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Biochemistry

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Purines, Pyrimidines and Nucleic Acid Metabolism

Question
12 out of 59
 

Triplex DNA is formed because of (AIIMS May 2010)



A In palindromic sequences

B Increase no. of guanosine repeat

C Increase in polypyrimidine sequences

D Hoogstein pairing

Ans. D Hoogstein pairing

a. A Hoogsteen base pair is a variation of base-pairing in nucleic acids such as the A•T pair. In this manner, two nucleobases on each strand can be held together by hydrogen bonds in the major groove.

b. A Hoogsteen base pair applies the N7 position of the purine base (as a hydrogen bond acceptor) and C6 amino group (as a donor), which bind the Watson-Crick (N3–N4) face of the pyrimidine base.

Chemical properties

a. Hoogsteen pairs have quite different properties from Watson-Crick base pairs. The angle between the two glycosylic bonds (ca. 80° in the A• T pair) is larger and the C12–C12 distance (ca. 860 pm or 8.6 Å) is smaller than in the regular geometry.

b. In some cases, called reversed Hoogsteen base pairs, one base is rotated 180° with respect to the other.

Triplex structures

This non-Watson-Crick base-pairing allows the third strands to wind around the duplexes, which are assembled in the Watson-Crick pattern, and form triple-stranded helices such as (poly(dA)•2poly(dT)) and (poly(rC)• 2poly(rC)). It can be also seen in three-dimensional structures of transfer RNA.

To form a triplex, a sequence must conform to unusual requirements. One can see that the guanine and thymine residues in the half of the purine-rich strand that turns back to make a triplex form Hoogsteen hydrogen bonds with guanine and adenine residues, respectively, in the other half of the same strand

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

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