Meiotic aspects of chromosome organization : (meiosis, pachytene DNA)
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Homologous chromosome pairing regulates the expression of a set of metabolic activities that is unique to the zygotene-pachytene interval , These activities probably relate to crossing-over and they involve specific proteins and specific chromosome regions. The recombination-related proteins include an endonuclease, a DNA-unwinding protein, and are association protein. These proteins act primarily on specific DNA sequences, their activity being manifest in the formation of nicks and gaps accompanied by repair synthesis. The site specificity of nick-repair activity is determined by site specific changes in chromatin organization. These changes do not occur in the absence of homologous pairing. The DNA undergoing nick-repair is housed in a set of families of moderately repeated sequences ("P-DNA"). They range in length from 800-4000 bp; they show very small sequence divergence and share homology with corresponding repeats across a broad phylogenetic spectrum. They are unevenly distributed in the genome. About 60% of the Lilium genome has no P-DNA sequences within at least 350 kb from one another; the remainder of the genome has P-DNA sequences spaced 30-350kb apart. Nick-repair activity is mainly confined to the end regions of P-DNA ("PsnDNA") which measure 125-400 bp in length . PsnDNA is housed in chromatin of distinctive composition. The chromatin has little, if any, histones; instead, the DNA is associated with a snRNA ("PsnRNA") and a non-histone protein. The PsnRNA is complementary to the PsnDNA and the protein binds specifically to PsnRNA. A model is briefly discussed to account for the regulation of pachytene DNA metabolism.