Question

The constant diameter of the DNA double helix is maintained by base pairing between

• A. Any purine and any pyrimidine
• B. Guanine and cytosine and adenine and thymine
• C. Two pyrimidines
• D. Two purines

Answer 1

Answer: (B)

Guanine and cytosine and adenine and thymine

• The constant diameter of the DNA can be maintained by nitrogenous base pairing, which is complimentary.
• Adenine is specific to pair with thymine and guanine with cytosine. Adenine forms double bond with thymine (A = T) and cytosine forms triple bond with guanine.
• In each complementary base pair, one purine is paired with one pyrimidine.
• There is not enough space for two purines to fit within the helix and too much space for two pyrimidines to get close enough together for hydrogen bonds to form between them.
  

So, the correct answer is 'Guanine and cytosine and adenine and thymine'