Retrotransposon

=Retrotransposon=

A **retrotransposon ** (also **Class I transposon **) is a genetic element capable of replicating itself via an RNA intermediate, and reinserting its progeny elsewhere in the genome. For this reason, it is known as the 'copy and paste' class of transposons.

Retrotransposons code for an mRNA that itself codes for the enzyme reverse transcriptase. Reverse transcriptase then takes the mRNA which synthesised it, converts to a complementary DNA (cDNA) strand and the enzyme DNA polymerase makes it double-stranded and ready for reinsertion into the genome.

Retrotransposons act similarly to retroviruses such as HIV. **Retroviruses** are viruses with an RNA genome that also use reverse transcriptase to create a cDNA copy for insertion into a DNA genome.

There are three major classes of retrotransposon:
 * Viral - encode reverse transcriptase and have long terminal repeats (LTRs), similarly to retroviruses
 * LINEs - encode reverse transcriptase, but lack LTRs, and are transcribed by RNA polymerase II
 * Non-viral - do not encode reverse transcriptase, and are transcribed by RNA polymerase III