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To determine: The positions in the purine ring of a purine
Introduction:
There are two types of nitrogenous bases: purines and pyrimidines. Adenine and guanine are the purine bases with double ring structures. Cytosine and thymine are the pyrimidines bases that have single ring structure. These bases pair with their complementary bases and form a nucleotide sequence through hydrogen bonding.
Explanation of Solution
Explanation:
Adenine is linked to thymine with double hydrogen bond and guanine is linked to cytosine with triple hydrogen bond. The two nucleotides are joined together by the phosphodiester bond.
Purine is a structure that contains nine member ring which is made up of a imidazole ring along with a pyrimidine ring. In adenine amino group present at the sixth position along with first position’s nitrogen is involved in Watson-Crick base pairing with complementary pyrimidine base which is thymine.
In guanine, nitrogen present at first position and amino group at second carbon along with oxo group at sixth carbon is involved in Watson-Crick base pairing with complementary pyrimidine base which is cytosine.
Thus, the positions of purine ring that have the property to form hydrogen bonds but do not participate in Watson-Crick base pairing are the nitrogen which is present at third and seventh position of purine ring.
Conclusion:
Third and seventh positions in purine ring are involved in hydrogen bonding but do not involve in Watson-Crick base pairing.
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