Cell Division
This domain examines how cells replicate and how chromosomes behave during mitosis and meiosis. Questions often ask you to count chromosomes and chromatids, identify stages, explain variation, and link division to growth, repair and gamete formation.
Cell cycle and mitosis
The cell cycle has interphase (G1, S, G2) and M phase. DNA replicates in S phase, not prophase. Mitosis proceeds through prophase (chromosomes condense, spindle forms), metaphase (chromosomes align on the equator), anaphase (sister chromatids separate) and telophase. Cytokinesis uses a cleavage furrow in animals and a cell plate in plants. Mitosis produces genetically identical diploid cells for growth, repair and asexual reproduction.
Meiosis
Meiosis has two divisions. In prophase I, homologous chromosomes synapse and crossing over occurs at chiasmata. In metaphase I, homologous pairs align independently, giving independent assortment. Meiosis I separates homologous chromosomes; meiosis II separates sister chromatids, producing four haploid cells. Variation comes from crossing over, independent assortment and random fertilisation.
Worked example
A diploid cell with 2n = 8 undergoes meiosis. At metaphase I there are four bivalents, so eight chromosomes and sixteen chromatids. Each gamete has n = 4 chromosomes, each consisting of a single chromatid. Without crossing over, the number of possible combinations of maternal and paternal chromosomes is 2⁴ = 16.
Traps
Do not say DNA replicates in prophase; it replicates in S phase. Distinguish chromosomes from chromatids when counting. Meiosis does not produce identical cells, and crossing over belongs to prophase I, not anaphase.