3.2Topic 3 · Voice of the Genome

Cell Division — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

20 min read 18 parts Lesson 10 of 26 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision

0%read

Lesson summary

Cell Division Edexcel A Level Biology SNAB notes: Describe the cell cycle, including DNA replication in interphase, and the stages of mitosis.Mitotic index = number of cells in mitosis ÷ total number of cells observed (no units).Number of possible chromosome combinations from ind…

Human body cells
Meiosis II
Independent assortment
Cell cycle control
G2 preparations
Chromosomes vs DNA molecules

Part 1 of 8Key ideas

Mitotic index = number of cells in mitosis ÷ total number of cells observed (no units).

Number of possible chromosome combinations from independent assortment = 2ⁿ, where n = number of homologous pairs (humans: 2²³ ≈ 8.4 million).

Combinations at fertilisation from random fusion (ignoring crossing over) = (2ⁿ)².

4Human body cells

2n = 46 (23 pairs); human gametes: n = 23.

DNA content doubles in S phase of interphase (e.g. from 2x to 4x) and halves at each nuclear division in meiosis (4x → 2x → x).

Time in a stage ≈ (cells in that stage ÷ total cells) × length of cell cycle.

Magnification = image size ÷ actual size (no units); 1 mm = 1000 µm.

Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids.

Part 2 of 8Learning objectives

Describe the cell cycle, including DNA replication in interphase, and the stages of mitosis.

Explain the importance of mitosis in growth, repair and asexual reproduction.

Describe the role of meiosis in producing haploid gametes and explain how it creates genetic variation.

Prepare and stain a root tip squash to observe mitosis and calculate a mitotic index.

Part 3 of 8Essential knowledge

In G1 the cell grows and makes proteins and organelles; in S phase DNA replicates semi-conservatively so each chromosome becomes two sister chromatids; in G2 the cell continues to grow, makes proteins needed for division and checks the replicated DNA.

Prophase

Prophase chromatin supercoils so chromosomes shorten and thicken; centrioles (animal cells) move to opposite poles and form the spindle; the nuclear envelope and nucleolus disappear.

Metaphase

Metaphase chromosomes are attached to spindle fibres by their centromeres and line up across the equator of the cell.

Anaphase

Anaphase centromeres divide and spindle fibres contract, pulling the sister chromatids apart to opposite poles, so each pole receives an identical set.

Telophase

Telophase chromosomes arrive at poles and uncoil; nuclear envelopes reform; cytokinesis then divides the cytoplasm (cleavage furrow in animals, cell plate/new cell wall in plants).

Mitosis produces genetically identical cells because DNA is replicated accurately and one copy of each chromatid goes to each daughter cell; this allows growth of multicellular organisms, repair/replacement of damaged tissue and asexual reproduction (e.g. runners, bulbs, yeast budding).

Meiosis I

Meiosis I homologous chromosomes pair up to form bivalents; crossing over occurs at chiasmata; homologous pairs line up randomly on the equator and separate, halving chromosome number.

Meiosis II

Meiosis II sister chromatids (no longer identical because of crossing over) are separated as in mitosis, giving four haploid nuclei.

Independent assortment

Independent assortment the orientation of each bivalent is random and independent of other pairs, so each gamete receives a random mix of maternal and paternal chromosomes.

Crossing over swaps alleles between non-sister chromatids, producing new combinations of alleles on the same chromosome (recombinants); this breaks up linked alleles.

Haploid gametes mean the diploid number is restored at fertilisation, keeping chromosome number constant between generations; random fertilisation adds further variation.

In the root tip practical, HCl stops division and separates cells; the stain makes chromosomes visible; squashing makes a thin layer so light passes through and individual cells can be seen.

Cell cycle control

Progress from one phase of the cell cycle to the next is regulated by proteins called cyclins, whose levels rise and fall to trigger each step. Cycle length varies with cell type, organism and conditions, e.g. gut lining cells divide far more often than many root cells.

G2 preparations

In G2 the cell makes proteins needed for division, such as tubulin for the spindle microtubules, and the centrosomes (centriole pairs in animal cells) are duplicated so one can move to each pole in prophase.

Chromosomes vs DNA molecules

After S phase a human cell still has 46 chromosomes, but each is two sister chromatids, so there are 92 DNA molecules. When chromatids separate in anaphase, each daughter nucleus receives 46 single-chromatid chromosomes (46 DNA molecules).

Part 4 of 8Process & sequence

Core practical: cut 5 mm of a root tip (e.g. garlic or onion) where the meristem is dividing.

Place in 1 mol dm^-3 hydrochloric acid at about 60 °C for ~5 minutes to break down the middle lamella so cells separate; rinse in water.

Put the tip on a slide, add a stain that binds to DNA (e.g. toluidine blue or acetic orcein) and leave for a few minutes.

Break the tissue up with a mounted needle, add a coverslip and squash firmly but gently (without sideways movement) to give a single layer of cells.

Observe under a light microscope at low then high power (×400); identify cells in each stage of mitosis.

Count cells in mitosis and total cells in several fields of view and calculate the mitotic index = cells in mitosis ÷ total cells.

Compare regions (e.g. tip vs further back) or treatments; repeat with several root tips for reliability.

Sequence of mitosiswrite these steps in this order; each step is a marking point.

Interphase: DNA replicates, each chromosome becomes two sister chromatids

Prophase: chromosomes condense, spindle forms, nuclear envelope breaks down

Metaphase: chromosomes line up on equator attached to spindle by centromeres

Anaphase: centromeres split, sister chromatids pulled to opposite poles

Telophase: nuclear envelopes reform around each set of chromosomes

Cytokinesis: cytoplasm divides to give two identical daughter cells

Part 5 of 8Worked examples

Worked examples

A root tip squash showed 42 cells in mitosis out of 350 cells counted. Calculate the mitotic index.

