repetitive DNA DNA whose base sequence is repeated many times throughout the genome of an organism. It is common in eukaryotes, accounting for about half of the total DNA in mammals, for example, and can be divided into various types. Some serves a useful purpose, but a significant proportion is of uncertain function, and may be ‘junk’, or selfish DNA. One important type consists of multiple copies of particular genes or gene sequences; these may represent the members of gene families or be duplicates of genes encoding histones or ribosomal RNAs, which often form tandem arrays. Repeats of short DNA sequences, typically less than 10 bp, flank the centromeres of each chromosome, stretching for hundreds of kilobases along either arm of the chromosome and forming centromeric heterochromatin. On centrifugation of the total DNA, this separates out as a distinct band, called satellite DNA. Tandemly repeated short sequences also occur at each chromosome tip (telomeric DNA). Both types are important for maintaining chromosome structure. Other distinct types of repetitive DNA lie dispersed throughout the genome, both in noncoding introns within genes and between genes, where they may act as ‘spacer’ DNA. Among these are variable number tandem repeats (VNTRs), sequences of 15–100 nucleotides repeated hundreds or thousands of times at numerous sites within the genome, and represented by minisatellite DNA. Repeats of shorter sequences (2–10 bp) form so-called microsatellite DNA. Many transposons also occur as numerous copies throughout the genome, and so contribute to repetitive DNA.
repetitive DNA See SATELLITE DNA.
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