D2.1 Cell and nuclear divisionIB Biology HL: Revision notes

Cells divide to form two daughter cells. Cytokinesis splits the cytoplasm: in animals a ring of actin and myosin pinches the membrane; in plants vesicles build new membrane and cell wall. It is usually equal, but unequal in oogenesis and yeast budding; each daughter needs at least one mitochondrion. Nuclear division must come first to avoid anucleate cells. Mitosis keeps the chromosome number and genome; meiosis halves it and generates diversity. Both need DNA replication first, giving chromosomes of two chromatids, and both use histones (supercoiling for condensation) and microtubules with motor proteins to move chromosomes.

Mitosis: prophase (condensation), metaphase (alignment at equator), anaphase (chromatids separate), telophase (nuclei re-form) — two genetically identical nuclei.

Meiosis is a reduction division: meiosis I separates homologous chromosomes, meiosis II separates chromatids, giving four haploid nuclei from one diploid nucleus; needed because fertilisation doubles chromosome number. Non-disjunction can give a gamete with two chromosome 21s and a child with Down syndrome. Variation comes from random orientation of bivalents ($2^{23}$ combinations in humans) and crossing over.

Cell proliferation is used for growth (plant meristems, early animal embryos), cell replacement (skin cells are continually replaced) and tissue repair (wound healing in skin). It is achieved by the cell cycle: interphase (G1, S, G2) followed by mitosis and then cytokinesis.

Interphase is metabolically active: in G1 and G2 cells grow by synthesising proteins and other components; in S the DNA is replicated. Numbers of mitochondria and chloroplasts increase by growth and division of these organelles.

Cyclins are proteins whose concentrations rise and fall during the cell cycle. A threshold concentration of a specific cyclin is needed to pass each checkpoint; when a cyclin reaches its threshold it triggers the next stage, then it is broken down. You do not need the roles of specific cyclins.

Proto-oncogenes stimulate division; mutation can turn one into an oncogene that stimulates division continuously. Tumour suppressor genes inhibit division; mutations that inactivate them remove this brake. Either can cause uncontrolled cell division and a tumour.

• Benign tumours divide slowly, stay localised and do not invade or spread — not cancer.
• Malignant tumours divide rapidly, invade neighbouring tissue and can metastasise — cancer.
• The primary tumour is where the cancer began; secondary tumours form elsewhere after metastasis.

The mitotic index is the proportion of cells in a sample that are in mitosis:

mitotic index = number of cells in mitosis ÷ total number of cells

A high index indicates rapid proliferation (e.g. a meristem or a tumour). The proportion of cells in any stage reflects how long that stage lasts.