A transposable element is a DNA sequence that can move to a new genomic location or create a new copy at another location. Movement of transposable elements is called transposition.

Transposition does not require a long region of matching DNA sequence at the destination.

Two broad strategies

DNA transposons

Many DNA transposons move through a DNA intermediate. An enzyme called a transposase recognizes sequences at the element's ends and catalyzes reactions that insert the element into a new DNA site.

Some mechanisms are effectively cut-and-paste: the element is excised from one location and inserted elsewhere. Other transposition mechanisms can create additional copies during the process.

Retrotransposons

A retrotransposon uses an RNA intermediate. The element is transcribed into RNA, reverse transcription produces a new DNA copy, and that copy is integrated at a new genomic location.

Schematically:

original DNA element
   ↓ RNA intermediate
new DNA copy
   ↓ integration
additional genomic copy

Because the original DNA copy need not be removed, this strategy can increase the number of element copies in a genome.

Transposition can alter genes and genome structure

Insertion into a protein-coding sequence can disrupt the encoded product. Insertion into a promoter or other regulatory region can alter gene expression. Repeated copies of a transposable element can also provide similar sequences at different genomic positions, creating opportunities for DNA exchange that produces deletions, duplications or other rearrangements.

Transposable elements can therefore create mutations and also act as long-term sources of new genome architecture.

Mobile does not mean constantly moving

A genome can contain many transposable-element copies even when only a small fraction are currently capable of moving. Cells possess mechanisms that can suppress mobile-element activity, because uncontrolled transposition threatens genome integrity.

Over long time scales, inserted elements can accumulate sequence changes or be co-opted into new regulatory and structural roles.

Transposable elements therefore illustrate an important tension in genome biology: mechanisms that destabilize existing sequence arrangements can also generate stable, copyable genome variation.