Transposable Elements: Evolution


Transposable elements are DNA segments that only encode the enzymatic activity necessary to make more copies of themselves. Most of the human genome is composed of inactive copies of three distinct classes of elements that have accumulated over millions of years. These classes differ in their mechanisms of integration, origins and modes of evolution.

Keywords: transposon; retrotransposon; LINEs; integration; reverse transcriptase

Figure 1.

Mechanism of expansion of the three classes of eukaryotic transposable elements. (a) Excision and integration of the transposon is carried out by the element‐encoded transposase. The cleaved donor site can be religated by DNA repair, which frequently generates small deletions. Alternatively the homologous chromosome can serve as template to fill in the gap, which results in resynthesis of the element at the donor site. (b) Long terminal repeat (LTR) retrotransposons are transcribed and the RNA used as template by the element‐encoded reverse transcriptase to generate a double‐stranded DNA intermediate. Integration of the element is carried out by the transposase‐like enzyme integrase. (c) Reverse transcription of the RNA transcript of a long interspersed nuclear element (LINE; non‐LTR retrotransposon) occurs directly onto the cleaved site of the chromosome. Most subsequent steps are assumed to be carried out by the host cellular repair machinery.

Figure 2.

Possible origin of eukaryotic transposable elements. Eukaryotic transposons are similar in sequence and mechanism of insertion to the transposons of bacteria and presumably evolved with little change directly from these prokaryotic elements. LINE elements are most similar in sequence and mechanism of insertion to prokaryotic group II introns. Group II introns are speculated to have been lost in eukaryotes, with aspects of their integration machinery surviving as LINEs. No equivalent of an LTR retrotransposon has been found in prokaryotes. Their mechanism of expansion suggests that they resulted from the fusion of a transposon and an LINE.


Further Reading

Boeke JD and Stoye JP (1997) Retrotransposons, endogenous retroviruses, and the evolution of retroelements. In: Coffin JM, Hughes SH and Varmus HE (eds.) Retroviruses, pp. 343–435. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press.

Craig N, Craigie R, Gellert M and Lambowitz A (eds.) (2001) Mobile DNA II. Washington, DC: American Society of Microbiology Press.

Doolittle WF and Sapienza C (1980) Selfish genes, the phenotype paradigm and genome evolution. Nature 284: 601–603.

International Human Genome Sequencing Consortium (2001) Initial sequencing and analysis of the human genome. Nature 409: 860–921.

Kazazian HH and Moran JV (1998) The impact of L1 retrotransposition on the human genome. Nature Genetics 19: 19–24.

Li W‐H, Gu Z, Wang H and Nekrutenko A (2001) Evolutionary analysis of the human genome. Nature 409: 847–849.

Malik HS and Eickbush TH (2001) Phylogenetic analysis of ribonuclease H domains suggests a late, chimeric origin of LTR retrotransposable elements and retroviruses. Genome Research 11: 1187–1197.

Malik HS, Henikoff S and Eickbush TH (2000) Poised for contagion: evolutionary origins of the infectious abilities of insect errantiviruses and nematode retroviruses. Genome Research 10: 1307–1318.

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Eickbush, Thomas H, and Eickbush, Danna G(Jul 2006) Transposable Elements: Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1038/npg.els.0005130]