unsynapsed

Unsynapsed refers to chromosomes or chromosomal regions that fail to pair with their homologous partners during meiosis, especially in prophase I when homologs normally synapse to form the synaptonemal complex. In cells, unsynapsed chromatin appears as unpaired axes and is a key feature monitored during meiotic progression.

Causes of unsynapsis include structural differences between homologs, such as inversions or translocations, which prevent proper alignment and synapsis. Heterozygosity for such rearrangements, defects in the machinery that promotes synapsis or recombination, or extensive DNA damage can also lead to unsynapsed regions.

Consequences of unsynapsis are tied to cellular surveillance mechanisms. The meiotic checkpoints can detect unsynapsed chromatin and may arrest meiosis or trigger apoptosis to prevent the formation of aneuploid gametes. A common cellular response is meiotic silencing of unsynapsed chromatin (MSUC), in which transcription is silenced and chromatin is compacted over unsynapsed regions. In many male animals, the sex chromosomes remain largely unsynapsed during pachytene, leading to meiotic sex chromosome inactivation (MSCI). Some species exhibit partial silencing or contexts in which certain genes on unsynapsed regions remain active.

Study and relevance: Unsynapsis is studied to understand chromosome behavior, fertility, and the origins of chromosomal disorders. It is investigated across model organisms, including mice, Drosophila, and C. elegans, to elucidate the molecular players governing synapsis, MSUC/MSCI, and meiotic checkpoint signaling.