Cell Division: Mitosis & Meiosis
Cells only come from pre-existing cells. Before any division, the cell doubles its DNA (S phase) so each daughter receives a complete copy.
Making identical cells
- Binary fission — how prokaryotes divide into two identical cells.
- Mitosis — in eukaryotic body cells: one division making two genetically identical, diploid cells for growth and repair (phases: prophase, metaphase, anaphase, telophase).
Making gametes — meiosis
Meiosis is two divisions producing four genetically different, haploid gametes. Variation comes from crossing over and independent assortment; fertilisation then restores the diploid number.
Mitosis → 2 identical diploid cells; meiosis → 4 varied haploid gametes.
So asexual reproduction (mitosis/binary fission) yields clones, while sexual reproduction (meiosis + fertilisation) generates the variation evolution needs.