Each homolog is organized around a protein axial core, and the synaptonemal axes are linked by rod-shaped transverse filaments. The axial core of each homolog also interacts with the cohesin complexes axial cores that hold the sister chromatids together of the (see Figure 9–35). (modified from homologs k. nasmyth, Annu. Rev. Genet. 35:673– 745, 2001.) cohesin chromatin loops of sister chromatids of one homolog along their entire lengths (see Figure 9–35). The pachytene stage can persist for days or longer, until desynapsis begins at diplotene with the disassembly of the synaptonemal complexes and the concomitant condensation and shortening of the chromosomes. It is only at this stage, after the complexes have disassembled, that the individual crossover events between nonsister chromatids can be seen as inter-homolog connections called chiasmata (singular chiasma), which now play a crucial part in holding the compact homologs together (Figure 17–57). The homologs are now ready to begin the process of segregation.
Figure 17–56 Homolog synapsis and desynapsis during the different stages of prophase I. (a) a single bivalent is shown schematically. at leptotene, the two sister chromatids coalesce, and their chromatid loops extend out from a common axial core. assembly of the synaptonemal complex begins in early zygotene and is complete in pachytene. The complex disassembles in diplotene. (B) an electron micrograph of a synaptonemal complex from a meiotic cell at pachytene in a lily flower. (C and d) Immunofluorescence micrographs of prophase I cells of the fungus Sordaria. partially synapsed bivalents at zygotene are shown in (C) and fully synapsed bivalents are shown in (d). Red arrowheads in (C) point to regions where synapsis is still incomplete. (B, courtesy of Brian Wells; C and d, from a. storlazzi et al., Genes Dev. 17:2675–2687, 2003. With permission from Cold spring Harbor Laboratory press.)
Figure 17–58 Comparison of chromosome behavior in meiosis I, meiosis II, and mitosis. Chromosomes behave similarly in mitosis and meiosis II, but they behave very differently in meiosis I. (a) In meiosis I, the two sister kinetochores are located side-by-side on each homolog and attach to microtubules from the same spindle pole. The proteolytic cleavage of cohesin along the sister-chromatid arms unglues the arms and resolves the crossovers, allowing the duplicated homologs to separate at anaphase I, while the residual cohesin at the centromeres keeps the sisters together. Cleavage of centromeric cohesin allows the sister chromatids to separate at anaphase II. (B) In mitosis, by contrast, the two sister kinetochores attach to microtubules from different spindle poles, and the two sister chromatids come apart at the start of anaphase and segregate into separate daughter nuclei.
localized at the kinetochores in meiosis I, but we do not know in any detail how these proteins work. They are removed from kinetochores after meiosis I, so that in meiosis II the sister-chromatid pairs can be bi-oriented on the spindle as they are in mitosis.
Second, crossovers generate a strong physical linkage between homologs, allowing their bi-orientation at the equator of the spindle—much like cohesion between sister chromatids is important for their bi-orientation in mitosis (and meiosis II). Crossovers hold homolog pairs together only because the arms of the sister chromatids are connected by sister-chromatid cohesion (see Figure 17–58A).