Synapsis or pairing is the union of homologous chromosomes that give tetrads, groups of 4 chromatids. Synapsis occurs through the formation of the synaptonemal complex, consisting of a long protein matrix on each side of which the two homologues are aligned. It only happens during meiosis.
During synapsis, adjacent homologous chromatid arms undergo spanning or otherwise intra-chromosomal mixing that occurs at one or more anchor points called chiasmata. In general, the longer the chromatids, the more chiasmas are numerous.
The synapsis phenomenon marks the end of the leptotene stage of prophase 1 of the reductional division of meiosis and the initiation of the 2 stage of prophase 1 which is zygotene. Synapsis often begins with a closer approximation of the homologous ends of the two chromosomes at the nuclear envelope, thus aligning the homologous chromosomes next to each other.
Note: Contrary to some documents that can mention it, the plural of chiasma is not "chiasmas", but chiasmata.
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