Cambridge International AS & A Level Biology 9700: The mitotic cell cycle – Study Guide

This study guide teaches topic 5, The mitotic cell cycle, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers sub-topics 5.1 (Replication and division of nuclei and cells) and 5.2 (Chromosome behaviour in mitosis). All of it is AS Level content, so it is tested on Paper 1 (multiple choice) and Paper 2 (AS Level structured questions), and Paper 4 can draw on it because A Level questions require AS knowledge. Outcome 5.2.2 asks you to interpret microscope slides, a skill that also belongs to the practical work assessed in Paper 3.

Use it with the 9700 course hub and the printable 9700 checklist. Then condense it with the mitotic cell cycle revision notes, test yourself with the mitotic cell cycle practice questions, and take the free 10-minute 9700 AS diagnostic.

What this topic covers

9700 outcome	What you must be able to do
5.1.1	Describe chromosome structure: DNA, histone proteins, sister chromatids, centromere, telomeres
5.1.2	Explain why mitosis matters for growth, replacement, repair and asexual reproduction
5.1.3	Outline the cell cycle: interphase (G1, S, G2), mitosis, cytokinesis
5.1.4	Outline how telomeres prevent loss of genes during DNA replication
5.1.5	Outline the role of stem cells in cell replacement and tissue repair
5.1.6	Explain how uncontrolled cell division can form a tumour
5.2.1	Describe chromosome, nuclear envelope, cell surface membrane and spindle behaviour in plant and animal cells, naming prophase, metaphase, anaphase and telophase
5.2.2	Interpret photomicrographs, diagrams and slides and identify the stages of mitosis

This topic uses the nucleus, centrioles and microtubules from Cell structure, and it sets up DNA replication in topic 6 (nucleic acids and protein synthesis). The cell surface membrane you meet in 5.2 is the one described in the cell membranes and transport study guide.

5.1 Replication and division of nuclei and cells

The structure of a chromosome

The syllabus limits you to five features.

• DNA: each chromatid contains one very long DNA molecule.
• Histone proteins: the DNA is wound around histones. This packs the DNA and, during division, lets the chromosome condense into a short, thick structure that can be moved without tangling.
• Sister chromatids: after DNA replication, a chromosome consists of two identical copies, the sister chromatids.
• Centromere: the region that holds the two sister chromatids together. Spindle microtubules attach here.
• Telomeres: repeated, non-coding base sequences at each end of the chromosome.

A chromosome in G1 is a single chromatid. It becomes two sister chromatids only after the S phase. Keep this in mind when you count chromosomes and chromatids.

Why mitosis matters

Mitosis produces two daughter nuclei that are genetically identical to each other and to the parent nucleus. This happens because every DNA molecule is copied exactly before division, and in anaphase each daughter nucleus receives one chromatid of every chromosome. Genetically identical cells are needed for:

• growth of multicellular organisms, such as the increase in cell number at a root tip;
• replacement of damaged or dead cells, such as red blood cells and cells lost from the skin surface;
• repair of tissues, such as new cells closing a wound;
• asexual reproduction, such as a potato tuber or a strawberry runner producing new plants identical to the parent.

The new cells carry the same genes as the cells they replace, so they can do the same job.

The mitotic cell cycle

Stage	What happens
G1 (interphase)	Cell grows; proteins and organelles are made
S (interphase)	DNA replicates, so each chromosome becomes two sister chromatids
G2 (interphase)	Further growth; the cell prepares for division
Mitosis	The nucleus divides into two genetically identical nuclei
Cytokinesis	The cytoplasm divides, forming two cells

Interphase is usually by far the longest stage, and it is not a resting phase: the cell is growing and copying its DNA.

Worked example 1: DNA content through one cycle. A cell contains 16 pg of DNA in G1.

G1: 16 pg
S phase: rises steadily from 16 pg to 32 pg as DNA replicates
G2 and mitosis: 32 pg (DNA doubled, still one cell)
after cytokinesis: each daughter cell 16 pg

On a graph of DNA per cell against time, the line rises during S, stays level through G2 and mitosis, and halves at cytokinesis.

Telomeres

When DNA is replicated, the very end of each DNA molecule cannot be fully copied, so a short length is lost from each end in every cycle. Telomeres are long repeated sequences that do not code for anything. The length lost comes from the telomere, not from a gene near the end of the chromosome. Telomeres therefore prevent the loss of genes from the ends of chromosomes during DNA replication. In most body cells telomeres get shorter each time the cell divides.

