Structure Of Genomes

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Eukaryotic genomes are composed of three types of sequences: non-repetitive sequences, that are unique; moderately repetitive sequences, that are dispersed and repeated a small number of times in the form of related but non identical copies; and highly repetitive sequences, that are short and usually repeated as a tandem array. Most structural genes are located in non-repetitive DNA. The coding sequences of the 23,000 human genes comprise about 1% of the genome, and about the same genome share can be assigned to their regulatory sequences, to give a total of only 2% of the full DNA.

Composition of the human genome. The percentage shares of various functional and non-functional sequences are shown.

The repetitive sequences are divided in interspersed repeats and tandem repeats. The latter include satellites, minisatellites and microsatellites, or variable number tandem repeats or short tandem repeats (STRs), which occupy 3% of the human genome. The STRs, which are short (from 1 to 6 nucleotides generally) and tandemly repetitive sequences, tend to be unstable at cell meiosis and mitosis, easily producing a variation in the number of repeats as DNA replication errors. In the human genome a combination of 13 STRs is enough to identify uniquely a single individual.

The human genome is basically uniform, and only about 0,3% of its sequence differs between individuals in a population. This small proportion of DNA is represented by small insertions and deletions, but most of all by single nucleotide polymorphisms (SNPs), binary changes in only one base that on average occur every 300 bases, and that in the human genome are 10 millions. SNPs can also be used to distinguish between different individuals.