Retrotransposons classes

There are two classes of retrotrotransposons those with long terminal repeats (LTRs) and those without (LTRs). The first group is closely related to retroviruses, but its members lack genes for envelope proteins. They do carry gag and pol genes similar to those of retroviruses (Fig. 28-3). Most retrotransposons are defective and do not move. Over evolutionary time they accumulate in the genome, sometimes to the extent that the genome size grows enormously. This... 

Studies of overall genome composition based on reassociation kinetics (Simpson et ai, 1982 Cox et ai, 1990 Marx et a/., 2000) and analysis of fully sequenced bacterial artificial chromosome (BAC) clones from the 5. mansoni genome project show that platyhelminth genomes contain abundant highly and moderately repetitive sequence (Fig. 2.1). Much of the repetitive DNA comprises two classes of integrated mobile elements class I elements, which include long terminal repeat (LTR) retrotransposons and retroviruses, non-LTR retro-transposons and short interspersed nuclear elements (SINES) and transpose via an RNA intermediate, and class II elements (trans-posons), which transpose as DNA (Brindley et ai, 2003). Additionally, small dispersed or tandemly repeated sequences are common. A wide variety of these sequences have been isolated and characterized from a variety of taxa (Table 2.4). 

Drew, A.C. and Brindley, P.J. (1 997) A retrotransposon of the non-long terminal repeat class from the human blood fluke Schistosoma mansoni. Similarities to the chicken-repeat-1-like elements of vertebrates. Molecular Biology and Evolution 14, 602-610. 

The major forms of transposable elements in eukaryotes that lack terminal repeats belong to the class of retrotransposons. The LINEs and SINEs in mammals (long interspersed elements and short interspersed... 

Retrotransposons are a class of mobile elements that use a RNA Intermediate. Contrast the mechanism of transposition between retrotransposons that contain and those that lack long terminal repeats (LTRs). 

Transposable elements in eukaryotic cells show striking resemblances to retroviruses in sequence organization. The term retrotransposon is used to denote this class of elements. These similarities are illustrated in Figure 25.39 for two retroviruses Ty, a transposon of yeast copia and 412, transposable elements in Drosophila, and LAP, a transposon found in the mouse genome. 

Duncan L, Bouckaert K, Yeh F, Kirk DL. Kangaroo, a mobile element from Volvox carteri, is a member of a newly recognized third class of retrotransposons. Genes. 2002 162 1617-30. 

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