[3H]Thymidine incorporation into DNA fractions with differing frequencies of repetitive nucleotide sequences in the sea urchin strongylocentrotus droebachiensis genome].  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.