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The dynamics of [3H]thymidine incorporation into DNA fractions with highly repetitive, moderately repetitive and unique nucleotide sequences in the genome of sea urchin embryos at the cleavage stage (8-32 blastomers) were studied. The ratio of specific radioactivity of the DNA fractions to the specific radioactivity of the original DNA showed that for each of the points of incubation with [3H]thymidine the relative specific radioactivities of different DNA fractions differed from each other. The highly repetitive DNA fraction has a very high relative specific radioactivity at the beginning of incubation (270% in 10 min), but in 40 min its the relative specific radioactivities of the DNA fractions with moderately repetitive and unique nucleotide sequences are not high at the beginning of incubation (40 and 75%, respectively) and then gradually increase up to 80 and 120%, respectively.

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.