How many hydrogen bonds form between guanine and cytosine in DNA?

How many hydrogen bonds form between guanine and cytosine in DNA?

• A. Two hydrogen bonds
• B. Four hydrogen bonds
• C. One hydrogen bond
• D. Three hydrogen bonds

Answer: (D)

Guanine and cytosine pair through three hydrogen-bond contacts, which gives this pair greater stability than other pairs. The three bonds form by using complementary donors and acceptors on each base: one bond from guanine’s N1–H to cytosine’s N3, a second from guanine’s exocyclic N2–H to cytosine’s carbonyl oxygen at C2, and a third from cytosine’s exocyclic N4–H to guanine’s carbonyl oxygen at O6. This trio of contacts explains why GC-rich DNA is more thermally stable, since there are more hydrogen bonds holding the strands together.

Guanine and cytosine pair through three hydrogen-bond contacts, which gives this pair greater stability than other pairs. The three bonds form by using complementary donors and acceptors on each base: one bond from guanine’s N1–H to cytosine’s N3, a second from guanine’s exocyclic N2–H to cytosine’s carbonyl oxygen at C2, and a third from cytosine’s exocyclic N4–H to guanine’s carbonyl oxygen at O6. This trio of contacts explains why GC-rich DNA is more thermally stable, since there are more hydrogen bonds holding the strands together.