The Cell Cycle and Mitosis — AQA A-level Biology

SPEC 3.2.2 - CELL DIVISION

The Cell Cycle, Mitosis & Cancer

Interphase, the four stages of mitosis, cancer biology, binary fission in prokaryotes, and Required Practical 2 — with full mitotic index calculation skills

The Cell Cycle

Definition

Cell cycle: The ordered sequence of events by which a eukaryotic cell grows and divides: interphase → mitosis → cytokinesis. Not all cells retain the ability to divide.

Key fact

DNA replication occurs during the S phase of interphase, not during mitosis. By the time mitosis begins, each chromosome consists of two identical chromatids joined at the centromere.

Interphase — Three Sub-Phases

Definition

Interphase: The stage of the cell cycle during which the cell grows, carries out normal metabolic activity, and replicates its DNA. Comprises three sub-phases: G₁, S, and G₂.

Sub-phaseFull nameKey events

G₁First gap (growth)Cell grows; organelles duplicated; proteins synthesised; cell prepares for DNA replication

SSynthesisDNA replication — each chromosome duplicated to form two identical sister chromatids joined at the centromere; histones also synthesised

G₂Second gap (growth)Cell continues to grow; centrioles replicate; final preparations for cell division; cell checks DNA integrity

Examiner Alert

The 2024 AQA Examiner Report noted that many students "did not appreciate that there are two stages in a cell cycle: mitosis and interphase." A common error is referring to only the stages of mitosis when asked about the cell cycle. Interphase takes up approximately 90% of the cell cycle duration.

Mitosis — Four Stages

PROPHASE

Chromosomes condense and become visible under a microscope. Spindle fibres form (from centrioles in animal cells; from a diffuse organising centre in plant cells). Nuclear envelope breaks down. Centrioles (where present) move to opposite poles.

METAPHASE

Chromosomes align along the cell equator (metaphase plate). Spindle fibres from each pole attach to centromeres of each chromosome. Chromosomes are maximally condensed here.

ANAPHASE

Centromeres split. Spindle fibres shorten, pulling chromatids (now called chromosomes) to opposite poles. Cell elongates. Each pole receives one copy of each chromosome.

TELOPHASE

Chromosomes arrive at poles and decondense. Nuclear envelopes reform around each set of chromosomes. Cytokinesis begins — cytoplasm divides to produce two daughter cells.

Critical Error — Anaphase

At anaphase, it is chromatids that separate — NOT chromosomes. Once the centromere splits, each chromatid becomes a chromosome. Writing “chromosomes separate at anaphase” is factually imprecise and loses marks. AQA mark schemes specifically look for “centromere splits” and “chromatids pulled to opposite poles.”

Mitosis — Visual Overview

Definition

Cytokinesis: The division of the cytoplasm that follows nuclear division (telophase), producing two separate daughter cells. In animal cells, the membrane pinches inward; in plant cells, a cell plate forms.

What Mitosis Produces & Why

Definition

Mitosis: A type of nuclear division that produces two genetically identical daughter cells, each with the same chromosome number as the parent cell (diploid, 2n).

Uses of mitosis

→ Growth of multicellular organisms

→ Repair of damaged tissues

→ Replacement of worn-out cells (e.g. skin, gut lining)

→ Asexual reproduction (e.g. vegetative propagation)

Key outcomes

Genetically identical: Both daughter cells contain identical copies of the parent cell’s DNA — because DNA replication in interphase (S phase) produces exact copies.

Diploid: Each daughter cell has the same chromosome number (2n) as the parent. Mitosis maintains the chromosome number.

Not meiosis: Mitosis does NOT produce haploid cells, does not involve homologous chromosome pairing, and does not produce genetic variation.

Synoptic Link → Lesson 4.04

Meiosis is covered in Lesson 4.04. The key distinction: mitosis produces 2 diploid identical cells; meiosis produces 4 haploid genetically different cells. AQA frequently tests whether students can correctly distinguish the two processes.

Cancer — Uncontrolled Cell Division

Definition

Tumour: A mass of abnormally dividing cells resulting from uncontrolled cell division. Caused by mutations in genes that control the cell cycle.

TypeBehaviourPrognosis

Benign tumourGrows but does NOT invade surrounding tissues; does not spread (no metastasis); contained within a capsuleUsually less serious; can often be removed surgically

Malignant tumour (cancer)Invades surrounding tissue; cells can break off and spread via blood or lymph ( metastasis ) to form secondary tumours elsewhereMore serious; harder to treat; may be fatal

Cancer treatments & the cell cycle

Many cancer treatments target the cell cycle to prevent uncontrolled division:

Chemotherapy — drugs that prevent spindle formation (blocking chromatid separation) or inhibit DNA replication; targets rapidly dividing cells

Radiotherapy — ionising radiation damages DNA, triggering cell death in rapidly dividing cells

Targeted therapies — drugs that block specific signalling molecules that drive uncontrolled division (covered in Lesson 8.04)

Required Practical 2 — Root Tip Squash

▶ Required Practical 2 — Mitosis in Root Tips

Aim: Prepare stained squashes of plant root tip cells; use an optical microscope to identify stages of mitosis; calculate a mitotic index.

