synapsis

Synapsis is the pairing and physical association of homologous chromosomes during meiosis, typically occurring in prophase I, and is associated with the formation of the synaptonemal complex. The process begins as chromosomes condense in leptotene, proceeds to zygotene when homologs begin to pair and synapse, and reaches a stage of complete synapsis along their lengths in pachytene, where crossing over between non-sister chromatids occurs. In diplotene, the synaptonemal complex disassembles and the homologs begin to separate, but remain connected at chiasmata, the cytological marks of crossover events.

The synaptonemal complex is a proteinaceous scaffold that holds homologous chromosomes together during pairing. In mammals, it includes transverse elements such as SYCP1 and axial elements such as SYCP2 and SYCP3; in other organisms, analogous components are described (for example, Zip proteins in yeast). The formation of synapsis facilitates homologous recombination, ensuring genetic exchange between homologs and the proper alignment of chromosomes for segregation at meiosis I.

Genetic recombination begins with DNA double-strand breaks introduced by Spo11 and is processed by recombination machinery including Rad51 and Dmc1, yielding crossovers that become chiasmata. These events promote genetic diversity and are essential for correct chromosome segregation.

Synapsis is meiosis-specific; mitosis does not involve extensive homolog pairing. In some species, desynapsis occurs during diplotene as the chromosomes prepare for later stages.