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

This guide teaches sub-topic D2.1, Cell and nuclear division, from the IB Biology guide, first assessment 2025. It covers statements D2.1.1 to D2.1.11 for SL and HL, and D2.1.12 to D2.1.17, which are HL only and are labelled each time they appear. For condensed recall use the revision notes; to test yourself use the practice questions. Course hub: IB DP Biology. Printable list: checklist. Not sure what to revise first? Try the free 10-minute diagnostics.

What this unit covers

Statement	What you must be able to do	Level
D2.1.1	State that a parent (mother) cell divides to give two daughter cells in all living organisms	SL and HL
D2.1.2	Describe cytokinesis in animal and plant cells	SL and HL
D2.1.3	Explain equal and unequal cytokinesis, with oogenesis and yeast budding	SL and HL
D2.1.4	Explain the roles of mitosis and meiosis in eukaryotes	SL and HL
D2.1.5	Explain why DNA replication must come first	SL and HL
D2.1.6	Describe chromosome condensation and movement	SL and HL
D2.1.7, D2.1.8	Name and identify the phases of mitosis	SL and HL
D2.1.9	Explain meiosis as a reduction division	SL and HL
D2.1.10	Explain Down syndrome as non-disjunction	SL and HL
D2.1.11	Explain how meiosis produces variation	SL and HL
D2.1.12 to D2.1.14	Cell proliferation, the phases of the cell cycle, growth in interphase	HL only
D2.1.15 to D2.1.17	Cyclins, cell cycle mutations, tumours and mitotic index	HL only

D2.1.1 to D2.1.3: Making new cells

In every living organism, a parent cell divides to give two daughter cells. In eukaryotes this needs two linked jobs. The nucleus divides first (mitosis or meiosis). The cytoplasm then splits between the daughters (cytokinesis).

Animal cells. A ring of contractile protein filaments, made of actin and myosin, forms under the cell membrane at the middle of the cell. It tightens like a drawstring and pinches the membrane inward until the cell is cut in two.

Plant cells. There is a rigid cell wall, so pinching is impossible. Vesicles gather across the middle of the cell and fuse. They build new sections of membrane and cell wall, which grow outward until they meet the existing walls.

Equal and unequal. Cytokinesis usually shares the cytoplasm about equally. It is not always equal. Two examples are named in the guide.

• Oogenesis in humans. The cytoplasm divides very unequally, so one large cell keeps almost all of it and the other products are tiny. The large cell is the egg, with food stores for early development.
• Budding in yeast. A small bud forms on the parent. The nucleus divides and one nucleus moves into the bud, which stays much smaller than the parent until it separates.

Whatever the split, each daughter must receive at least one mitochondrion. Mitochondria cannot be made from scratch; they arise only by dividing a pre-existing one. The same applies to any other organelle made only by division of an existing structure. A daughter without them could not make ATP and could not recover.

D2.1.4 to D2.1.6: Why divide the nucleus, and what must happen first

Nuclear division must come before cell division. If the cytoplasm split without it, one daughter would have no nucleus (an anucleate cell) and could not make new proteins from its genes.

• Mitosis keeps the chromosome number and the genome the same, so daughter cells match the parent.
• Meiosis halves the chromosome number and produces genetic diversity.

DNA replication comes first in both. After replication each chromosome is made of two identical, elongated DNA molecules called chromatids, joined together until anaphase. Dividing without replicating would halve the DNA content at every division.

Condensation and movement. Long DNA threads would tangle and break if pulled apart. Histones are proteins that DNA winds around. In prophase the DNA-histone strands are coiled and supercoiled, so each chromosome becomes short and thick. Chromosomes are then moved by microtubules of the spindle, using microtubule motors (proteins that use energy to move along the tubules). Condensation and movement are shared features of mitosis and meiosis.

D2.1.7 and D2.1.8: The phases of mitosis

Mitosis produces two nuclei that are genetically identical to each other and to the parent.

Phase	What you see
Prophase	Chromosomes condense and become visible as two chromatids; the nuclear envelope breaks down; spindle microtubules form
Metaphase	Chromosomes line up across the equator of the cell, attached to spindle microtubules
Anaphase	Centromeres split; the chromatids (now chromosomes) are pulled to opposite poles
Telophase	Chromosomes reach the poles and decondense; nuclear envelopes re-form; cytokinesis follows

Identifying phases (D2.1.8). Use these clues on a micrograph or diagram. Chromosomes spread through an intact nucleus mean interphase. Visible threads with no nuclear envelope mean prophase. A line of chromosomes across the middle means metaphase. V-shaped or I-shaped groups moving apart mean anaphase. Two separated clusters with new envelopes forming mean telophase.

Worked example 1: mitotic index and phase counts (the mitotic index itself is HL only, D2.1.17). A student scans 400 cells of a growing shoot tip and records 34 in prophase, 6 in metaphase, 4 in anaphase and 8 in telophase.

1. Cells in mitosis: 34 + 6 + 4 + 8 = 52.
2. Mitotic index = cells in mitosis ÷ total cells = 52 ÷ 400 = 0.13, or 13%.
3. Most mitotic cells were in prophase (34 of 52, which is 65%). Prophase is long, so more cells are caught in it than in the brief anaphase. This is a reasonable inference, not a measurement of durations.

D2.1.9: Meiosis as a reduction division

A diploid (2n) nucleus holds two full sets of chromosomes, so its chromosomes form homologous pairs, one member of each pair from each parent. A haploid (n) cell has one set. In a sexual life cycle fertilisation fuses two haploid gametes. Without a halving step the chromosome number would double every generation, so meiosis is needed.

