Direct repeat

Direct repeats are a type of genetic sequence that consists of two or more repeats of a specific sequence.^[1] In other words, the direct repeats are nucleotide sequences present in multiple copies in the genome. Generally, a direct repeat occurs when a sequence is repeated with the same pattern downstream.^[1] There is no inversion^{[clarification needed]} and no reverse complement associated with a direct repeat. It may or may not have intervening nucleotides. The nucleotide sequence written in bold characters signifies the repeated sequence.

5´ TTACGnnnnnnTTACG 3´

3´ AATGCnnnnnnAATGC 5´

Linguistically, a typical direct repeat is comparable to saying "bye-bye".^[1]

Types

There are several types of repeated sequences:

• Interspersed (or dispersed) DNA repeats (interspersed repetitive sequences) are copies of transposable elements interspersed throughout the genome.
• Flanking (or terminal) repeats (terminal repeat sequences) are sequences that are repeated on both ends of a sequence, for example, the long terminal repeats (LTRs) on retroviruses. Direct terminal repeats are in the same direction and inverted terminal repeats are opposite to each other in direction.
• Tandem repeats (tandem repeat sequences) are repeated copies which lie adjacent to each other. These can also be direct or inverted repeats.^{[citation needed]} The ribosomal RNA and transfer RNA genes belong to the class of middle repetitive DNA.

Microsatellite DNA

A tract of repetitive DNA in which a motif of a few base pairs is tandemly repeated numerous times (e.g. 5 to 50 times) is referred to as microsatellite DNA. Thus direct repeat tandem sequences are a form of microsattelite DNA. The process of DNA mismatch repair plays a prominent role in the formation of direct trinucleotide repeat expansions.^[2] Such repeat expansions underlie several neurological and developmental disorders in humans.^[2]

Homologous recombination

In directly repeated sequences of the tobacco plant genome, DNA double-strand breaks can be efficiently repaired by homologous recombination between the repeated sequences.^[3]

See also

• Inverted repeat