Structured illumination microscopy imaging reveals localization of replication protein A between chromosome lateral elements during mammalian meiosis

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. Cell division: Recombination in action Using very high resolution microscopy, Korean researchers are able to visualize meiosis, a two-step division process that ensures that each egg or sperm cell has a single set of chromosomes, in unprecedented detail. After DNA replication and before the first division cycle, pairs of chromosomes align to exchange genetic material, a process known as recombination. This process is facilitated by multi-protein structures called synaptonemal complexes (SCs) that hold the chromosomes together. Keun Kim and colleagues at Chung-Ang University, Seoul, South Korea used structured illumination microscopy to observe SCs during meiosis in mouse sperm-forming cells. They found that the position of replication protein A, a key regulator of recombination, on the SC changes as recombination takes place. These findings suggest that RPA location on SCs could be used to monitor the progression of meiotic recombination.