Revision sheet
Meiosis and the two shufflings
Two divisions, one replication — and the two mechanisms that make every gamete unique.
Why a reduction division is necessary
Fertilisation doubles the chromosome number. Without a compensating halving, the number would double each generation, so gametogenesis must include a division that halves it.
Meiosis does exactly that: one round of DNA replication followed by two divisions, giving four haploid cells from one diploid cell.
That is the arithmetic. But meiosis does something a simple halving would not: it shuffles the parental chromosomes, so no two gametes carry the same combination. Reduction is the requirement; variation is the point.
- One replication, two divisionsfour haploid cells
- Mitosis conserves; meiosis halves and shuffles.
The first division: reductional
In prophase I the homologous chromosomes pair up — synapsis — forming bivalents of four chromatids, and crossing-over takes place. Metaphase I aligns the bivalents on the equatorial plate. In anaphase I the homologues separate and travel to opposite poles, each still made of two chromatids.
This is the division that halves the chromosome number: each daughter cell receives one chromosome of each pair, not one chromatid. Telophase I ends with two haploid cells whose chromosomes are still double.
- Anaphase I separates homologues, not chromatids.
- Ploidy halves here, at the first division.
The two shufflings
Interchromosomal shuffling happens at metaphase I: each bivalent orients independently of the others, so for n pairs there are 2ⁿ possible combinations — 2²³, over eight million, in humans. It redistributes whole chromosomes.
Intrachromosomal shuffling happens earlier, in prophase I: crossing-over exchanges segments between the chromatids of two homologues, producing chromosomes that are themselves mosaics of maternal and paternal DNA.
It is what creates recombinant gametes for two genes carried on the same chromosome, and it is why linkage is rarely absolute.
- Intrachromosomal = crossing-over, prophase I, within a chromosome.
- Interchromosomal = independent assortment, metaphase I, between chromosomes.
- Fertilisation then adds a third shuffling, by combining two gametes at random.
The second division: equational
The second division follows without any new replication. It is mechanically a mitosis performed on a haploid cell: the chromosomes align in metaphase II, the centromeres split in anaphase II, and the sister chromatids separate.
Each of the two cells gives two, so meiosis ends with four haploid cells whose chromosomes are now single. Because of crossing-over, the two chromatids of one chromosome are no longer identical — which is why the four products are genetically different from each other.
- Anaphase II separates chromatids — like mitosis, but on a haploid cell.
- No replication between the two divisions.