Meiosis is two divisions after one round of replication. It gives four haploid nuclei from one diploid nucleus.

• Meiosis I separates homologous chromosomes. Homologues pair up as a bivalent, then one chromosome of each pair goes to each pole. This is the first round of segregation and the reduction. Each nucleus is now haploid, though each chromosome still has two chromatids.
• Meiosis II separates sister chromatids, much like mitosis. This is the second round of segregation.

Worked example 2: DNA content. A diploid cell with 6 chromosomes has 6.0 arbitrary units (au) of DNA in G1.

Stage	Chromosomes	Chromatids	DNA per nucleus
G1	6	6 (one per chromosome)	6.0 au
After replication	6	12	12.0 au
After mitosis (each)	6	6 (one per chromosome)	6.0 au
After meiosis I (each)	3	6	6.0 au
After meiosis II (each)	3	3 (one per chromosome)	3.0 au

Meiosis I halves the chromosome number but not the DNA amount, because each chromosome still has two chromatids. The DNA halves again in meiosis II. Four nuclei of 3.0 au come from one nucleus of 6.0 au.

D2.1.10: Down syndrome and non-disjunction

Non-disjunction is failure of chromosomes to separate in meiosis, so both go to the same pole. In meiosis I the two members of a homologous pair stay together. In meiosis II the two chromatids of one chromosome stay together.

Down syndrome is an example. A gamete with an extra chromosome 21 has 24 chromosomes. When it fuses with a normal gamete (23), the zygote has 47 chromosomes with three copies of chromosome 21 (trisomy 21).

Worked example 3. Explain how one egg could carry 24 chromosomes.

• During meiosis I the two chromosome 21s did not separate (or in meiosis II the two chromatids did not).
• Both went into one nucleus, so that gamete had 23 + 1 = 24 chromosomes.
• The other gamete from that division lacks a chromosome 21 (22 chromosomes).
• Fertilisation by a normal sperm (23): 24 + 23 = 47.

D2.1.11: Meiosis as a source of variation

Two events make the gametes different from one another.

Crossing over. In prophase I, chromatids of homologues swap sections at points called chiasmata. New combinations of alleles appear on a single chromosome.

Random orientation of bivalents. In metaphase I each bivalent faces either pole at random, independently of the others. The number of possible combinations of maternal and paternal chromosomes is 2ⁿ for n pairs, ignoring crossing over.

Worked example 4. A species has 2n = 8. Then n = 4 and 2⁴ = 16 combinations. For humans, n = 23, so 2²³ = 8 388 608. Crossing over multiplies the possibilities further, and random fertilisation adds more.

D2.1.12 to D2.1.14 (HL only): Proliferation and the cell cycle

Cell proliferation (increase in cell number by division) has three uses:

• Growth. Examples are plant meristems, where cells divide near root and shoot tips, and the early animal embryo, where repeated divisions build a multicellular body.
• Replacement. Skin cells are lost and replaced all the time.
• Repair. After a wound, cells near the edge divide to close the gap. You do not need the structure of skin.

The cell cycle (D2.1.13). The sequence is: G1, S and G2 (together called interphase), then mitosis, then cytokinesis.

Stage	Main events
G1	Cell growth, protein synthesis, organelle increase
S	DNA replication
G2	Further growth; preparation for mitosis
M	Mitosis, then cytokinesis

Interphase is not a resting stage (D2.1.14). It is metabolically very active. The cell makes proteins and new DNA, and builds other components. Mitochondria and chloroplasts increase in number by growing and dividing.

D2.1.15 (HL only): Control by cyclins

Cyclins are proteins whose concentrations rise and fall in a regular pattern through the cell cycle. A cell passes each checkpoint only when a specific cyclin has reached a threshold concentration. The guide does not ask for individual cyclins.

Worked example 5. A cell’s cyclin X (invented for the example) is 8 au at the start of G1, rises to 44 au by hour 9, and the checkpoint threshold is 40 au. The cell can pass the checkpoint once the level is at or above 40 au. The level crosses 40 au shortly before hour 9, so the cell cannot proceed earlier than that. Later the level falls, which resets the control for the next cycle.

D2.1.16 and D2.1.17 (HL only): Mutations, cancer and tumours

Genes control the cycle. Proto-oncogenes normally stimulate division. A mutation can convert one into an oncogene, which keeps stimulating division. Tumour suppressor genes normally restrain division; a mutation can switch them off. Either change can give uncontrolled division, and a tumour.

Term	Meaning
Tumour	An abnormal mass of cells
Benign	Stays in one place and does not invade; does not cause cancer
Malignant	Grows quickly, invades neighbouring tissue and can spread; this is cancer
Primary tumour	Where the tumour began
Secondary tumour	New tumour from cells that spread by metastasis

Worked example 6. Sample P has 62 cells in mitosis out of 250. Sample Q (normal tissue) has 52 out of 400.

• P: 62 ÷ 250 = 0.248, so 24.8%.
• Q: 52 ÷ 400 = 13.0%. The mitotic index of P is about 1.9 times that of Q (24.8 ÷ 13.0 = 1.9).
• A higher index shows faster division, but not by itself whether the tumour is benign or malignant. You would need evidence of invasion or secondary tumours.

Common errors

• Writing that chromosomes “split” in anaphase I. In meiosis I whole chromosomes separate; centromeres split in anaphase of mitosis and meiosis II.
• Saying meiosis II is the reduction. The chromosome number halves in meiosis I.
• Confusing cytokinesis with mitosis.
• Calling G1, S and G2 “rest phases”.
• Writing that every tumour is cancer.

Official syllabus

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