Interspersed repeat
Interspersed repetitive DNA is found in all eukaryotic genomes. Certain classes of these sequences propagate themselves by RNA mediated transposition, they have been called retrotransposons, and they constitute 25–40% of most mammalian genomes. Interspersed repetitive DNA elements allow new genes to evolve. They do this by uncoupling similar DNA sequences from gene conversion during meiosis.[1] 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, a process called concerted evolution. 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. Gene conversion is the underlying mechanism at the level of DNA creating the natural kind called species.[2] In effect 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.
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.[3] 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.[4] 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.
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References
- ↑ Schimenti JC, Duncan CH (February 1984). "Ruminant globin gene structures suggest an evolutionary role for Alu-type repeats". Nucleic Acids Res. 12 (3): 1641–55. doi:10.1093/nar/12.3.1641. PMC 318605. PMID 6322113.
- ↑ Stephen J. Gould (August 1979). "Species are not Specious". New Scientist 83 (1166): 374–376.
- ↑ Hess JF, Fox M, Schmid C, Shen CK (October 1983). "Molecular evolution of the human adult alpha-globin-like gene region: insertion and deletion of Alu family repeats and non-Alu DNA sequences". Proc. Natl. Acad. Sci. U.S.A. 80 (19): 5970–4. doi:10.1073/pnas.80.19.5970. PMC 390199. PMID 6310609.
- ↑ Brunner AM, Schimenti JC, Duncan CH (September 1986). "Dual evolutionary modes in the bovine globin locus". Biochemistry 25 (18): 5028–35. doi:10.1021/bi00366a009. PMID 3768329.
- Interspersed Repetitive Sequences at the US National Library of Medicine Medical Subject Headings (MeSH)
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