synapsis

thumb|Synapsis during meiosis. The circled area is the part where synapsis occurs, where the two chromatids meet before crossing over Synapsis or syzygy is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. When homologous chromosomes synapse, their ends are first attached to the nuclear envelope. These end-membrane complexes then migrate, assisted by the extranuclear cytoskeleton, until matching ends have been paired. Then the intervening regions of the chromosome are brought together, and may be connected by a protein-DNA complex called the synaptonemal complex (SC). The SC protein scaffold stabilizes the physical pairing of homologous chromosomes by polymerizing between them during meiotic prophase. During synapsis, autosomes are held together by the synaptonemal complex along their whole length, whereas for sex chromosomes, this only takes place at one end of each chromosome.

This is not to be confused with mitosis. Mitosis also has prophase, but does not ordinarily do pairing of two homologous chromosomes. In contrast to the mitosis cycle, during meiosis, the number of chromosomes is reduced by half to create haploid gametes; this reduction is called Haploidization; after fertilization, diploidy is restored. Homologous chromosomes – two copies inherited from each parent – recognize one another and pair before reductional segregation, which is essential for crossover recombination and forms chiasmata, a stable physical connection that hold homologous chromosomes together until metaphase. In most species, every homologous chromosome experiences at least one meiotic crossover referred to as the obligate crossover.