Synapsis and crossing-over mechanisms increase genetic diversity by recombining combinations of gene alleles on a single chromosome which always segregate together.

The coming together and pairing of homologous chromosomes form a synapsis. This takes place during the first stage of meiosis, prophase I. It helps to ensure that each cell divides a full set of chromosomes.

When synapsis happens, the homologous chromosomes end up next to each other. Each set of homologous chromosomes contains the same genes on them in the same location. One of the sister chromatids from each chromosome will have the chance to exchange genetic material with a sister chromatid from the homologous chromosome and is called crossing over. 

The points where homologs crossover and exchange genetic material are known as a chiasmata. These are chosen more or less at random, and they will be different in each cell that goes through meiosis. When a chiasma forms a synaptonemal complex, the protein complex that glues the tetrad together supports the crossing over. At the end of prophase I, the pairs are held together only at the chiasmata and are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible. Tetrads form for crossing over to occur. It breaks apart when the homologous chromosomes separate in meiosis I. The breaking apart forms recombinant chromosomes. 

If meiosis happens many times, as it does in human ovaries and testes, crossovers will occur at many different points. This repetition produces a wide variety of recombinant chromosomes, chromosomes where fragments of DNA exchanged between homologs.

Key Points

• Synapsis and crossing over increase the genetic diversity by recombining combinations of gene alleles on a single chromosome which will always segregate together. 

Key Terms

synapsis: the fusion of chromosome pairs at the start of meiosis

crossing-over: exchange of genetic material between homologous chromosomes after the tetrad forms

homologous chromosomes: are made up of chromosome pairs of approximately the same length. One homologous chromosome is inherited from the organism’s mother; the other is inherited from the organism’s father

prophase: the first stage of cell division, before metaphase, during which the chromosomes become visible as paired chromatids and the nuclear envelope disappears

chromatid: one of two strands of a chromosome. Chromatids that are joined together at their centromeres are called sister chromatids.

chiasmata: a point at which paired chromosomes remain in contact during the first metaphase of meiosis, and at which crossing over and exchange of genetic material occur between the strands

tetrad: two pairs of sister chromatids (a dyad pair) aligned in a certain way and often on the equatorial plane during the meiosis process

synaptonemal complex: is a protein structure that forms between homologous chromosomes to mediate synapsis and recombination during meiosis