Size classes of replication units in DNA from sea urchin embryos.

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Abstract
Sea urchin [Arbacia punctulata] DNA containing replication structures isolated from 2-4 cell stage and blastula stage embryos was examined by EM. The expected eye forms and molecules with large internal single-stranded gaps were observed. Such structures were not present in DNA devoid of replicating molecules such as that isolated from sea urchin sperm. When the size of eye forms and interbubble distances between the 2 stages were compared, there was no detectable difference. In both stages, 2 distinct size classes of bubbles and of interbubble distances were observed. In the case of bubble sizes, the smaller size class was comprised of clustered microbubbles that ranged from 200 base pairs [bp] to 1 kilobase (kb) with a mean of 432 bp. The large eye forms measured 1-35 kb with a mean of 6.8 kb. Interbubble distances also yielded 2 distinct populations, with the smaller class ranging from 400 bp to 2.3 kb (mean = 1.1 kb) and the larger population ranging from 2.8-36 kb (mean = 10.9 kb). A substantial fraction of the larger eye-form population may arise from the fusion of the clustered microbubbles.

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