The highly condensed and tightly packaged DNA of hamster spermatozoa was found to be organized into topologically constrained DNA loop domains attached at their bases to a nuclear matrix. The loop domains of the sperm nuclei differed from somatic cell loop domains from the same animal in two aspects. Sperm loop domains were 60% smaller than somatic cell loop domains, with an average DNA length of 46±7 kb in sperm as compared with 16±11 kb in brain. Secondly, unlike virtually all somatic cell DNA known which is negatively supercoiled, sperm DNA was devoid of detectable supercoiling. The presence of the loop domain structure in the highly condensed DNA of mammalian spermatozoa suggests that this motif is a fundamental aspect of eukaryotic DNA organization.
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