Which of these complementary base pairs form the strongest or "tightest" association?
A)Adenine and thymine
B)Cytosine and guanine
C)Guanine and thymine
D)Cytosine and adenine

Question

Quizplus · Accepted Answer

Cytosine and guanine normally pair together, whereas adenine and thymine normally pair together. The reason for this specific and complementary pairing of bases is related to the forces that hold the two DNA strands together. The two strands are held loosely together, most of the time, by weak hydrogen bonds. Within a base pair, the hydrogen bonds form between the two nucleotides. Adenine and thymine each have a site for two hydrogen bonds to form, whereas cytosine and guanine each have three sites for hydrogen bonds to form. With three hydrogen bonds instead of just two, the cytosine-guanine pair is tighter (stronger) than an adenine-thymine pair. Note, a purine must always pair with a pyrimidine: they each can only pair with the base that can form the same number of hydrogen bonds. Adenine and cytosine do not pair, and neither do guanine and thymine.

Which of These Complementary Base Pairs Form the Strongest or "Tightest