted and 45 are in prophase. The cycle lasts 16 h. How long is prophase in minutes?
11. A cell is 18 mm long on a photomicrograph at × 600. Calculate its actual length in µm.
12. State why a mutation in one cell can lead to a tumour.

Answers

1. Histones (histone proteins).
2. S phase of interphase.
3. Any two: growth; replacement of damaged or dead cells; repair of tissues; asexual reproduction.
4. 40 pg.
5. 48 chromatids (24 chromosomes × 2).
6. 24.
7. Anaphase.
8. Centrioles.
9. A little DNA is lost from each end at every replication; that length comes from the non-coding telomere, not from genes.
10. 45 / 900 × 16 = 0.8 h = 48 min.
11. 18 / 600 = 0.03 mm = 30 µm.
12. The mutation can stop the cell responding to controls on division; the cell divides by mitosis without control, and every daughter cell inherits the mutation, forming a mass of cells.

Where marks are usually lost

• Writing “DNA replicates in prophase”. It happens in S phase.
• Giving “chromosomes separate” for anaphase instead of “sister chromatids separate, centromeres divide”.
• Missing the nuclear envelope or the spindle when asked for the behaviour of other structures, not only the chromosomes.
• Forgetting that plant cells lack centrioles, or saying plant cells do not form a spindle.
• Describing plant cytokinesis as “the membrane pinches in”.
• Saying telomeres “protect DNA from damage” without linking it to replication and the loss of genes.
• Defining a stem cell as “a cell that can become any cell” with no mention of mitosis or differentiation.
• Omitting the assumption (random, unsynchronised division) when calculating time from counts.
• Forgetting to convert mm to µm in actual size calculations.

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.