Meiosis: two divisions, four cells

Meiosis serves one purpose only: making reproductive cells. It halves the number of chromosomes, and that is mathematically indispensable.

The problem to solve

If gametes had the normal number of chromosomes, fertilisation would double it every generation:

generation 1:  46 + 46 = 92
generation 2:  92 + 92 = 184
generation 3:  368 ...                impossible

So a division that halves the count is needed: each gamete brings 23, and fertilisation restores 46.

DIPLOID CELL (2n = 46)   ->  MEIOSIS  ->  HAPLOID GAMETES (n = 23)

                fertilisation: 23 + 23 = 46

Two successive divisions

   2n cell (chromosomes already replicated)
            |
            |  MEIOSIS I  -- REDUCTIONAL division
            |  the HOMOLOGOUS chromosomes separate
            v
      2 cells, n  (chromosomes with 2 chromatids)
            |
            |  MEIOSIS II -- EQUATIONAL division
            |  the chromatids separate (like a mitosis)
            v
      4 cells, n  (chromosomes with 1 chromatid)

The key point: it is the first division that reduces the number, by separating homologous chromosomes — the two copies of a pair, one from the father, one from the mother. The second resembles an ordinary mitosis.

MITOSIS                             MEIOSIS
--------------------------------    --------------------------------
1 division                          2 divisions
2 daughter cells                    4 daughter cells
2n -> 2n (identical)                2n -> n (halved)
IDENTICAL cells                     all DIFFERENT cells
everywhere in the body              reproductive organs only

Homologous chromosomes

   pair of homologous chromosomes

     ||        ||
     ||        ||          same SIZE, same GENES in the same places
     ||        ||          but not necessarily the same VERSIONS (alleles)
   from father from mother

An individual has 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes (XX or XY). Each pair carries the same genes in the same order — but the alleles may differ, for instance "blue eyes" on one and "brown eyes" on the other.

When meiosis goes wrong

Separation can fail: one gamete then gets two copies of a chromosome and the other none. This is non-disjunction.

normal gamete   : 23 chromosomes
abnormal gamete : 24 or 22 chromosomes
                     |
                     v
   after fertilisation: 47 or 45 chromosomes  ->  trisomy or monosomy

Trisomy 21 corresponds to three copies of chromosome 21. Its frequency rises markedly with maternal age, because oocytes remain arrested in prophase of meiosis I from fetal life — sometimes for forty years — and the separation machinery degrades over time.

Summary

• Meiosis produces haploid gametes (n) from diploid cells (2n).
• Without it, the chromosome number would double each generation.
• Two divisions: the first separates the homologues (reduction), the second the chromatids.
• One 2n cell gives four n cells, all different.
• Homologous chromosomes carry the same genes, not necessarily the same alleles.
• A non-disjunction produces unbalanced gametes (trisomies).