Characterization of long and short repetitive sequences in the sea urchin genome

Long and short repetitive sequences were purified from the DNA of Paracentrotus lividus under conditions designed to optimize the yield of complete, end to end sequences. Double-stranded long repeat DNA prepared in this manner ranged in length from approximately 3000 to 15 000 nucleotide pairs with average sizes of approximately 6000 base pairs. In the electron microscope, long repeat DNA was observed to possess continuous sequences that often appeared to be terminated by one or more loops and/or fold backs. Long repeat DNA sequences, resheared to 300 base pairs, were found to have an average melting point identical to that for sheared native DNA. Thus, the reassociated duplexes of long repetitive DNA seem to possess very few mismatched base pairs. Reassociation kinetic analyses indicate that the majority of the long repeat sequences are reiterated only 4--7 times per haploid amount of DNA. Melt-reassociation analyses of short repetitive DNA, at several criteria, support the previously held concept that these sequences belong the sets or families of sequences which are inexact copies of one another. Our studies also support hypotheses suggesting that short repetitive sequences belong to families which may have arisen via distinct salttatory events. The relationships between long and short repetitive DNA sequences are considered with respect to widely held concepts of their sequence organization, evolution, and possible functions within eucaryotic genomes. A model for the possible organization of short repeats within long repetitive DNA sequences is also presented.