Repetitive DNA Elements

Most eukaryotic genomes contain repetitive DNA. Although most repeated sequences have no known function, their impact and importance on genomes is evident. Mobile repeated elements have been a critical factor in gene evolution. It has been suggested that some types of repeats may be linked to speciation, since during the evolutionary period when there was a high activity of mobile elements, radiation of different species occurred.

Figure 3. Representation of the division of the major types of mobile elements. Mobile elements can be subdivided into groups depending on their mechanism of amplification (DNA or RNA based), their structure (presence of LTR elements) and their ablilty for autonomous and nonautonomous amplification.

There are several diseases linked to—or caused by—repetitive elements. Expansion of triplet repeats has been tied to fragile X syndrome (a common cause of mental retardation), Huntington's disease, myotonic muscular dystrophy, and several other diseases. In addition, the discovery of STR instability in certain cancers suggests that sequence instability may play a role in cancer progression.

Mobile elements have caused diseases when a new mobile element disrupts an important gene. Neurofibromatosis type 1, for example, is caused by the insertion of an Alu element in the gene NF1. Alternatively, recombination between two repeated elements within a gene will alter its function, also causing disease. Many examples of cancers (e.g., acute myelogenous leukemia) and inherited diseases (e.g., alpha thalassemia) are caused by mobile-element-based recombinations.