Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

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
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
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

Section 1 of 18Cell Division

1
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.
2
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.
3
Metaphase
Metaphase chromosomes are attached to spindle fibres by their centromeres and line up across the equator of the cell.
4
Anaphase
Anaphase centromeres divide and spindle fibres contract, pulling the sister chromatids apart to opposite poles, so each pole receives an identical set.
5
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).
6
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).
7
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.
8
Meiosis II
Meiosis II sister chromatids (no longer identical because of crossing over) are separated as in mitosis, giving four haploid nuclei.
9
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.
10
Crossing over swaps alleles between non-sister chromatids, producing new combinations of alleles on the same chromosome (recombinants); this breaks up linked alleles.
11
Haploid gametes mean the diploid number is restored at fertilisation, keeping chromosome number constant between generations; random fertilisation adds further variation.
12
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.
13
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.
14
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.
15
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).

Section 2 of 18Process & sequence

1
Core practical: cut 5 mm of a root tip (e.g. garlic or onion) where the meristem is dividing.
2
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.
3
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.
4
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.
5
Observe under a light microscope at low then high power (×400); identify cells in each stage of mitosis.
6
Count cells in mitosis and total cells in several fields of view and calculate the mitotic index = cells in mitosis ÷ total cells.
7
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.

1
Interphase: DNA replicates, each chromosome becomes two sister chromatids
2
Prophase: chromosomes condense, spindle forms, nuclear envelope breaks down
3
Metaphase: chromosomes line up on equator attached to spindle by centromeres
4
Anaphase: centromeres split, sister chromatids pulled to opposite poles
5
Telophase: nuclear envelopes reform around each set of chromosomes
6
Cytokinesis: cytoplasm divides to give two identical daughter cells

Section 3 of 18Compare

Mitosis compared with meiosis
FeatureMitosisMeiosis
Number of divisionsOneTwo (meiosis I and II)
Daughter cellsTwoFour
Chromosome numberSame as parent (2n → 2n)Halved (2n → n)
Genetic identityIdentical to each other and parentGenetically different
Homologous pairs associateNoYes, pair up in prophase I
Crossing overNoYes, prophase I
RoleGrowth, repair, asexual reproductionGamete formation, genetic variation
Stages of mitosis
StageMain events
ProphaseChromosomes condense and become visible as two sister chromatids; centrioles move to poles and spindle forms; nuclear envelope breaks down; nucleolus disappears
MetaphaseChromosomes line up on the equator; spindle fibres attach to centromeres
AnaphaseCentromeres split; spindle fibres shorten and pull sister chromatids to opposite poles, centromere first
TelophaseChromatids (now chromosomes) reach poles and uncoil; nuclear envelopes reform around each set; nucleolus reappears

Cytokinesis usually begins in late telophase.

Section 4 of 18Worked 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.

Section 5 of 18Keep 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

1
Revise the main Cell Division Edexcel A Level Biology SNAB definitions until you can write them from memory.
2
Practise Cell Division Edexcel A Level Biology SNAB exam questions under timed conditions.
3
Link each Cell Division Edexcel A Level Biology SNAB idea to a diagram or a worked example.
4
List the common Cell Division Edexcel A Level Biology SNAB mistakes and how to avoid them.
5
Explain one Cell Division Edexcel A Level Biology SNAB idea out loud as if teaching a friend.
6
Summarise Cell Division Edexcel A Level Biology SNAB in five key points on a revision card.
1
Practice: Cell Division Edexcel A Level Biology SNAB practice questions
2
Previous lesson: 3.1 Cell Structure & Organisation — 9BI0 Notes
3
Next lesson: 3.3 Reproduction & Inheritance — 9BI0 Notes
4
Course: Edexcel A Level Biology A (snab) 9BI0 — all units
5
Official syllabus: Pearson Edexcel A Levels

Section 6 of 18The cell cycle

Mitosis is part of the regulated sequence of events, the cell cycle, between one cell division and the next. Its length varies with environmental conditions, cell type and organism. The cycle consists of interphase (a period of growth and DNA replication), mitosis (nuclear division) and cytokinesis (cytoplasm division).

1G1 phase
G1 phase cell growth, protein synthesis (including transcription of mRNA) and organelle production.
  • A signal to divide again is received at some point during G1.
2S phase
DNA replicates, so each chromosome consists of two identical sister chromatids.
3G2 phase
G2 phase further growth, the cell checks the replicated DNA and prepares for division.

An exam question might ask during which stage a cell would be producing the most mRNA; G1 (and G2) when protein synthesis is occurring.

Section 7 of 18Stages of mitosis

Mitosis produces two genetically identical daughter nuclei that are also identical to the parent nucleus, with the same number of chromosomes. It is one continuous process but is divided into four stages. (Diagrams often use four chromosomes for simplicity; humans have 46.)

1Prophase
Prophase chromosomes condense and become visible, each made of two sister chromatids joined at a centromere; the nucleolus disappears and the nuclear envelope breaks down; the centrosomes move to opposite poles and spindle fibres (protein microtubules) extend from them.
2Metaphase
Metaphase chromosomes line up at the equator of the spindle (the metaphase plate), equidistant from the two centrosome poles, attached by centromeres to spindle fibres.
3Anaphase
Anaphase the centromere divides and sister chromatids separate; spindle fibres shorten and pull the chromatids to opposite poles.
4Telophase
Telophase chromosomes arrive at opposite poles and begin to decondense; nuclear envelopes re-form around each set; the cell begins to split into two.

Cytokinesis is often mistaken for a stage of mitosis, but it is a separate part of the cell cycle.

