Interspersed repeats, the second type of repetitious DNA in eukaryotic genomes, is composed of a very large number of copies of relatively few sequence families (see Table 8-1). Also known as moderately repeated DNA, or intermediate-repeat DNA, these sequences are interspersed throughout mammalian genomes and make up 25–50 percent of mammalian DNA (~45 percent of human DNA).

Because interspersed repeats have the unique ability to “move” in the genome, they are collectively referred to as transposable DNA elements or mobile DNA elements (we use these terms interchangeably). Although transposable DNA elements were originally discovered in eukaryotes, they are also found, although less frequently, in prokaryotes. The process by which these sequences are copied and inserted into a new site in the genome is called transposition. Transposable DNA elements are essentially molecular symbionts that in most cases appear to have no specific function in the biology of their host organisms, but exist only to maintain themselves. For this reason, Francis Crick referred to them as “selfish DNA.”

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When transposition occurs in germ cells, the transposed sequences at their new sites are passed on to succeeding generations. In this way, mobile elements have multiplied and slowly accumulated in eukaryotic genomes over evolutionary time. Since mobile elements are eliminated from eukaryotic genomes very slowly, they now constitute a significant portion of the genomes of many eukaryotes.

Mobile elements are not only the source for much of the DNA in our genomes, but also provide a second mechanism, in addition to meiotic recombination, for bringing about chromosomal DNA rearrangements during evolution (see Figure 8-2). One reason for this is that during transposition of a particular mobile element, adjacent DNA is sometimes also mobilized (see Figure 8-19). Transpositions occur rarely: in humans, there is about one new germ-line transposition for every eight individuals. Since 97 percent of our DNA is noncoding, most transpositions have no deleterious effects. But over time, they have played an essential part in the evolution of genes that have multiple exons and of genes whose expression is restricted to specific cell types or developmental periods. In other words, although transposable elements probably evolved as cellular symbionts, they have had an important function in the evolution of complex multicellular organisms.

Transposition may also occur within a somatic cell; in this case, the transposed sequence is transmitted only to the daughter cells derived from that cell. In rare cases, such somatic-cell transposition may lead to a somatic-cell mutation with detrimental phenotypic effects, such as the inactivation of a tumor-suppressor gene (see Chapter 24). In this section, we first describe the structure and transposition mechanisms of the major types of transposable DNA elements and then consider their likely role in evolution.