![]() ![]() ![]() The G-C Hoogsteen base pair was first observed via X-ray crystallography years later, in 1986, by co-crystallizing DNA with triostin A (an antibiotic). Five years after Hoogsteen proposed the A-T Hoogsteen base pair, optical rotary dispersion spectra which provided evidence for a G-C Hoogsteen base pair were reported. Ten years later, in 1963, Karst Hoogsteen reported that he had used single crystal X-ray diffraction to investigate alternative base pair structures, and he found an alternative structure for the nucelobase pair adenine-thymine in which the purine (A) takes on an alternative conformation with respect to the pyrimidine (T). James Watson and Francis Crick published the double helical structure of DNA and proposed the canonical Watson-Crick base pairs in 1953. The presence of non-canonical base pairs in double stranded DNA results in a disrupted double helix. They are typically less stable than standard base pairings. pairing of G with U), and in tRNA recognition. Non-canonical base pairings commonly occur in the secondary structure of RNA (e.g. The first discovered non-canonical base pairs are Hoogsteen base pairs, which were first described by American biochemist Karst Hoogsteen. There are three main types of non-canonical base pairs: those stabilized by polar hydrogen bonds, those having interactions among C−H and O/N groups, and those that have hydrogen bonds between the bases themselves. Non-canonical base pairing occurs when nucleobases hydrogen bond, or base pair, to one another in schemes other than the standard Watson-Crick base pairs (which are adenine (A) - thymine (T) in DNA, adenine (A) - uracil (U) in RNA, and guanine (G) - cytosine (C) in both DNA and RNA).
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