synapsed

Synapsed is an adjective used in genetics and cell biology to describe homologous chromosomes that have undergone synapsis, the pairing and alignment of homologs during meiotic prophase I. The term derives from synapsis, the process by which homologous chromosomes become physically connected by proteinaceous structures, forming the basis for recombination.

During meiosis, homologous chromosomes begin to pair in the leptotene and zygotene stages and become synapsed as the synaptonemal complex forms. By pachytene, the homologs are typically fully synapsed into tetrads, with four chromatids held together by the complex and sister chromatid cohesion. This synapsed state facilitates crossing over between non-sister chromatids and ensures proper alignment for subsequent segregation. The synapsed configuration is temporary and is dissolved during diplotene as chiasmata become visible and chromosomes begin to separate.

Significance of synapsis lies in accurate homolog segregation and the promotion of genetic recombination, which increases diversity in gametes. In research and diagnostics, synapsed chromosomes can be observed in meiotic spreads and analyzed using cytogenetic techniques. Immunostaining for synaptonemal complex components (for example, SYCP1, SYCP3 in vertebrates) helps assess the extent of synapsis. Variations in synapsis, including partial or failed synapsis (asynapsis), can result from genetic defects and are associated with fertility issues in various organisms.

Synapsed contrasts with asynapsed states, and the term is primarily used to describe the meiotic pairing of homologous chromosomes.