Nucleic acid bases are aromatic heterocycles (heteroatom:Nitrogen,present in the aromatic ring). There are two types: pyrimidines (1 ring) and purines (2 rings -one smaller).

 

Adenine (A)

Guanine (G)

These are the purine bases.

Guanine was first observed in guano (bird excretion). It is insoluble in water.

Adenine is present in various fundamental biomolecules, in addition to nucleic acids. It is part of adenosine triphosphate (ATP), which is the bio energy currency, and also in the cofactors nicotinamide adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD), which are essential in redox metabolic pathways.

Thymine (T)

Cytosine (C)

Uracil (U)

These are the pyrimidine bases.

Uracil is only present in RNA, where it takes the place of Thymine which is absent.

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The bases pair up in a well defined manner:

Thymine (or Uracil in RNA) pairs with Adenine

Cytosine pairs with Guanine

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3D views of G and C

Guanine

 

Cytosine

 

Next: Look at the hydrogen bonded pair G - C >>