Meiosis is a special type of cell division in which two rounds of chromosome segregation follow one round of DNA replication (Figure 2A). This serves a fundamental function by halving the number of chromosomes which is required for sexual reproduction. The typical number of chromosomes for a given species is then reestablished after fertilization. The two meiotic divisions are known as meiosis I and meiosis II. The first segregation, meiosis I, is reductional as homologous chromosomes, instead of sister chromatids, migrate to opposite poles of the spindle. Meiosis II is an equational division analogous to mitosis, in which the sister chromatids are segregated, creating four haploid daughter cells.
Figure 2. Mouse spermatogenesis. (A) Schematic of male mouse spermatogenesis. (B) Synaptonemal complex components and structure. (C) Stages of meiotic prophase I and the subsequent stage of metaphase I.
Prophase of meiosis I is the longest phase of meiosis. During prophase I, homologous chromosomes are prepared to segregate by events in prophase, when the partners form synaptonemal complex (Figure 2B) to synapse and undergo recombination. The first stage of prophase I is the leptotene stage. In this stage, individual homologous chromosomes condense from the diffuse interphase conformation into visible strands within the nucleus. During leptotene, lateral elements of the synaptonemal complex assemble. Following leptotene, the zygotene stage occurs as the chromosomes line up (pair) with each other in homology dependent process to form homologous chromosome pairs. At this stage, the synapsis of homologous chromosomes takes place, facilitated by assembly of central element of the synaptonemal complex. Here, axial elements begin to zipper together (marked by Sycp1 staining, Figure 2A and B), as homologous chromosomes synapse, to form a tripartite synaptonemal complex. The pachytene stage is characterized by fully synapsis of the homologous chromosome pairs and is the stage when CO occurs. At this stage sex chromosomes, however, are not wholly identical and only exchange information over a small region of homology (the PAR region). At the sites where exchange happens, chiasmata form. At the end of prophase I, the diplotene stage is easily recognized by the typical separation of the homologous chromosomes consequence of synaptonemal complex degradation.
Given that meiotic recombination and homologous chromosome synapsis are co-dependent processes, mutations in one pathway invariably affect the other. The co-dependence of these fundamental meiotic processes allow us to assess in what recombination intermediate and stage of chromosome synapsis the specific proteins we study function.