Meiosis: spindle, synapsis, bivalents and chiasmata

1Half the chromosomes

In 1883 the Belgian biologist Édouard van Beneden was studying the fertilised eggs of a roundworm of horses, a species with only four chromosomes in its body cells. He saw that the sperm nucleus and the egg nucleus each brought just two chromosomes to the zygote: the gametes carried half the number. Some special division must halve the chromosome number before gametes form, or it would double in every generation. That division is meiosis. In Senior 3 you followed its stages in outline. Here you study what makes meiosis different from mitosis: the pairing of homologous chromosomes, the chiasmata that hold them together, and the exchange of genes between them.

2The stages of meiosis

3Why synapsis, bivalents, chiasmata and the spindle matter

• The spindle forms twice. In meiosis I it separates homologous chromosomes; in meiosis II it separates sister chromatids. This is how each gamete receives exactly one chromosome of every pair.
• Synapsis and bivalents bring each chromosome alongside its homologue. Only then can the homologues be attached to opposite poles and sent to different cells.
• Chiasmata hold the homologues together until anaphase I. Without at least one chiasma, a pair can separate too early and both homologues may go to the same pole.
• Crossing over produces chromatids with new combinations of alleles (Figure 2).