retrotransposons

Retrotransposons are a subgroup of transposable elements that occur only in eukaryotic genomes, and which employ reverse transcriptase for their operation. Retrotransposons are particularly common in plants.

▼ Alu elements : biological evolution : cDNA : deletions : eukaryotic vs prokaryotic : integrase : inversions : inverted repeats : LINEs : LTRs : long terminal repeats : mutations : non-replicative transposition : recombination : replicative transposition : retroposons : retrotransposons : retrovirus : reverse transcriptase : RT : SINEs : stable mutations : translocations : transposable elements : transposition types I & II : transposase : transposons : viral retroposons : ▼

Retroposons are repetitive DNA fragments that were inserted into chromosomes after they had been reverse transcribed from any RNA molecule. In contrast to retrotransposons, they never encode reverse transcriptase (RT). Therefore, these elements are not capable of autonomous transposition activity, unlike transposons.

Transposable elements are relatively long DNA sequences that can act as mobile genetic elements in prokaryotic and eukaryotic genomes. Transposable elements represent a large part of the genomes in many species, and transpose by a mechanism that involves DNA synthesis followed by random integration at a new target location in the genome.

Most transposable elements contain inverted repeats at their termini. All transposable elements encode for transposase, a special enzyme activity that assists in the insertion of transposons at a new site. Prokaryotic transposable elements differ from their eukaryotic counterparts in the mechanism of transposition. Only eukaryotic genomes contain retroposons, the special type of transposable elements that employ reverse transcriptase to transpose an RNA intermediate. Alu elements are about 300 nucleotides in length and include a distinctive sequence that ends in a poly-A tail. The human gene's protein-generating capacity is considerably increased by the presence of Alu elements. Alu sequences have the potential to continue to greatly enrich the stock of meaningful genetic information available for producing new human proteins.

Transposition may result in splicing of DNA fragments into or out of the genome. During replicative transposition, the transposon is first replicated giving a new copy that is transferred to a new site, while the old copy is retained at the original site (type I transposition). Nonreplicative transposition however describes the movement of a transposon that is excised from a donor site and is integrated in a new site, usually generating a double (type II transposition). Transposons normally influence the expression of the genes in proximity of their insertion sites.

Transposons and retroposons seem to play a mechanism for biological evolution by promoting rearrangement and restructuring of genomes. Retrotransposon-induced mutations are relatively stable, because the sequence at the insertion site is retained as they transpose via the replication mechanism. Transposition may directly cause both deletion and inversion mutagenesis. Furthermore, transposable elements mediate the movement of host DNA sequences to new locations, enriching the genome with identical sequences positioned at different locations, and promoting homologous recombination. Such recombination may eventually result in deletions, inversions, and translocations.

In eukaryotes, retroposons are more common than transposons. They are either retroviral or nonviral. Viral retroposons encode for the enzymes reverse transcriptase and integrase and, in the same manner as retroviruses, are flanked by long terminal repeats (LTRs). Short interspersed elements (SINEs) and long interspersed elements (LINEs) are the most abundant nonviral retroposons. These are usually repeated many times in the mammalian genome.

Both SINEs and LINEs lack LTRs and are thought to transpose through a special retrotransposition mechanism that involves transcription of one strand of the retroposon into RNA. The RNA then undergoes conformation change (looping) and provides a primer for the synthesis of single stranded cDNA (cellular DNA). The cDNA later serve as template for the synthesis of a double stranded DNA that is inserted in the genome by as yet unknown mechanisms.

▲ Alu elements ₪ Alu elements : biological evolution »» Biological Evolution : cDNA : deletions »» Deletion : eukaryotic vs prokaryotic : integrase : inversions »» Inversion : inverted repeats : LINEs : LTRs : long terminal repeats : mutations »» Mutation : non-replicative transposition : recombination »» Recombination : replicative transposition : retroposons : retrotransposons : retrovirus ۰۰ retroviruses : reverse transcriptase : RT : SINEs : stable mutations : translocations »» Translocation : transposable elements ~ transposable elements : transposition types I & II : transposase : transposons : viral retroposons : ▲

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External : Transposons part 1, transposons part 2 : Barbara McClintock and mobile genetic elements :