Editing Transposable element (section)

=== Retrotransposon ===
{{Main|Retrotransposon}}
Class I TEs are copied in two stages: first, they are [[Transcription (genetics)|transcribed]] from DNA to [[RNA]], and the RNA produced is then [[reverse transcription|reverse transcribed]] to DNA. This [[cDNA|copied DNA]] is then inserted back into the genome at a new position. The reverse transcription step is catalyzed by a [[reverse transcriptase]], which is often encoded by the TE itself. The characteristics of retrotransposons are similar to [[retrovirus]]es, such as [[HIV]].

Despite the potential negative effects of retrotransposons, like inserting itself into the middle of a necessary DNA sequence, which can render important genes unusable, they are still essential to keep a species' [[ribosomal DNA]] intact over the generations, preventing infertility.<ref>[https://wi.mit.edu/news/not-so-selfish-genetic-parasite-helps-preserve-fertility A not-so-selfish “genetic parasite” helps to preserve fertility]</ref>

Retrotransposons are commonly grouped into three main orders:
* Retrotransposons, with [[long terminal repeat]]s (LTRs), which encode reverse transcriptase, similar to retroviruses
* Retroposons, [[LINEs|long interspersed nuclear elements]] (LINEs, LINE-1s, or L1s), which encode reverse transcriptase but lack LTRs, and are transcribed by [[RNA polymerase II]]
* [[Short interspersed nuclear element]]s (SINEs) do not encode reverse transcriptase and are transcribed by [[RNA polymerase III]]

Retroviruses can also be considered TEs. For example, after the conversion of retroviral RNA into DNA inside a host cell, the newly produced retroviral DNA is integrated into the [[genome]] of the host cell. These integrated DNAs are termed ''[[provirus]]es''. The provirus is a specialized form of [[eukaryotic]] retrotransposon, which can produce RNA intermediates that may leave the host cell and infect other cells. The transposition cycle of retroviruses has similarities to that of [[prokaryotic]] TEs, suggesting a distant relationship between the two.