Repetitive Dna Elements
The human genome contains approximately three billion base pairs of DNA. Within this there are between 30,000 and 70,000 genes, which together add up to less than 5 percent of the entire genome. Most of the rest is made up of several types of noncoding repeated elements.
Most gene sequences are unique, found only once in the genome. In contrast, repetitive DNA elements are found in multiple copies, in some cases thousands of copies, as shown in Table 1. Unlike genes, most repetitive elements do not code for protein or RNA. Repetitive elements have been found in most other eukaryotic genomes that have been analyzed. What functions they serve, if any, are mainly unknown. Their presence and spread causes several inherited diseases, and they have been linked to major events in evolution.
Types of Repetitive Elements
Repetitive elements differ in their position in the genome, sequence, size, number of copies, and presence or absence of coding regions within them. The two major classes of repetitive elements are interspersed elements and tandem arrays.
Interspersed repeated elements are usually present as single copies and distributed widely throughout the genome. The interspersed repeats alone constitute about 45 percent of the genome.
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