Changes in the mode of DNA packaging in nuclei during spermatogenesis were studied by measuring of the fluorescence anisotropy decay of an ethidium dye intercalated in the DNA in whole nuclei. The nuclei were isolated from boar spermatid or sperm cells at three distinct stages of spermatogenesis: just before the completion of a maturation process in the testis (late spermatid), immediately after a subsequent transformation into spermatozoa (caput spermatozoon), and after full maturation (cauda spermatozoon). Although these three kinds of nucleus were morphologically indistinguishable from each other, the anisotropy decay detected a clear difference. In the late spermatid nuclei, in which the replacement of histones by protamine was still in progress, the anisotropy decayed extensively. The decay suggested that the DNA in the spermatid nuclei contained very flexible regions, in which the interaction of the DNA and proteins may be weak. The rapid and extensive anisotropy decay was absent in the caput and cauda nuclei. The flexible portions must have turned into very rigid structures during transformation from the late spermatid into the caput spermatozoon.
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