The mechanism of meiotic homologue pairing

Homologous chromosome pairing involves the moving together of matching chromosomes or chromosome segments across substantial distances within a nucleus. Although the time in the life cycle of initial association of homologues varies among organisms, it may well be that similar underlying mechanisms for its occurrence prevail throughout sexually reproducing eukaryotes. The means by which pairing its accomplished is in no case understood. In the apparent absence of a long range specific force of attraction, simple partial models have been proposed which relay for the most part upon interactions of chromosome ends (telomeres) with specialized portions of the nuclear envelope. While such interactions, as well as the persistence of chromosome orientation established by mitotic anaphase poleward movement of centromere regions, may provide in many cases for closer than random positioning of some parts of homologues, the distances remaining to be traversed are still long range in physical-chemical terms. Also, the specific pairing observed in some kinds of rearranged segments is not facilitated by such circumstances, even if synapsis is initiated at available homologous telomere pairs and proceeds to completion by a "zip-up" process. A unified, more complex model is considered which is designed to accommodate the various relevant findings. It invokes the interaction of intranuclear structures with intercalary and/or terminal chromosomal pairing sites, e.g. filamentous structures which specifically bind to these, and a contractile system involving proteins such as actin and myosin to draw homologues together.