Nucleic acids are composed of a combination of 5 nitrogenous bases:
- Guanine and adenine are double-ringed purine molecules
- Cytosine, thymine and uracil are single-ringed pyrimidine molecules
Thymine and uracil are chemically similar molecules – thymine is present in DNA, while uracil is used in RNA
The Five Nitrogenous Bases
Complementary Base Pairing
The nitrogenous bases may form hydrogen bonds according to complementary base pairing:
- Adenine always forms two hydrogen bonds with thymine / uracil
- Guanine always forms three hydrogen bonds with cytosine
Complementary Base Pairs
