Noncoding Tandemly Repeated DNA Sequences

Abstract

Noncoding, tandemly repeated dna sequences form a substantial fraction of the genomes of eukaryotes. In bacteria, however, large tandem arrays are not present. Some of the variation in genome size between different eukaryotic species, which bears little relation to differences in organismal complexity or the numbers of protein-coding genes (the c-value paradox), appears to be due to these nongenic sequences. This class of DNA includes satellite DNA (very highly repetitive sequences), minisatellite DNA (moderately repetitive sequences), and microsatellite DNA (short tandem arrays). In many cases, tandem arrays of noncoding DNA sequences seem to be maintained solely by their ability to replicate quickly within the genome (the selfish DNA hypothesis). Their behavior can result in mutations that cause human genetic disorders (e.g. fragile X syndrome). Features of the organization of tandemly repeated DNA sequences in eukaryotic genomes reflect the genetic mechanisms and evolutionary pressures acting on selfish DNA. New data from the human genome project provide a first glimpse on centromeric satellite DNA, a hitherto unexplored bastion of our genomes, and allow us to test previously proposed models of the evolution of highly repetitive sequences.