THE SYNAPTONEMAL COMPLEX - THE CHAPERONE OF CROSSING-OVER

During meiosis homologous chromosomes pair and exchange homologous chr omosome segments. The synaptonemal complex (SC) forms between paired c hromosomes. The role of the SC in the process of reciprocal exchange o f flanking markers is a matter of debate. I propose a dual pathway for reciprocal exchange of flanking markers (REFM). In the first, SC-inde pendent, path, two 'half-nodules' and an independent REFM protein comb ine to form a functional recombination nodule (RN). The RN binds to pa ired chromosomes and accomplishes reciprocal exchange of flanking mark ers. In the other, SC-dependent, pathway 'half-nodules' occur at pairi ng initiation sites. 'Half-nodules' move along the SC as it forms. Ass isted by an SC-bound REFM protein, 'half-nodules' combine to form func tional RNs. I propose that different organisms rely to different exten ts on the two pathways, and hence rely to different extents on the SC.