Shortest nucleosomal repeat lengths during sea urchin development are found in two-cell embryos

Abstract

Prior to fertilization, sperm possess one of the longest nucleosome repeat lengths yet determined [.apprx. 250 base pairs (bp) for the sea urchin S. purpuratus]. The 2-cell embryo has an average repeat size of 189 .+-. 2 bp as probed by micrococcal nuclease; this is the shortest average nucleosomal subunit reported for S. purpuratus. By the 8 cell stage, the average nucleosome repeat increases to 201 .+-. 2 bp; it subsequently increases further during development. A dramatic rearrangement of chromatin occurs upon fertilization. This chromatin remodeling continues through early development. When 2 cell embryos are labeled for 30 min with [3H]thymidine and digested briefly, they exhibit nuclease-hypersensitive fragments averaging 308 bp in size, which are consistent with the size of protected DNA units in replication intermediate complexes at blastula stage. These results are consistent with 2 general propositions: long repeat lengths are found in highly differentiated cells, and short repeat lenghts are characteristic of cells more active in cell division. These data would also imply that a rapid increase in the DNA complement, e.g., in the transition from haploid to diploid state folllowing fertilization, is accompanied by a shortening of the average size of DNA in a nucleosome after replication.