Mitotic index = cells in mitosis ÷ total = 42 ÷ 350 = 0.12 (or 12%). No units.

In a sample of 500 cells, 20 were in metaphase. The cell cycle lasts 24 hours. Estimate the time spent in metaphase in minutes.

Proportion = 20 ÷ 500 = 0.04. Time = 0.04 × 24 h = 0.96 h = 0.96 × 60 = 57.6 min ≈ 58 minutes (2 s.f.).

A species has 2n = 8. How many different chromosome combinations can occur in its gametes as a result of independent assortment alone?

n = 4 homologous pairs. Combinations = 2⁴ = 16.

Part 6 of 8Summary

The cell cycle is interphase (G1, S, G2) then mitosis and cytokinesis.

Mitosis gives two genetically identical diploid cells for growth, repair and asexual reproduction.

Meiosis gives four genetically different haploid cells for sexual reproduction.

Variation in meiosis comes from independent assortment and crossing over.

Root tip squash: mitotic index = cells in mitosis ÷ total cells.

Part 7 of 8Keep going

Cell Division Edexcel A Level Biology SNAB: what to do next

These Cell Division Edexcel A Level Biology SNAB notes covered the key ideas, worked examples and exam technique for this topic. Check your Cell Division Edexcel A Level Biology SNAB understanding with the practice questions below, then reread any Cell Division Edexcel A Level Biology SNAB point you were unsure about before moving on.

Cell Division Edexcel A Level Biology SNAB: revision checklist

Revise the main Cell Division Edexcel A Level Biology SNAB definitions until you can write them from memory.

Practise Cell Division Edexcel A Level Biology SNAB exam questions under timed conditions.

Link each Cell Division Edexcel A Level Biology SNAB idea to a diagram or a worked example.

List the common Cell Division Edexcel A Level Biology SNAB mistakes and how to avoid them.

Explain one Cell Division Edexcel A Level Biology SNAB idea out loud as if teaching a friend.

Summarise Cell Division Edexcel A Level Biology SNAB in five key points on a revision card.

Practice: Cell Division Edexcel A Level Biology SNAB practice questions

Previous lesson: 3.1 Cell Structure & Organisation — 9BI0 Notes

Next lesson: 3.3 Reproduction & Inheritance — 9BI0 Notes

Course: Edexcel A Level Biology A (snab) 9BI₀ — all units

Official syllabus: Pearson Edexcel A Levels

Part 8 of 8Compare: Compare

Mitosis compared with meiosis
Feature	Mitosis	Meiosis
Number of divisions	One	Two (meiosis I and II)
Daughter cells	Two	Four
Chromosome number	Same as parent (2n → 2n)	Halved (2n → n)
Genetic identity	Identical to each other and parent	Genetically different
Homologous pairs associate	No	Yes, pair up in prophase I
Crossing over	No	Yes, prophase I
Role	Growth, repair, asexual reproduction	Gamete formation, genetic variation

Stages of mitosis
Stage	Main events
Prophase	Chromosomes condense and become visible as two sister chromatids; centrioles move to poles and spindle forms; nuclear envelope breaks down; nucleolus disappears
Metaphase	Chromosomes line up on the equator; spindle fibres attach to centromeres
Anaphase	Centromeres split; spindle fibres shorten and pull sister chromatids to opposite poles, centromere first
Telophase	Chromatids (now chromosomes) reach poles and uncoil; nuclear envelopes reform around each set; nucleolus reappears

Cytokinesis usually begins in late telophase.

Key terms

Command terms

Give / State / Name

Recall a fact; one idea per mark.

Define

Give the exact meaning using the specification keywords.

Describe

Give an account of what happens / the pattern in data, quoting values; no reasons needed.

Explain

Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.

Suggest

Use knowledge in a new context; more than one answer may be accepted.

Compare

Similarities and differences; each point must refer to both items.

Calculate

Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.

Discuss

Explore the issue from different angles and link the points.

Evaluate / Assess

Weigh strengths and weaknesses and reach a supported judgement.

Deduce

Draw a conclusion from the information given.

Justify

Support a statement/choice with evidence.

Exam technique

Keywords the mark scheme looks for in this lesson

Cell cycle · Interphase · Mitosis · Meiosis · Chromatid · Centromere · Homologous chromosomes · Diploid (2n) · Haploid (n) · Cytokinesis · Crossing over · Independent assortment · Mitotic index · Asexual reproduction

Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

Use specification keywords exactly (e.g. enzyme–substrate complex, active site, complementary).

2Link ideas

Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.

3Use the information in the question

“discuss/evaluate” answers must use the data provided and match the number of marks.

Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.

5Avoid vague words

“it”, “stuff”, “goes up”, “energy is made”. Name the substance and the process.

Extended (6-mark) responses are marked on content AND a logical, linked line of reasoning.

Words to avoid

chromosomes separate in anaphase of mitosissister chromatids separate and move to opposite poles

DNA replicates during mitosisDNA replicates during S phase of interphase, before mitosis

meiosis makes identical cellsmeiosis produces four genetically different haploid cells

crossing over happens between sister chromatidscrossing over happens between non-sister chromatids of homologous chromosomes

the acid kills the cells so they can be seenthe acid breaks down the middle lamella so cells separate and can be squashed into a single layer

spindle fibres attach to the chromosome endsspindle fibres attach to the centromere

Last updated: 18 October 2026

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.