VariableDetail

Independent variableStage of the cell cycle being identified (prophase, metaphase, anaphase, telophase, interphase)

Dependent variableNumber of cells in each stage; mitotic index (calculated)

Stain usedAceto-orcein or toluidine blue — stains chromosomes (and chromatids) visible because they are composed of DNA + histones; acidic stains bind basic histone proteins

Why root tip?Root tip meristem is a region of rapid, continuous mitotic cell division — high mitotic index; cells are small and accessible

Procedure

1. Cut ~1 cm from root tip → 2. Place in aceto-orcein for 5 min (stains chromosomes) → 3. Place on microscope slide → 4. Squash gently with coverslip (spreads cells to single layer) → 5. View under optical microscope → 6. Count cells in each stage

Mitotic Index — Calculation

Definition

Mitotic index (MI): The ratio of cells undergoing mitosis to the total number of cells observed. Expressed as a proportion (not a percentage).

Mitotic index = Number of cells in mitosis ÷ Total number of cells observed

Worked Example

Cells in mitosis = 32

Total cells = 200

MI = 32 ÷ 200

MI = 0.16

Interpreting the MI

High MI → rapid cell division (e.g. root tip meristem, tumour tissue, embryo, wound healing)

Low MI → slow or no division (e.g. mature nerve cells, muscle fibres, red blood cells)

2024 Examiner Report

Students who expressed mitotic index as a percentage (multiplied by 100) did not receive the mark when the question asked for a proportion. Always express MI as a decimal (e.g. 0.16) unless the question specifically asks for a percentage. Also: count cells with visible chromosomes — not individual chromosomes.

Binary Fission in Prokaryotes

Prokaryotes divide by binary fission — a simpler, faster process than mitosis. No spindle fibres, no condensed chromosomes, no nuclear envelope breakdown.

StepEventKey detail

1Circular DNA replicationThe single circular chromosome is replicated; both copies attach to the cell membrane

2Plasmid replicationPlasmids are also replicated; daughter cells receive a variable number of plasmid copies

3Cell elongationCell grows; the two DNA copies are pulled to opposite ends as the cell membrane grows between them

4Cytoplasm divisionNew cell wall and membrane form across the centre; two daughter cells produced, each with one circular DNA molecule

Never write

“Bacteria undergo mitosis” — mitosis requires a nucleus, spindle fibres, and condensed chromosomes. Prokaryotes have none of these. Always write binary fission.

Viruses — no cell division

Viruses do NOT undergo cell division of any kind. Being non-living, they inject their nucleic acid into a host cell, which then replicates the virus particles using host cell machinery.

Comparing Mitosis & Binary Fission

FeatureMitosis (eukaryotes)Binary fission (prokaryotes)

DNA typeLinear chromosomes associated with histonesCircular chromosome; not associated with histones

Spindle fibresYes — from centrioles; pull chromatids apartNo spindle; DNA separated by membrane growth

Nuclear envelopeBreaks down during prophase; reforms in telophaseNo nuclear envelope present

PlasmidsNot present (in most eukaryotes)Replicated; variable number in daughter cells

Daughter cells2 genetically identical diploid cells2 genetically identical cells, each with one circular DNA

SpeedHours (varies by cell type)As fast as 20 minutes (e.g. E. coli )

Genetic variation produced?No (barring mutation)No (barring mutation)

Exam Tip

“Compare” questions require explicit statements about both organisms per point. e.g. “Mitosis involves spindle fibres formed from centrioles; binary fission does not — DNA copies are separated by cell membrane growth.”

Identifying Mitosis Stages — How to Score

AQA questions provide microscope images (or diagrams) and ask you to identify the stage. Use these diagnostic features:

StageWhat you seeDiagnostic feature

InterphaseLarge nucleus visible; no individual chromosomes visible; chromatin is diffuseNo condensed chromosomes; intact nuclear envelope

ProphaseChromosomes begin to appear as dark threads; nuclear envelope gone or disappearingChromosomes visible but not yet aligned; no nuclear envelope

MetaphaseChromosomes clearly visible, aligned in a line across the middleChromosomes at equator; most condensed — easiest stage to count chromosomes

AnaphaseV-shaped chromosomes moving toward poles; cell elongatedChromosomes at both poles; cell appears stretched; V-shape visible

TelophaseTwo groups of chromosomes at poles; new nuclear envelopes forming; cell dividingTwo nuclei forming; constriction visible

Examiner Alert

Some students describe stages from meiosis (e.g. homologous chromosomes pairing, crossing over) when asked about mitosis. These features are absent in mitosis. In mitosis, homologous chromosomes do NOT pair up — each chromosome behaves independently.

