These DSBs facilitate homology search by creating a single-strand section of DNA that searches for the appropriate homologous sequence (or repair template) on other chromosomes. In some organisms, like yeast, Arabidopsis, and mice, the process of identifying the homolog is initiated by the formation of programmed DSBs (reviewed in Zickler and Kleckner 2015 Mercier et al. elegans, homolog pairing occurs prior to the formation of DSBs and DSBs are made within the context of assembled synaptonemal complexįollowing the establishment of the meiotic chromatin structure, homologous chromosomes must identify each other and pair. For simplicity, the other end of the DSB is not depicted, but it will also be resected and coated in RAD51/DMC1. This single-strand DNA is coated in the recombinase proteins RAD51/DMC1 (purple) and underdoes a search process for homologous sequences. To facilitate homolog pairing and form crossovers, double-strand DNA breaks (DSBs, represented by the yellow star) are made and resected to reveal a region of single-strand DNA (ssDNA). The loop-axis structure is formed by cohesins (maroon rings) and axial elements (green). For simplicity, only one sister chromatid of each homolog is shown. Depicted is a pair of homologous chromosomes in dark and light blue with the lines of the chromosomes representing double-strand DNA (dsDNA) wrapped around histones. During leptotene, meiotic chromosomes begin to organize into a loop-axis DNA structure where loops of DNA extend out from a chromosome axis. In humans, meiotic errors are the leading causes of miscarriages and birth defects (reviewed in Nagaoka et al. When errors occur during meiosis, the resulting gametes are frequently aneuploid, with either too many or too few chromosomes. As part of the crossover regulation process, crossovers undergo a “designation” to limit the number of DSB sites licensed to mature into crossovers (Yokoo et al. Thus, all the preceding steps to the formation of a crossover are highly regulated to guarantee that at least one crossover is formed between each pair of homologous chromosomes. A crossover allows for accurate segregation of the homologs during meiosis I. To ensure a successful meiosis in most organisms, there are three events that must occur: (1) homologous chromosomes must pair (2) homologous chromosomes must repair double-strand DNA breaks (DSBs) to form crossovers, which forge a physical connection between the chromosomes and (3) homologous chromosomes must undergo two successive segregation events. Meiosis produces gametes, such as sperm and eggs, with exactly half the number of chromosomes as the original parent germ cell. Furthermore, we consider how sex-specific changes in the meiotic chromosome axes and the epigenetic landscape may function together to regulate crossing over in each sex, indicating that the mechanisms controlling crossing over may be different in oogenesis and spermatogenesis. Here we explore the sexually dimorphic features of both the chromosome axis and crossing over for each stage of meiotic prophase I in Mus musculus, Caenorhabditis elegans, and Arabidopsis thaliana. Additionally, epigenetic regulation of meiosis may contribute to sexually dimorphic recombination landscapes. Moreover, these changes likely cause sex-specific changes in the recombination landscape with the sex that has the longer chromosomes usually obtaining more crossovers. These sexually dimorphic features of meiosis include the meiotic chromosome architecture, in which both the lengths of the chromosomes and the requirement for specific meiotic axis proteins being different between the sexes. In addition, these unique end products demonstrate sex-specific differences that occur throughout meiosis to produce the final genetic material that is packaged into distinct gametes with unique extracellular morphologies and nuclear sizes. Males and females produce different end products of meiosis: eggs (females) and sperm (males). Meiosis is a conserved cell division process that is used by sexually reproducing organisms to generate haploid gametes.
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