Question

Which of the following base pairings is/are possible?

• A. Purine-Purine
• B. Pyrimidine-Pyrimidine
• C. Purine-pyrimidine
• D. All of these

Answer 1

The correct option is C Purine-pyrimidine

Nitrogen bases are nitrogen containing compounds with a ring structure. They are divided into purines and pyrimidines depending on the number of rings they possess. Purines are double ringed structures and include adenine and guanine. One of the rings is 6 membered and the other is 5 membered. Pyrimidines are single ringed structures and include cytosine, thymine and uracil. They are 6 membered rings. Uracil is found in RNA only and not in DNA. Thymine is found in DNA only and not in RNA. Adenine, guanine and cytosine are found in both DNA and in RNA.
Purines of one strand bond with pyrimidines of the other strand with the help of hydrogen bonds. This is done so that the uniformity in distance separating both the strands can be maintained. Purines (double ringed structures) always pair with pyrimidines (single ringed structures). A purine-purine or pyrimidine-pyrimidine pairing will distort the double helical structure. Adenine pairs with thymine with the help of 2 hydrogen bonds and guanine pairs with cytosine with the help of 3 hydrogen bonds.