Biology (university) · Part 06
Cell cycle, mitosis and meiosis
How cells divide, how division is controlled, and how sex cells are made.
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- Cell cycleThe ordered series of events from one cell division to the next. Interphase (G1, S, G2) is when the cell grows and copies its DNA in S phase. The M phase is mitosis plus cytokinesis.
- Cell cycle checkpointA control point where the cell checks that conditions are right before moving on. Main checkpoints: G1 (size, nutrients, DNA damage), G2 (DNA fully copied) and M (chromosomes attached to the spindle). Cyclins and CDK enzymes drive the cycle.
- MitosisDivision of the nucleus that produces two genetically identical daughter nuclei. Stages: prophase, metaphase, anaphase, telophase. It is used for growth and repair.
- CytokinesisThe splitting of the cytoplasm after mitosis. Animal cells pinch in two with a ring of actin (a cleavage furrow). Plant cells build a new cell wall (cell plate) down the middle.
- CancerUncontrolled cell division caused by mutations in genes that control the cycle. Proto-oncogenes (accelerators) mutated into oncogenes are stuck on. Tumour suppressor genes (brakes) such as p53 are switched off.
- MeiosisTwo divisions that turn one diploid cell into four haploid cells, each with half the chromosomes. It makes gametes (eggs and sperm). Meiosis I separates homologous pairs. Meiosis II separates sister chromatids.
- Crossing over and independent assortmentTwo sources of variation in meiosis. In crossing over, homologous chromosomes swap pieces in prophase I. In independent assortment, each pair lines up and separates at random.
- NondisjunctionA failure of chromosomes to separate in meiosis. A gamete gets an extra chromosome or is missing one. After fertilisation the baby has the wrong number.