Chromatin organization in nuclei of sea urchin embryos. Comparison with the chromatin organization of the sperm.

The chromatin in sea urchin embryo nuclei and that in sperm heads are both organized in nucleosomes but show marked differences when analyzed by endonuclease digestion. Sperm chromatin DNA appears to be totally organized in nucleosomes that are highly resistant to nuclease hydrolysis. The kinetics of formation of acid-soluble oligonucleotides is slow and concerns only about 50% of the total DNA. In contrast, the DNA of embryo chromatin does not appear to be totally organized in nucleosomes since 5 to 10% is rapidly and preferentially hydrolysed into acid-soluble oligonucleotides without any appreciable fragmentation of the remaining parts. Futher digestion causes the formation of the usual pattern of DNA bands, as detected by gel electrophoresis. The length of the DNA segment associated with the embryo nucleosomes appears to be shorter than that of the DNA segment associated with the sperm nucleosomes. The kinetics of formation of acid-soluble oligonucleotides upon digestion of embryo chromatin is much faster than that of sperm chromatin and concerns almost all the chromatin DNA.