Synapsis

Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. Synapsis takes place during prophase I. When homologous chromosomes synapse, they come closer together until they are connected by a protein complex called the synaptonemal complex, which contains central and lateral elements. While autosomes undergo synapsis during meiosis sex chromosomes usually remain unpaired.

When the non-sister chromatids intertwine, segments of chromatids with the same sequence break apart at and are exchanged in a process known as genetic recombination or "crossing-over". Recombination exchanges genetic material between homologous chromosomes and increases the genetic variability of the offspring. This exchange produces a chiasma, a region that is shaped like an X, where the two chromosomes are physically joined.

The opposite of synapsis is disjunction.