The synaptonemal complex--the chaperone of crossing over.

During meiosis homologous chromosomes pair and exchange homologous chromosome segments. The synaptonemal complex (SC) forms between paired chromosomes. The role of the SC in the process of reciprocal exchange of flanking markers is a matter of debate. I propose a dual pathway for reciprocal exchange of flanking markers (REFM). In the first, SC-independent, path, two 'half-nodules' and an independent REFM protein combine to form a functional recombination nodule (RN). The RN binds to paired chromosomes and accomplishes reciprocal exchange of flanking markers. In the other, SC-dependent, pathway 'half-nodules' occur at pairing initiation sites. 'Half-nodules' move along the SC as it forms. Assisted by an SC-bound REFM protein, 'half-nodules' combine to form functional RNs. I propose that different organisms rely to different extents on the two pathways, and hence rely to different extents on the SC.