Cells divide to allow growth, repair, replacement and reproduction. There are two types of nuclear division: mitosis and meiosis. They differ in the number of divisions and the type of cells produced.
Mitosis
Mitosis produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell (diploid). Before division the DNA is copied so each chromosome forms two identical strands. Mitosis is used for growth, repair of damaged tissue, replacement of worn-out cells and asexual reproduction.
Key idea
Mitosis: 1 division → 2 identical diploid cells. Meiosis: 2 divisions → 4 genetically different haploid cells (gametes).
Meiosis
Meiosis is a reduction division that halves the chromosome number, producing gametes. One diploid cell divides twice to give four haploid cells that are genetically different from each other. This is essential for sexual reproduction: when two haploid gametes fuse, the diploid number is restored.
Worked example
A human cell has 46 chromosomes. After mitosis each daughter cell has 46. After meiosis each of the four cells has 23. When a sperm (23) fertilises an egg (23), the zygote returns to 46 — the diploid number.
Remember
- Mitosis → identical, diploid, growth & repair.
- Meiosis → different, haploid, gametes.
- Meiosis introduces genetic variation.