# Direct repeat

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'''Direct repeats''' are a type of genetic sequence that consists of two or more repeats of a specific sequence.<ref name="Ussery2008">{{cite book |title=Computing for Comparative Microbial Genomics: Bioinformatics for Microbiologists |chapter=Word Frequencies and Repeats §Word Frequencies, Repeats, and Repeat-related Structures in Bacterial Genomes |chapter-url=https://link.springer.com/chapter/10.1007/978-1-84800-255-5_8 |doi=10.1007/978-1-84800-255-5_8 |series=Computational Biology |volume=8 |pages=133–144 <!-- Different pagination --> |first1=David W. |last1=Ussery |first2=Trudy |last2=Wassenaar |first3=Stefano |last3=Borini |publisher=Springer |edition=1 |date=2009 |isbn=978-1-84800-254-8}}</ref> In other words, the direct repeats are [nucleotide](/source/nucleotide) sequences present in multiple copies in the [genome](/source/genome). Generally, a direct repeat occurs when a sequence is repeated with the same pattern downstream.<ref name=Ussery2008 />  There is {{Clarify | text = no inversion | date = November 2021 | reason = What is an inversion on a single-strand sequence?  This may need to be dropped.}} and no [reverse complement](/source/inverted_repeat) associated with a direct repeat. It may or may not have intervening nucleotides. The nucleotide sequence written in bold characters signifies the repeated sequence. 
:::{{color|blue |5´ '''TTACG'''nnnnnnTTACG 3´}}
:::{{color|green|3´ '''AATGC'''nnnnnnAATGC 5´}}
Linguistically, a typical direct repeat is comparable to saying "bye-bye".<ref name=Ussery2008 />

==Types==
There are several types of repeated sequences:
*Interspersed (or dispersed) DNA repeats (interspersed repetitive sequences) are copies of [transposable elements](/source/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 repeat](/source/long_terminal_repeat)s (LTRs) on [retroviruses](/source/retroviruses). Direct terminal repeats are in the same direction and inverted terminal repeats are opposite to each other in direction.
*[Tandem repeats](/source/Tandem_repeats) (tandem repeat sequences) are repeated copies which lie adjacent to each other. These can also be direct or [inverted repeats](/source/inverted_repeats).{{Citation needed | date = November 2021 | reason = This is in conflict with the [Tandem repeat](/source/Tandem_repeat) article itself which seems to be defined as only a direct repeat with no intervening nucleotide.}} The [ribosomal RNA](/source/ribosomal_RNA) and [transfer RNA](/source/transfer_RNA) genes belong to the class of middle repetitive DNA.

===Microsatellite DNA===
A tract of [repetitive DNA](/source/Repeated_sequence_(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](/source/microsatellite) DNA.  Thus direct repeat tandem sequences are a form of microsattelite DNA.  The process of [DNA mismatch repair](/source/DNA_mismatch_repair) plays a prominent role in the formation of direct trinucleotide repeat expansions.<ref name = Richard2021>{{cite journal|pmid=33925919 |date=2021 |last1=Richard |first1=G. F. |title=The Startling Role of Mismatch Repair in Trinucleotide Repeat Expansions |journal=Cells |volume=10 |issue=5 |page=1019 |doi=10.3390/cells10051019 |pmc=8145212 |doi-access=free }}</ref>  Such repeat expansions underlie several neurological and developmental disorders in humans.<ref name = Richard2021/>

==Homologous recombination==
In directly repeated sequences of the [tobacco](/source/tobacco) plant [genome](/source/genome), [DNA double-strand breaks](/source/DNA_damage_(naturally_occurring)) can be efficiently repaired by [homologous recombination](/source/homologous_recombination) between the repeated sequences.<ref>{{cite journal |vauthors=Siebert R, Puchta H |title=Efficient repair of genomic double-strand breaks by homologous recombination between directly repeated sequences in the plant genome |journal=Plant Cell |volume=14 |issue=5 |pages=1121–31 |date=May 2002 |pmid=12034901 |pmc=150611 |doi=10.1105/tpc.001727 }}</ref>

==See also==
*[Inverted repeat](/source/Inverted_repeat)

==References==
{{reflist}}

{{Repeated sequence}}

Category:Repetitive DNA sequences
Category:Genetics

{{genetics-stub}}

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