DEEP Answer — Explaining Anaphase

“Describe and explain the events of anaphase in mitosis. [4 marks]”

DEEP Framework Answer

D

DefineAnaphase is the stage of mitosis during which sister chromatids are separated and pulled to opposite poles of the cell. [1]

E

Explain mechanismThe centromere of each chromosome splits. Spindle fibres, which are attached to the centromeres, shorten by depolymerisation of tubulin subunits. This pulls the chromatids (which are now chromosomes) toward opposite poles. The cell elongates as the spindle extends. [2]

E

Evidence/precisionEach pole receives one copy of every chromosome, ensuring that both daughter cells will have identical, complete sets of genetic information — the same as the parent cell. [1]

P

Practical linkIn a root tip squash (Required Practical 2) stained with aceto-orcein, anaphase cells appear V-shaped as the chromatids are pulled toward the poles — this distinctive shape allows identification of the stage under the light microscope.

Top Examiner Errors — Lesson 2.04

Error 1 — “Chromosomes separate at anaphase”

Must say chromatids separate at anaphase (once the centromere splits, each chromatid becomes a chromosome). “Chromosomes separate” loses the mark. AQA mark schemes require: “centromere splits” and “chromatids pulled to opposite poles.”

Error 2 — DNA replication during mitosis

DNA replication occurs during the S phase of interphase, NOT during mitosis. Many students incorrectly state that DNA is replicated during prophase or metaphase. By the time mitosis begins, each chromosome is already two chromatids.

Error 3 — Mitotic index expressed as percentage

When asked for the mitotic index as a proportion, do NOT multiply by 100. MI = cells in mitosis ÷ total cells. Express as a decimal (e.g. 0.16), not a percentage (16%), unless specifically asked.

Error 4 — Describing meiosis features in mitosis

Crossing over and homologous chromosome pairing occur in meiosis, not mitosis. In mitosis, chromosomes are never paired up. Confusing the two processes is heavily penalised in mark schemes.

Practice Question

A student examines a stained squash preparation of onion root tip cells under an optical microscope. They count 240 cells in total. Of these, 18 are in prophase, 12 are in metaphase, 6 are in anaphase and 4 are in telophase.

(a) Calculate the mitotic index for this preparation. Show your working. [2 marks]

(b) The student claims that “during anaphase, chromosomes separate and move to opposite poles.” Identify the error in this statement and correct it. [2 marks]

(c) Explain why the root tip was chosen for this practical and why the cells were stained with aceto-orcein. [3 marks]

D:

(a) Cells in mitosis = 18 + 12 + 6 + 4 = 40. Mitotic index = 40 ÷ 240 = 0.167 (accept 0.17). [2 — 1 for summing correctly, 1 for correct division and answer as a proportion not %]

E:

(b) Error: “chromosomes separate.” Correction: it is chromatids that separate at anaphase — the centromere splits and spindle fibres pull sister chromatids to opposite poles. Once separated, each chromatid is then called a chromosome. [2]

P:

(c) Root tip is chosen because the meristem region undergoes continuous rapid mitotic cell division, giving a high mitotic index — cells in all stages of mitosis are present. [1] Aceto-orcein is an acidic stain that stains chromosomes (DNA + histone proteins), making them visible under the light microscope. [1] Without staining, chromosomes cannot be distinguished from the rest of the cell contents. [1]

Summary

Stages of Mitosis — Quick Reference

StageKey eventsDiagnostic feature

ProphaseChromosomes condense; nuclear envelope breaks down; spindle formsChromosomes visible; no nuclear envelope

MetaphaseChromosomes align at equator; spindle fibres attach to centromeresChromosomes in line at centre

AnaphaseCentromere splits; chromatids pulled to poles by spindle fibresV-shaped chromatids; cell elongated

TelophaseNuclear envelopes reform; chromosomes decondense; cytokinesisTwo nuclei forming; cell dividing

Mitotic Index

MI = cells in mitosis ÷ total cells

Express as a decimal proportion. High MI = rapid division (tumour, meristem, embryo).

Binary Fission

Prokaryotes: circular DNA replicated → plasmids replicated (variable number) → cell elongates → cytoplasm divides → 2 daughter cells. No spindle, no condensed chromosomes.

Three Most Dropped Marks

(1) “Chromosomes separate” at anaphase — must say chromatids and centromere splits. (2) Stating DNA replication happens during mitosis — it happens during S phase of interphase. (3) Expressing MI as a percentage when a proportion is asked for.