Section 8 of 18Counting DNA molecules and why mitosis matters

In a human diploid cell with 46 chromosomes (46 DNA molecules) in G1, replication in S phase gives 92 DNA molecules (46 chromosomes, each of two sister chromatids). After mitosis each daughter nucleus has 46 again. Only human diploid cells have 46 chromosomes; use the organism's own diploid number in other questions.

Importance of mitosis

  • the two daughter cells are genetically identical (clones) with the same chromosome number as the parent;
    • it allows a unicellular zygote to grow into a multicellular organism;
    • it replaces damaged and worn-out cells and repairs tissues;
    • and it is the basis of asexual reproduction in many organisms.

Section 9 of 18Interpreting data on the cell cycle

A graph of DNA content against time shows the amount doubling during S phase, staying constant through G2 and falling back to the original level at the end of mitosis when the cell divides.

Interpreting data on the cell cycle
If the proportion of cells in each stage is counted in a tissue sample, the stage that lasts longest has the highest proportion of cells.
For example, in a root tip most cells are in interphase because it is far longer than mitosis.

Section 10 of 18Worked examples

  1. State what happens to the chromosomes in anaphase. [2]2 marks

  2. A human cell has 46 chromosomes. How many DNA molecules are present at the start of mitosis? Explain. [2]2 marks

  3. Explain why cytokinesis is not part of mitosis. [1]1 mark

Section 11 of 18Why use root tips

Growth in plants occurs in specific regions called meristems, in which cells divide by mitosis. The root tip meristem can be used to study mitosis because many cells are dividing. Garlic or onion (Allium cepa) root tips are most commonly used; bulbs can be encouraged to grow roots by suspending them over water for a week or two.

Section 12 of 18Preparing a root tip squash

1
Prepare a boiling tube of 1 M hydrochloric acid and place it in a water bath at 60 °C for about 10 minutes.
2
Cut off the tip of the root (about 1 cm) and place it in the warmed hydrochloric acid for about 5 minutes.
  • The acid breaks down the middle lamella that glues cells together and softens the tissue (and stops the cells at that stage: it is a fixative).
3
Rinse the root tip with water, place it on a slide and cut the very end (the actively dividing zone, about 2 mm) to use.
4
Add a stain that binds to DNA, such as acetic orcein or toluidine blue, and leave for a few minutes.
5
Cover with a coverslip, place a paper towel on top and gently press (squash) the tissue so that the cells spread into a single layer.
  • Avoid sideways movement.
6
View under a light microscope, starting on low power then high power,
  • and identify cells in the stages of mitosis: prophase, metaphase, anaphase and telophase,
  • as well as non-dividing cells in interphase.

Safety

  • hydrochloric acid is corrosive and stains can be irritants;
    • wear eye protection and gloves.

Section 13 of 18Identifying stages and the mitotic index

In prophase the chromosomes are condensed and visible; in metaphase they line at the equator; in anaphase the chromatids are pulled apart in two groups; in telophase two groups of chromosomes are at the poles. Cells in interphase have a visible nucleus with no visible individual chromosomes.

The mitotic index is the proportion of cells in a sample or tissue that are undergoing mitosis: mitotic index = number of cells with visible chromosomes / total number of cells. Multiply by 100 for a percentage. A high mitotic index suggests rapid growth or, in tissues such as tumours, uncontrolled division.

Worked example 1: 42 cells counted, 32 with visible chromosomes: 32 / 42 = 0.76.

Worked example 2: in an onion root tip micrograph, 20 cells (green) have visible chromosomes and 55 do not, so the total is 20 + 55 = 75 and the mitotic index is 20 / 75 = 0.27.

Section 14 of 18Sources of error in the practical

The tissue must be a single layer, or chromosomes overlap and cannot be counted; you must count enough fields of view (at least 100 to 200 cells) for a representative mitotic index; the stage of cell cycle at which the roots were cut affects the result; and counting is subjective when judging borderline stages. Because the cells are killed and fixed by the acid, you see only a snapshot of the continuous process, and the relative number of cells in each stage reflects how long each stage lasts.

Section 15 of 18Worked examples

  1. Calculate the mitotic index of a sample with 18 dividing cells in 90 cells. Give as a percentage. [2]2 marks

  2. Explain why the root tip is warmed in hydrochloric acid. [2]2 marks

  3. Why is a squash rather than a thick section used? [1]1 mark

Section 16 of 18Key terms

TermMeaning
Cell cycleRegulated sequence of events from one cell division to the next.
InterphaseG1, S and G2 phases between divisions.
Sister chromatidsTwo identical copies of a chromosome joined at a centromere.
Spindle fibresProtein microtubules that move chromosomes in mitosis.
Metaphase plateEquator of the cell where chromosomes line up.
DiploidHaving two sets of chromosomes.
TermMeaning
MeristemRegion of plant tissue where cells divide by mitosis.
Mitotic indexFraction of cells in a sample that are in mitosis.
SquashPreparation in which tissue is pressed to a single cell layer.
OrceinStain that binds to chromosomes.

Section 17 of 18Command 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.

Section 18 of 18Exam technique

1
Saying chromosomes replicate in prophase — it happens in S phase.
2
Calling cytokinesis a stage of mitosis.
3
Saying sister chromatids separate in metaphase — in anaphase.
4
Saying interphase is a resting phase.
Avoid these mistakes
Counting only one stage of mitosis for the numerator — all cells with visible chromosomes are counted.
Using the number of non-dividing cells as the total.
Using too large a root section so cells overlap.

Last updated: