DNA repair replication has been previously demonstrated to occur in mouse spermatocytes during the pachytene stage. The results reported in this study provide a more detailed characterization of pachytene repair by focusing upon specific properties of the sites of replication. Our data demonstrate that single-strand breaks persist within replicated sequences throughout a period which corresponds to a defined interval of the pachytene stage. A large fraction of the sites may be nicked more than once within the same DNA strand, allowing the selective release of replicated DNA sequences from gently denatured spermatocyte DNA. DNA fragments thus prepared from pachytene spermatocytes are not of random sequence composition, but are derived from a specific subset of the mouse genome. Sites of replication are also associated with chromatin of distinctive structure in pachytene spermatocytes, as evidenced by the sensitivity of replicated chromatin to DNase II, and its solubility in the presence of Mg2+. In each of these respects, sequences replicated in pachytene spermatocytes closely resemble their counterparts in the LiHum genome.
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