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Why do purines and purines do not pair?

April 26, 2017answer Biology India Mathematics pair Purines

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Why do purines and purines do not pair?

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The fact is purines CAN pair with purines in non-conventional pairing systems (ie: not the AT CG Watson-Crick pairings that we are all taught). Within the Watson-Crick system, the reason purines do not pair with purines is because of the overall structure of DNA. A purine-purine base pair is larger than a purine-pyrimidine base pair and disrupts the overall structure of the DNA helix (at least locally). This has larger impacts on the stability of the molecule and also its ability to be scanned, repaired, and replicated by the cellular machinery. One of the reasons Watson and Crick figured out that it had to be a purine-pyrimidine base pairing system was because of the X-ray crystallographic data solved by Rosalind Franklin.