Indicate whether each functional group of the five heterocyclic bases in nucleic acids is a hydrogen bond acceptor (A), a hydrogen bond donor (D), or both (D/A).

Nucleic acids are biosynthesized from nucleoside triphosphates, using enzymes called DNA polymerases (for the synthesis of DNA) or RNA polymerases (for the synthesis of RNA).

The functional group of five heterocyclic bases in nucleic acids may be hydrogen bond acceptor A, a hydrogen bond donor D, or both A/D.

In heterocyclic bases, amine groups could serve either as hydrogen bond acceptors by using non-bonding electrons on nitrogen or as hydrogen bond donors by using a hydrogen bond to the nitrogen.

The A, D, and A/D designations show that the maximum numbers o hydrogen bonds forming between bases are two hydrogen bonds between thymine and adenine and three between cytosine and guanine.

Pyrimidine based heterocyclic bases

Purine based heterocyclic bases