IB DP Biology: Cell and nuclear division: mitosis, meiosis and the cell cycle (DP Biology) – Revision Notes

These are condensed revision notes for sub-topic D2.1, Cell and nuclear division, from the IB Biology guide, first assessment 2025. They cover D2.1.1 to D2.1.11 (SL and HL) and D2.1.12 to D2.1.17 (HL only). For full explanations read the study guide, then test yourself with the practice questions. Links: IB DP Biology hub, checklist, free 10-minute diagnostics.

Core ideas in one place

• A parent (mother) cell divides into two daughter cells in all living organisms.
• Eukaryotes divide the nucleus first, then the cytoplasm. Skipping the first step gives anucleate cells.
• Mitosis: same chromosome number, same genome. Meiosis: half the chromosome number, new combinations.
• DNA replication comes first in both. After it, each chromosome has two chromatids joined until anaphase.

Cytokinesis (D2.1.2 and D2.1.3)

	Animal cell	Plant cell
Mechanism	Ring of actin and myosin contracts and pinches the membrane	Vesicles fuse across the middle and build new membrane and wall
Direction	Pinching inward from the edge	Building outward from the centre
Shape change	A furrow forms	A cell plate forms (new wall between daughters)

Unequal cytokinesis. Usually near-equal, but not always. Oogenesis in humans: one large egg cell, other products tiny. Budding in yeast: a small bud and a larger parent.

Must be shared. Every daughter cell receives a mitochondrion at minimum, plus any further organelle that arises only by division of an existing one.

Condensation and movement (D2.1.5 and D2.1.6)

• Histones: DNA winds round them and is then supercoiled, so chromosomes shorten and thicken.
• Microtubules of the spindle pull chromosomes; microtubule motors use energy to move along them.
• Why: untangled, compact chromosomes can be moved without breaking.

Mitosis (D2.1.7 and D2.1.8)

Phase	Key identification clue	What is happening
Prophase	Visible condensed chromosomes, no envelope	Condensation; envelope breaks down; spindle forms
Metaphase	Chromosomes along the equator	Attached to spindle microtubules
Anaphase	Two sets moving apart	Centromeres split; chromatids pulled to poles
Telophase	Two clusters, envelopes forming	Chromosomes decondense; cytokinesis follows

Method for identifying phases in a micrograph.

1. Find the nuclear envelope. Present with scattered material means interphase.
2. If no envelope, find where the chromosomes are.
3. Random and condensed: prophase. At the equator: metaphase. Moving to poles: anaphase. Grouped at poles: telophase.
4. State your reason from the visible structure, not from guessing.

Mitotic index (HL only, D2.1.17) = cells in mitosis ÷ total cells counted. Example: 23 of 310 cells gives 0.0742, or 7.42%.

Meiosis (D2.1.9)

	Mitosis	Meiosis
Divisions	One	Two
Daughter nuclei	Two	Four
Chromosome number	Same as parent	Halved
Homologues pair up	No	Yes, as bivalents in meiosis I
Crossing over	No	Yes, in prophase I
Genetic result	Identical	Different from each other

• Diploid (2n): two sets of chromosomes. Haploid (n): one set.
• Meiosis I separates homologous chromosomes (reduction). Meiosis II separates sister chromatids.
• Needed in a sexual life cycle so that fertilisation restores the diploid number rather than doubling it each generation.

DNA tracker for a diploid cell with 2n = 4 and 4.0 au in G1.

Stage	Chromosomes per nucleus	DNA per nucleus
G1	4	4.0 au
After replication	4 (8 chromatids)	8.0 au
After meiosis I	2 (4 chromatids)	4.0 au
After meiosis II	2 (2 chromatids)	2.0 au

Non-disjunction and Down syndrome (D2.1.10)

• Non-disjunction: chromosomes or chromatids fail to separate in meiosis.
• Meiosis I: homologues stay together. Meiosis II: sister chromatids stay together.
• Gamete with 24 chromosomes + normal gamete with 23 = zygote with 47, three copies of chromosome 21.
• This is trisomy 21, or Down syndrome.

Variation from meiosis (D2.1.11)

• Crossing over in prophase I: chiasmata, exchange between non-sister chromatids.
• Random orientation of bivalents in metaphase I: 2ⁿ combinations, where n is the haploid number. For n = 6, 2⁶ = 64.
• Random fertilisation adds more variation, although it is not part of meiosis itself.

HL only: proliferation, cell cycle, interphase (D2.1.12 to D2.1.14)

Uses of proliferation: growth (plant meristems, early animal embryos), replacement (skin cells), repair (wound healing in skin).

Cell cycle order: G1, then S, then G2 (together interphase), then mitosis, then cytokinesis.

