Abstract
Sexual reproduction requires the production of haploid gametes (sperm and egg) with only one copy of each chromosome; fertilization then restores the diploid chromosome content in the next generation. This reduction in genetic content is accomplished during a specialized cell division called meiosis, in which two rounds of chromosome segregation follow a single round of DNA replication. In preparation for the first meiotic division, homologous chromosomes pair and synapse, creating a context that promotes formation of crossover recombination events. These crossovers, in conjunction with sister chromatid cohesion, serve to connect the two homologs and facilitate their segregation to opposite poles during the first meiotic division. During the second meiotic division, which is similar to mitosis, sister chromatids separate; the resultant products are haploid cells that become gametes. In Caenorhabditis elegans (and most other eukaryotes) homologous pairing and recombination are required for proper chromosome inheritance during meiosis; accordingly, the events of meiosis are tightly coordinated to ensure the proper execution of these events. In this chapter, we review the seminal events of meiosis: pairing of homologous chromosomes, the changes in chromosome structure that chromosomes undergo during meiosis, the events of meiotic recombination, the differentiation of homologous chromosome pairs into structures optimized for proper chromosome segregation at Meiosis I, and the ultimate segregation of chromosomes during the meiotic divisions. We also review the regulatory processes that ensure the coordinated execution of these meiotic events during prophase I.
Highlights
In Caenorhabditis elegans homologous pairing and recombination are required for proper chromosome inheritance during meiosis; the events of meiosis are tightly coordinated to ensure the proper execution of these events
Crossover (CO) recombination events must be completed between the DNA molecules of the aligned and synapsed homologs, a process started by the deliberate formation of DNA double strand breaks (DSBs)
REC-8 is present during meiotic DNA replication—it provides cohesion independently of DSBs, and its loading onto chromosomes depends on the HORMA-domain protein HTP-3, and on TIM-1 (Pasierbek et al, 2001; Chan et al, 2003; Severson et al, 2009; Severson and Meyer, 2014)
Summary
Sexual reproduction requires the generation of haploid gametes from diploid precursors through the specialized cell division program of meiosis. This reduction in ploidy is essential to ensure the restoration of diploidy upon fertilization and requires completion of several key events (Figure 1). During early prophase (leptotene and zygotene stages), each chromosome must locate and recognize its appropriate homologous pairing partner and align with it. Crossing over is essential for the formation of chiasmata, connections between homologs that become evident upon structural remodeling of chromosomes during later stages of meiotic prophase (diplotene and diakinesis). The availability of worms expressing GFP::histone has made it possible to screen for mutants based on lack of chiasmata connecting homologs at the end of meiotic prophase. We round off the chapter with an overview of surveillance mechanisms that monitor meiotic events for proper completion (Section 7) and a description of the events that occur during meiotic chromosome segregation (Section 8)
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