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This non-Watson-Crick base-pairing allows the third strands to wind around the duplexes, which are assembled in the [[base pair|Watson-Crick pattern]], and form [[triple-stranded DNA|triple-stranded helices]] such as (poly(dA)•2poly(dT)) and (poly(rG)•2poly(rC)).<ref>{{cite journal |last1=Kim |first1=SK |last2=Takahashi |first2=M |last3=Nordén |first3=B |title=Binding of RecA to anti-parallel poly(dA).2poly(dT) triple helix DNA. |journal=Biochimica et Biophysica Acta |date=17 October 1995 |volume=1264 |issue=1 |pages=129–33 |pmid=7578246|doi=10.1016/0167-4781(95)00137-6 }}</ref> It can be also seen in three-dimensional structures of [[transfer RNA]], as T54•A58 and U8•A14.<ref name="pmid12853610">{{cite journal |last1=Zagryadskaya |first1=EI |last2=Doyon |first2=FR |last3=Steinberg |first3=SV |title=Importance of the reverse Hoogsteen base pair 54-58 for tRNA function. |journal=Nucleic Acids Research |date=15 July 2003 |volume=31 |issue=14 |pages=3946–53 |pmid=12853610 |pmc=165963|doi=10.1093/nar/gkg448 }}</ref><ref>{{cite book |last1=Westhof |first1=Eric |last2=Auffinger |first2=Pascal |title=Encyclopedia of life sciences. |publisher=Nature Pub. Group |isbn=9780470015902 |chapter-url=http://www-ibmc.u-strasbg.fr/upr9002/westhof/PDF/r2001_EWesthof_ELS.pdf |access-date=28 March 2019 |chapter=Transfer RNA Structure|date=2005-09-09 }}</ref>
===Triple-helix base pairing===▼
==Quadruplex structures==▼
Hoogsteen pairs also allows formation of secondary structures of single stranded DNA and RNA G-rich called [[G-quadruplex]]es (G4-DNA and G4-RNA). Evidence exists for both in vitro and in vivo formation of G4s. Genomic G4s have been suggested to regulate gene transcription and at the RNA level inhibit protein synthesis through steric inhibition of ribosome function. It needs four triplets of G, separated by short spacers. This permits assembly of planar quartets which are composed of stacked associations of Hoogsteen bonded guanine molecules.<ref name="Smith">{{cite journal |title=''In vivo veritas'': Using yeast to probe the biological functions of G-quadruplexes|vauthors=Johnson JE, Smith JS, Kozak ML, Johnson FB |journal=Biochimie|year=2008|volume=90|issue=8|pages=1250–1263|doi=10.1016/j.biochi.2008.02.013|pmid=18331848|pmc=2585026}}</ref>▼
▲==Triple-helix base pairing==
Watson-Crick base pairs are indicated by a "•", "-", or a "." (example: A•T, or poly(rC)•2poly(rC)).
Hoogsteen [[triple-stranded DNA]] base pairs are indicated by a "*" or a ":" (example: C•G*C+, T•A*T, C•G*G, or T•A*A).
▲==Quadruplex structures==
▲Hoogsteen pairs also allows formation of secondary structures of single stranded DNA and RNA G-rich called [[G-quadruplex]]es (G4-DNA and G4-RNA). Evidence exists for both in vitro and in vivo formation of G4s. Genomic G4s have been suggested to regulate gene transcription and at the RNA level inhibit protein synthesis through steric inhibition of ribosome function. It needs four triplets of G, separated by short spacers. This permits assembly of planar quartets which are composed of stacked associations of Hoogsteen bonded guanine molecules.<ref name="Smith">{{cite journal |title=''In vivo veritas'': Using yeast to probe the biological functions of G-quadruplexes|vauthors=Johnson JE, Smith JS, Kozak ML, Johnson FB |journal=Biochimie|year=2008|volume=90|issue=8|pages=1250–1263|doi=10.1016/j.biochi.2008.02.013|pmid=18331848|pmc=2585026}}</ref>
==See also==
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