The organization and abundance of repetitive DNA sequences among great apes and humans

Most studies of the human genome are focused upon coding DNA sequences which provide the structural genes and enzymatic machinery necessary for life. However, repetitive DNA sequences constitute a substantial portion of eukaryotic genomes. Formerly referred to as "junk" DNA, their importance is just now being realized as many of these repetitive sequences have been implicated in human disease, chromosomal structure, and gene regulation. Our view of human genome organization and human evolution is incomplete, without an understanding of how repetitive DNA has evolved to its present abundance and distribution. To address this problem genomic libraries were constructed from DNA samples of a human (Homo sapiens) and representatives from each of the four extant great apes; gorilla (Gorilla gorilla) chimpanzee (Pan troglodvtes)bonobo Pan paniscus). and orangutan (Pongo pvgmaeus). The libraries were screened with radiolabeled, synthetic oligonucleotides for the presence of the major classes of repetitive DNAs found in humans. These repetitive classes included tandemly repeated microsatellites, a minisatellite. and satellite DNAs as well as dispersed repetitive sequences represented by a LINE-1 element, a SINE, and a human retrotransposon, THE1. Addressing genome organization with this technique has proven useful because it provided a minimal estimate of human repetitive element copy number similar to the estimates obtained in previous studies through examination of genomic and chromosome 16 specific DNA libraries. However, the abundance of some repetitive probes varied considerably among the ape libraries, indicating variation in repetitive element copy number can be substantial among closely related species. The technique also indicated non-random associations between selected repetitive sequences and supported a previously erected hypothesis that a-satellite DNA and Alu sequences are negatively associated.