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Also know, why does a purine always pair with a pyrimidine?
The molecular structure of both pyrimidinesandpurines allow them to only be able to bond with eachotherand not within the group. Thymine (pyrimidine)andadenine(purine) both have two atoms that can either providea Hbond or receive it. Cytosine (pyr.) and guanine (pur.) canstablishthree H bonds.
Similarly one may ask, why do purines bond with pyrimidines in DNA?
Purines (adenine and guanine) have twocarbonnitrogen ring bases. I think purines bond withpyrimidinesin the DNA ladder because adenine moleculesonly pairthymine molecules and guanine molecules only pair withcytosinemolecules. A and T bond with 2 hydrogen bonds, C andGbond with 3 hydrogen bonds.
The rules of base pairing (ornucleotidepairing) are: A with T: the purineadenine (A)always pairs with the pyrimidine thymine (T) Cwith G: thepyrimidine cytosine (C) always pairs with thepurineguanine (G)