Stem cells

A stem cell is an unspecialised cell that can divide by mitosis again and again, and whose daughter cells can differentiate into specialised cells. Stem cells are how the body replaces cells and repairs tissues.

• Stem cells in bone marrow divide by mitosis to replace red and white blood cells, which have limited lifespans.
• Stem cells in the lower layer of the skin divide to replace cells lost from the surface and to repair a cut.

One daughter cell can stay as a stem cell while the other differentiates. This keeps the stem cell population going.

How a tumour forms

The cell cycle is controlled by genes. A mutation in a gene that controls cell division can mean the cell no longer responds to the signals that normally stop it dividing. The cell then divides by mitosis repeatedly and without control. All its descendants carry the same mutation and also divide uncontrollably. The result is a mass of cells called a tumour. Factors that increase the chance of mutation, such as some chemicals and ionising radiation, increase the risk. A tumour that invades other tissues is a cancer.

5.2 Chromosome behaviour in mitosis

The four stages

Stage	Chromosomes	Nuclear envelope and spindle
Prophase	Condense and become visible; each is two sister chromatids joined at the centromere	Nucleolus disappears; in animal cells the centrioles move to opposite poles; the spindle forms; the nuclear envelope breaks down at the end of prophase
Metaphase	Line up at the equator of the spindle	Spindle microtubules attach to each centromere
Anaphase	Centromeres divide; sister chromatids are pulled to opposite poles, centromere first	Spindle microtubules shorten
Telophase	Chromatids (now chromosomes) reach the poles and uncoil	A nuclear envelope forms around each set; the spindle breaks down; nucleoli reappear

Cytokinesis usually follows telophase.

• In animal cells, the cell surface membrane pinches inwards around the middle until the cell splits in two.
• In plant cells, the cell wall prevents pinching. Vesicles collect at the equator and fuse to form a cell plate, which becomes the new cell surface membranes and a new cell wall between the two cells.

Plant and animal cells compared

Feature	Animal cell	Plant cell
Centrioles	Present; organise the spindle	Absent; the spindle still forms
Cytokinesis	Cell surface membrane pinches in	Cell plate forms; new cell wall laid down
Shape during division	Rounds up	Stays roughly the same shape inside the wall

Identifying stages on photomicrographs

Look for one feature at a time.

1. Is there a nuclear envelope with no visible chromosomes? That is interphase.
2. Are chromosomes visible but scattered, with a nuclear outline still just visible? Prophase.
3. Are chromosomes in a line across the middle? Metaphase.
4. Are two groups of V-shaped chromatids moving apart? Anaphase.
5. Are two groups at the poles, perhaps with a new membrane or cell plate between them? Telophase.

Root tips (for example, onion or garlic) are used for slides because cells there divide often.

Worked example 2: time spent in each stage. A student counts 500 cells in a root tip slide. The cell cycle takes 24 hours.

Stage	Cells	Proportion	Time
Interphase	440	88 %	21.1 h
Prophase	30	6.0 %	1.44 h (86 min)
Metaphase	12	2.4 %	0.576 h (35 min)
Anaphase	8	1.6 %	0.384 h (23 min)
Telophase	10	2.0 %	0.48 h (29 min)

For prophase: 30 / 500 × 24 = 1.44 h, and 1.44 × 60 = 86.4 min. The method assumes that cells are dividing at random times, so the fraction of cells in a stage equals the fraction of the cycle spent in it. Here 60 of 500 cells, or 12 %, are in mitosis.

Worked example 3: actual size. A cell in prophase measures 42 mm across on a photomicrograph at × 1500.

actual size = image size / magnification
            = 42 mm / 1500 = 0.028 mm = 28 µm

Convert mm to µm by multiplying by 1000.

Common errors

• Saying DNA replicates “during mitosis” or “in prophase”. It replicates in the S phase of interphase.
• Calling interphase a resting stage.
• Saying chromosomes line up in anaphase, or that chromatids separate in metaphase.
• Saying “chromosomes are pulled apart” in anaphase when you mean sister chromatids.
• Forgetting that plant cells have no centrioles but still form a spindle.
• Describing cytokinesis in a plant cell as the membrane pinching in.
• Saying telomeres stop DNA from being lost. DNA is lost; telomeres stop genes being lost.
• Defining a tumour as “a cancer” without saying cells divide by mitosis without control.
• Counting chromatids instead of chromosomes (or the reverse) after S phase.

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.