[Incorporation at [3H]thymidine into the repetitive and unique nucleotide sequences in DNA of the sea urchin Strongylocentrotus droebachiensis at the stage of synchronous cleavage divisions].

The DNA synthesis in the sea urchin Strongylocentrotus droebachiensis at the stage of 4-8 blastomers starts in the telophase and is completed in the prophase. The highest [3H]thymidine incorporation was observed in telophase chromosomes. The changes in the relative rate of [3H] thymidine incorporation during DNA replication are not the same for the repetitive and unique nucleotide sequences. Highly repetitive nucleotide sequences incorporate [3H]thymidine mainly at the beginning and at the end of DNA synthesis. Moderately repetitive nucleotide sequences do not demonstrate any significant changes in the rate of [3H]thymidine incorporation during DNA replication. The unique nucleotide sequences have the lowest rate of [3H]thymidine incorporation at the beginning (telophase) and at the end (prophase) of DNA synthesis and the highest rate--in the middle of the interphase. The data obtained suggest that there exists some temporal organization of replication process of repetitive and unique nucleotide sequences in the sea urchin genome at the stage of the synchronous cleavage divisions.