Meiosis: Making Sex Cells

🧬 Meiosis: Making Sex Cells

While mitosis produces identical body cells, meiosis produces gametes — the sex cells needed for sexual reproduction. The result is very different: four genetically unique haploid cells rather than two identical diploid ones.

Where does meiosis happen?

• Testes — producing sperm cells
• Ovaries — producing egg cells (ova)

Two rounds of division

Meiosis involves two successive divisions of the nucleus:

1. Meiosis I — Separating homologous pairs: The cell starts with 46 chromosomes arranged in 23 pairs (each pair contains one chromosome from each parent). In Meiosis I, these homologous pairs are separated and pulled to opposite poles. The result is two cells, each with 23 chromosomes — but each chromosome still consists of two chromatids. The chromosome number is halved here.
2. Meiosis II — Separating chromatids: Each of the two cells from Meiosis I divides again, separating the chromatids. This is similar to mitosis. The result is four cells, each with 23 single chromosomes (haploid, n).

Why is meiosis essential?

• Halves the chromosome number: Human body cells are diploid (2n = 46 chromosomes). Gametes must be haploid (n = 23). When a sperm (23 chromosomes) fuses with an egg (23 chromosomes) at fertilisation, the diploid number (46) is restored in the zygote. Without meiosis halving the number first, every generation would double the chromosome count.
• Creates genetic variation: The four gametes produced are all genetically different from each other and from the parent cell. This occurs because homologous pairs are randomly shuffled when they separate (independent assortment), so each gamete receives a different mix of maternal and paternal chromosomes. This variation is crucial for evolution and the uniqueness of every individual.

Must memorise: Meiosis produces 4 genetically different haploid cells. Meiosis uses two divisions. It occurs in the testes and ovaries.