Interspersed repeat

Interspersed repetitive DNA is a form of repetitive DNA and is found in all eukaryotic genomes. Certain classes of these sequences propagate themselves by RNA mediated transposition, and they have been called retrotransposons. Interspersed repetitive DNA elements allow new genes to evolve. They do this by uncoupling similar DNA sequences from gene conversion during meiosis. The recombinational events of meiosis create heteroduplexes composed of strands from each parental chromosome. These heteroduplexes lead to mismatch repair. The net result is the homogenization and elimination of sequence differences during meiosis. Gene conversion can be viewed as the force acting to create sequence identity within the gene pool of a species. This is a cohesive force acting to match up DNA sequences of individual organisms that comprise a species. In effect the gene conversion causes the DNA sequences to clump together within a species and by doing so creates the natural boundaries between species. The gene pool of a species consists of DNA sequences linked in a network by gene conversion events. Interspersed repeat DNA constitutes 25–40% of most mammalian genomes.

Intrachromosomal and interchromosomal gene conversion
Gene conversion acts on DNA sequence homology as its substrate. There is no requirement that the sequence homologies lie at the allelic positions on their respective chromosomes or even that the homologies lie on different chromosomes. Gene conversion events can occur between different members of a gene family situated on the same chromosome. When this happens, it is called intrachromosomal gene conversion as distinguished from interchromosomal gene conversion. The effect of homogenizing DNA sequences is the same.

Role of Interspersed Repetitive DNA
Repetitive sequences play the role of uncoupling the gene conversion network, thereby allowing new genes to evolve. The shorter Alu or SINE repetitive DNA are specialized for uncoupling intrachromosomal gene conversion while the longer LINE repetitive DNA are specialized for uncoupling interchromosomal gene conversion. In both cases, the interspersed repeats block gene conversion by inserting regions of non-homology within otherwise similar DNA sequences. The homogenizing forces linking DNA sequences are thereby broken and the DNA sequences are free to evolve independently. This leads to the creation of new genes and new species during evolution. By breaking the links that would otherwise overwrite novel DNA sequence variations, interspersed repeats catalyse evolution, allowing the new genes and new species to develop.



Interspersed DNA elements catalyze the evolution of new genes
DNA sequences are linked together in a gene pool by gene conversion events. Insertion of an interspersed DNA element breaks this linkage, allowing independent evolution of a new gene. The interspersed repeat is an isolating mechanism enabling new genes to evolve without interference from the progenitor gene. Because insertion of an interspersed repeat is a saltatory event the evolution of the new gene will also be saltatory. Because speciation ultimately depends on the creation of new genes, this naturally causes punctuated equilibria. Interspersed repeats are thus responsible for punctuated evolution and rapid modes of evolution.