Nitrogenous Bases

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

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

base pairs