Abstract
An important event enabling meiotic prophase I to proceed is the close juxtaposition of conjoined chromosome axes of homologs and their assembly via an array of transverse filaments and meiosis-specific axial elements into the synaptonemal complex (SC). During meiosis, recombination requires the establishment of a platform for recombinational interactions between the chromosome axes and their subsequent stabilization. This is essential for ensuring crossover recombination and proper segregation of homologous chromosomes. Thus, well-established SCs are essential for supporting these processes. The regulation of recombination intermediates on the chromosome axis/SC and dynamic positioning of double-strand breaks are not well understood. Here, using super-resolution microscopy (structured illumination microscopy), we determined the localization of the replication protein A (RPA) complex on the chromosome axes in the early phase of leptonema/zygonema and within the CEs of SC in the pachynema during meiotic prophase in mouse spermatocytes. RPA, which marks the intermediate steps of pairing and recombination, appears in large numbers and is positioned on the chromosome axes at the zygonema. In the pachynema, RPA foci are reduced but do not completely disappear; instead, they are placed between lateral elements. Our results reveal the precise structure of SC and localization dynamics of recombination intermediates on meiocyte chromosomes undergoing homolog pairing and meiotic recombination.
Highlights
During meiosis, replicated chromosomes search for their homologous templates and form synapses to undergo meiotic recombination, a process that is essential for producing crossover (CO) and assuring correct chromosome separation during the first meiotic division[1,2]
We demonstrated that SYCP1 appeared between SYCP3 at the zygonema and axis elements were fully synapsed (Fig. 2)
At the zygonema and pachynema stages, the two lateral elements (LEs) were observed as separated lines in the electron microscopy (EM) images, and the central element (CE) stained with SYCP1 was localized in an internal border between LEs
Summary
During meiosis, replicated chromosomes search for their homologous templates ( known as homologs) and form synapses to undergo meiotic recombination, a process that is essential for producing crossover (CO) and assuring correct chromosome separation during the first meiotic division[1,2]. The SC begins to form during the leptonema stage of meiotic prophase I as the first homologs become connected by a central region composed of TFs, which become visible between axial elements. The axial element forms a proteinaceous structure related to the two sister chromatids of the homologs. During the zygonema-to-pachynema transition, the LEs and the CEs are completely assembled in a process known as synapsis. Synapsis is fully completed at the mid-pachynema of meiosis I; subsequently, the homologs separate, and the SCs disassemble in diplonema[3,8]
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