# Cell Division: Mitosis and Meiosis
Cell division is essential for growth, repair, and reproduction. Mitosis produces genetically identical cells, while meiosis produces genetically varied gametes.
1. The Cell Cycle
Interphase (90% of cycle)
- G₁: cell grows; organelles replicate; protein synthesis
- S phase: DNA replication (each chromosome becomes two sister chromatids joined at centromere)
- G₂: continued growth; preparation for division
Mitotic Phase
- Mitosis: nuclear division
- Cytokinesis: cytoplasm divides
2. Mitosis (PMAT)
| Stage | Events |
|---|---|
| Prophase | Chromosomes condense; nuclear envelope breaks down; spindle forms from centrioles |
| Metaphase | Chromosomes line up at cell equator; spindle fibres attach to centromeres |
| Anaphase | Centromeres divide; sister chromatids pulled to opposite poles by shortening spindle fibres |
| Telophase | Nuclear envelopes reform; chromosomes decondense; 2 nuclei visible |
Result: 2 genetically identical diploid cells
3. Meiosis
Meiosis I (Reduction Division)
- Homologous pairs synapse (pair up)
- Crossing over: exchange of genetic material between homologous chromosomes (chiasma)
- Homologous pairs separate (independent assortment)
- 2 haploid cells formed
Meiosis II
- Similar to mitosis: sister chromatids separate
- 4 genetically different haploid cells (gametes)
Sources of Genetic Variation
- Crossing over (prophase I)
- Independent assortment (metaphase I)
- Random fertilisation
4. Cancer and Cell Division
- Cancer: uncontrolled cell division due to mutations in genes controlling the cell cycle
- Proto-oncogenes → oncogenes: stimulate excessive division
- Tumour suppressor genes (e.g. p53): normally stop division; mutations remove this brake
5. Practice Questions
- Describe the events of prophase in mitosis.
- Explain how meiosis produces genetic variation.
- Compare mitosis and meiosis.
- What happens during S phase of interphase?
- Explain how mutations in tumour suppressor genes can lead to cancer.
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Summary
- Cell cycle: G₁ → S → G₂ → mitosis → cytokinesis
- Mitosis: PMAT → 2 identical diploid cells
- Meiosis: 2 divisions → 4 different haploid cells
- Variation: crossing over, independent assortment, random fertilisation
- Cancer: uncontrolled division from mutations in cell cycle genes