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

New cells are generated by cell division: a parent (mother) cell divides into two daughter cells. Cytokinesis is the splitting of the cytoplasm between them.

• Animal cells: a ring of contractile actin and myosin proteins pinches the plasma membrane inwards.
• Plant cells: vesicles assemble sections of membrane and cell wall across the equator (the cell plate).

Cytokinesis is usually, but not always, equal. Each daughter cell must receive at least one mitochondrion and any other organelle (e.g. chloroplasts) that can only be made by dividing a pre-existing one. Examples of unequal cytokinesis: oogenesis in humans (the egg keeps most cytoplasm, polar bodies get very little) and budding in yeast.

Nuclear division must happen before cell division, or anucleate cells would be produced. Mitosis maintains the chromosome number and genome, producing genetically identical nuclei. Meiosis halves the chromosome number and generates genetic diversity. Both require prior DNA replication: afterwards each chromosome consists of two chromatids held together until anaphase.

Shared features: condensation of chromosomes by supercoiling of DNA around histones, and movement of chromosomes by microtubules and microtubule motor proteins.

• Prophase: chromosomes condense and become visible; nuclear membrane breaks down; spindle forms.
• Metaphase: chromosomes line up at the equator, attached to spindle microtubules from both poles.
• Anaphase: sister chromatids separate and are pulled to opposite poles (often V-shaped).
• Telophase: chromosomes reach the poles and decondense; nuclear membranes re-form.

The result is two genetically identical nuclei.

Diploid nuclei have two sets of chromosomes (2n); haploid nuclei have one (n). In meiosis I, homologous chromosomes pair and are segregated to opposite poles (the reduction). In meiosis II, sister chromatids are segregated. One diploid nucleus gives four haploid nuclei. Meiosis is needed in a sexual life cycle because fertilisation doubles the chromosome number.

Non-disjunction is the failure of homologous chromosomes (meiosis I) or sister chromatids (meiosis II) to separate. A gamete may receive two copies of chromosome 21; after fertilisation the zygote has three copies (trisomy 21) — Down syndrome. Risk increases with maternal age.

• Random orientation of bivalents at metaphase I: with n pairs there are $2^n$ combinations; for humans $2^{23}$ ≈ 8.4 million.
• Crossing over in prophase I exchanges sections between non-sister chromatids, making new combinations of alleles.

Together with random fertilisation, this produces enormous genetic diversity.