Editing Transposable element (section)

=== DNA transposons ===
{{Main|DNA transposon}}
[[File:DNA Transposon.png|thumb|upright=1.4|'''A'''. Structure of DNA transposons (Mariner type). Two inverted tandem repeats (TIR) flank the transposase gene. Two short tandem site duplications (TSD) are present on both sides of the insert.<br />
'''B'''. Mechanism of transposition: Two transposases recognize and bind to TIR sequences, join and promote DNA double-strand cleavage. The DNA-transposase complex then inserts its DNA cargo at specific DNA motifs elsewhere in the genome, creating short TSDs upon integration.<ref>{{Cite thesis | vauthors = Walter M |year=2016 |title=Transposon regulation upon dynamic loss of DNA methylation |publisher=[[Université Pierre et Marie Curie]]|doi=10.13140/rg.2.2.18747.21286}}</ref>
]]

The cut-and-paste transposition mechanism of class II TEs does not involve an RNA intermediate. The transpositions are catalyzed by several [[transposase]] enzymes. Some transposases non-specifically bind to any target site in DNA, whereas others bind to specific target sequences. The transposase makes a staggered cut at the target site producing [[sticky ends]], cuts out the DNA transposon and ligates it into the target site. A [[DNA polymerase]] fills in the resulting gaps from the sticky ends and [[DNA ligase]] closes the sugar-phosphate backbone. This results in target site duplication and the insertion sites of DNA transposons may be identified by short direct repeats (a staggered cut in the target DNA filled by DNA polymerase) followed by [[inverted repeat]]s (which are important for the TE [[DNA repair|excision]] by [[transposase]]).

Cut-and-paste TEs may be duplicated if their transposition takes place during [[S phase]] of the [[cell cycle]], when a donor site has already been replicated but a target site has not yet been replicated.{{citation needed|date=September 2022}} Such duplications at the target site can result in [[gene duplication]], which plays an important role in genomic [[evolution]].<ref name="Brock">{{cite book |veditors=Madigan M, Martinko J |title=Brock Biolog of Microorganisms |edition=11th |publisher=Prentice Hall |year=2006 |isbn=978-0-13-144329-7}}</ref>{{rp|284}}

Not all DNA transposons transpose through the cut-and-paste mechanism. In some cases, a [[replicative transposition]] is observed in which a transposon replicates itself to a new target site (e.g. [[helitron (biology)|helitron]]).

Class II TEs comprise less than 2% of the human genome, making the rest Class I.<ref name="The impact of L1 retrotransposons o">{{cite journal | vauthors = Kazazian HH, Moran JV | s2cid = 33460203 | title = The impact of L1 retrotransposons on the human genome | journal = Nature Genetics | volume = 19 | issue = 1 | pages = 19–24 | date = May 1998 | pmid = 9590283 | doi = 10.1038/ng0598-19 }}</ref>