Fundamentals of Molecular Biology - V.I. Rezyapkin 2009

Mobile Genetic Elements
Retrotransposons

Retroposons are DNA copies integrated into The Genome that are synthesized from various RNAs (Fig. 10.17). They are flanked by short direct repeats. The existence of retroposons demonstrates that Information Flow from RNA to DNA is indeed possible.

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Fig. 10.17. Diagram of retroposon formation

Retroposons are widely distributed among eukaryotes, having been found in the genomes of mammals, birds, amphibians, and insects. In mammals, retroposons account for more than 10% of the total DNA.

Some retroposons function as pseudogenes. Unlike normal genes, the latter are not expressed to produce a functionally active product. These particular pseudogenes—retroposons—arose through the reverse METABOLISM/31.html">Transcription of processed (or partially processed) mRNAs followed by the insertion of the resulting DNA copies into the genome (Fig. 10.18). These pseudogenes feature flanking poly-dA-poly-dT tracts, lack introns, and are flanked by short direct repeats. Their reading frames are often disrupted, and they may lack a promoter of their own. Pseudogenes can range from a single copy to several dozen copies within the genome.

In some cases, retroposons can be expressed to yield a functionally active product. This occurs if the retroposon was formed via the reverse transcription of a properly processed mRNA, possesses an intact reading frame, and contains a promoter. Such retroposons are referred to as retrogenes. For example, mice and rats have been found to possess two preproinsulin genes: one contains 2 introns, while the other contains a single intron and is flanked by short direct repeats. Evidently, the second Gene originated from the reverse transcription of a partially processed transcript of the first gene, followed by the Integration of the DNA copy into the genome.

Fig. 10.18. Diagram of pseudogene formation

Retroposons serve as DNA copies not only of mRNA, but also of other RNA types, such as tRNA, snRNA involved in splicing, and 7S RNA involved in protein secretion. For instance, the repetitive sequences of the human Alu family (3*10⁵ repeats, accounting for 5 - 6 % of total DNA) are homologous to specific regions of 7S RNA and apparently represent retroposons.

In summary, it should be noted that MGEs are highly heterogeneous in Structure and function, and are widely represented in living nature. They account for a significant portion of the genetic material in the genomes of many organisms. MGEs can trigger a variety of genetic effects: they are capable of regulating Gene Expression, inducing genetic rearrangements in their vicinity, transferring genes between Bacteria, and so forth.

Typically, MGE movements occur quite infrequently. At the same time, under certain conditions, their frequency can increase dramatically, resulting in so-called transposition bursts that lead to the simultaneous mobilization of several different MGEs.

Despite the wealth of knowledge gathered about MGEs to date, their complete biological nature will not be fully elucidated for quite some time.