Cell division
Cell division tests mitosis, meiosis, the cell cycle, chromosome behaviour and the significance of genetic variation. Expect questions on chromosome counts, stage identification and comparing mitosis with meiosis.
Mitosis and the cell cycle
- Interphase includes G1, S and G2; DNA replicates in S phase. Checkpoints monitor DNA damage and chromosome attachment.
- Mitosis produces two genetically identical diploid nuclei for growth, repair and asexual reproduction. Prophase, metaphase, anaphase and telophase describe chromosome behaviour.
- Cytokinesis divides the cytoplasm.
Meiosis
Meiosis has two divisions and produces four haploid cells. Homologous chromosomes pair in prophase I; crossing over and independent assortment create genetic variation. Separation in meiosis I reduces chromosome number; meiosis II separates sister chromatids.
Worked example
A diploid cell has 2n = 24 chromosomes. After DNA replication it still has 24 chromosomes, each with two chromatids. Meiosis I produces two cells with 12 chromosomes; meiosis II produces four haploid cells with 12 chromosomes each. Fertilisation restores 24.
Traps
Chromosome number is counted by centromeres, not chromatids. Meiosis II does not involve DNA replication. Mitosis does not create genetic variation by crossing over. Cancer results from uncontrolled mitosis, not meiosis.