Stage	Main events
G1	Growth, protein synthesis, organelle numbers rise
S	DNA replication
G2	Growth, preparation for mitosis
M and cytokinesis	Nuclear division, then cytoplasm splits

Interphase is active. Biosynthesis of proteins and DNA. Mitochondria and chloroplasts increase by growth and division of existing ones.

HL only: control and cancer (D2.1.15 to D2.1.17)

• Cyclin concentrations rise and fall in a pattern through the cycle.
• A specific cyclin must reach a threshold to pass each checkpoint.
• Proto-oncogene mutated to oncogene: stimulates division when it should not.
• Tumour suppressor gene mutated: the brake is lost.

Term	Meaning
Benign tumour	Does not invade neighbouring tissue or spread; does not cause cancer
Malignant tumour	Invades and can spread; a cancer
Primary tumour	Where the cancer began
Secondary tumour	A new tumour formed from cells that have spread (metastasis)

Compare tumours by rate of division and growth, ability to invade neighbours and ability to metastasise.

Worked reminders

Non-disjunction outcomes. If one pair fails to separate in meiosis I, all four gametes are abnormal: two have an extra chromosome (n + 1) and two lack one (n - 1). If one chromosome’s chromatids fail to separate in meiosis II, two gametes are normal, one is n + 1 and one is n - 1. For a human, n + 1 = 24 and n - 1 = 22.

Mitotic index comparison (HL only). Sample A has 45 of 300 cells in mitosis and sample B has 9 of 300. A: 45 ÷ 300 = 15.0%. B: 9 ÷ 300 = 3.0%. A divides at five times the rate shown by B, judged by this snapshot. Always count enough cells, and the same number from each sample, for a fair comparison.

Embryo growth (HL only). A zygote that divides five times in a row, with every cell dividing each time, gives 2⁵ = 32 cells. Early divisions mostly add cells, and the cells get smaller because the embryo has not yet grown much overall.

Threshold reasoning (HL only). If a checkpoint needs 40 au of a cyclin and a table shows 25 au at 4 h and 42 au at 8 h, the cell passes between those times. Name the first reading at or above the threshold, and say that the pattern is a rise followed by a fall.

Must-know distinctions

• Cytokinesis is division of cytoplasm; mitosis and meiosis are division of the nucleus.
• Chromosome versus chromatid: after replication one chromosome has two chromatids.
• Meiosis I (homologues separate) versus meiosis II (chromatids separate).
• Crossing over (within a bivalent, prophase I) versus random orientation (between bivalents, metaphase I).
• Benign (stays put) versus malignant (invades).

Quick self-test

1. Name the two proteins in the contractile ring of an animal cell.
2. How does a plant cell split its cytoplasm?
3. Give one example of unequal cytokinesis.
4. A cell has 2n = 12. How many chromatids are present after replication?
5. In which phase of mitosis do centromeres split?
6. (HL only) 23 of 310 cells are in mitosis. Calculate the mitotic index as a percentage to 3 significant figures.
7. A species has 2n = 14. How many combinations arise from random orientation of bivalents?
8. A zygote has 47 chromosomes with three copies of chromosome 21. State the condition and its cause.
9. (HL only) Name the three stages of interphase, in order.
10. (HL only) What must happen to a cyclin concentration for a cell to pass a checkpoint?
11. (HL only) Distinguish a primary tumour from a secondary tumour.
12. (HL only) Which type of gene mutates to become an oncogene?

Answers

1. Actin and myosin.
2. Vesicles assemble new membrane and cell wall across the middle of the cell.
3. Oogenesis in humans, or budding in yeast.
4. 24 chromatids (12 chromosomes with two each).
5. Anaphase.
6. 23 ÷ 310 = 0.07419, so 7.42%.
7. n = 7, so 2⁷ = 128.
8. Down syndrome (trisomy 21), caused by non-disjunction in meiosis.
9. G1, S, G2.
10. It must reach the threshold concentration for that checkpoint.
11. A primary tumour is where the tumour began; a secondary tumour grows from cells that spread from it.
12. A proto-oncogene.

Where marks are usually lost

• Describing mitosis phases without naming what the chromosomes are doing.
• Saying “the cell splits in two” for cytokinesis without naming actin and myosin or vesicles and wall.
• Mixing up which meiotic division halves the chromosome number.
• Saying DNA halves in meiosis I. The chromosome number halves, but each chromosome still has two chromatids.
• Writing “chromosomes” instead of “chromatids” after replication.
• Stating random orientation without saying it happens in metaphase I and is independent between bivalents.
• Giving only “more variation” as the answer instead of the mechanism.
• Writing that all tumours are cancer.
• Stating the mitotic index as a count rather than a ratio or percentage.

Official syllabus

IB Biology guide, first assessment 2025, published by the International Baccalaureate Organization (sub-topic D2.1).