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There is remarkable variability in genome size among eukaryotes that has little correlation with organismal complexity, ploidy or number of coding genes. For example, a newt has six times the genome size of a human. Much of this variation is due to non-coding, tandemly repeated DNA. Indeed, a substantial fraction of the genomes of many eukaryotes is composed of repetitive DNA in which short sequences are tandemly repeated in small to huge arrays.
Tandemly repetitive sequences, commonly known as "satellite DNAs" are classified into three major groups:
- Satellites are very highly repetitive with repeat lengths of one to several thousand base pairs. These sequences typically are organized as large (up to 100 million bp !) clusters in the heterochromatic regions of chromosomes, near centrosomes and telomeres; these are also found abundantly on the Y chromsome.
- Minisatellites are moderately repetitive, tandemly repeated arrays of moderately-sized (9 to 100 bp, but usually about 15 bp) repeats, generally involving mean array lengths of 0.5 to 30 kb. They are found in euchromatic regions of the genome of vertebrates, fungi and plants and are highly variable in array size.
- Microsatellites are moderately repetitive, and composed of arrays of short (2-6 bp) repeats found in vertebrate, insect and plant genomes. The human genome contains at least 30,000 microsatellite loci located in euchromatin. Copy numbers are characteristically variable within a population, typically with mean array sizes on the order of 10 to 100.
In general, satellite DNAs show exceptional variability among individuals, particularly with regard to the number of repeats at a given locus. Minisatellite loci are the most highly polymorphic sequence elements yet discovered in the human genome, and delineating the repeat lengths of these loci is the basis of most DNA typing systems used in forensic medicine.
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