Medicine & USMLE


Nucleic Acids
  1. Adenine
  2. Guanine
  3. Cytosine
  4. Thymine
  5. Uracil
  6. Nucleotides vs Nucleosides


Adenine, abbreviated as the single letter A, is a nitrogenous base that serves as a building block for nucleotides found in both DNA and RNA. Adenine’s chemical structure contains two fused rings, meaning that it belongs to the purine family of double-ringed, nitrogen-containing molecules. Adenine’s molecular structure is composed of one five-membered and one six-membered nitrogenous ring that have been fused together. Branching out from the six-membered ring is an amino group, located at the sixth position.

Key Points

  • Adenine (A)
    • Characteristics
      • Nitrogenous Base
        • Organic molecules made up of nitrogen-containing ring structures
        • Each base has a unique structure, with its own set of functional groups attached to the ring structure 
      • Building block of Nucleosides and Nucleotides
        • Adenosine is a nucleoside with adenine as its base
        • Adenosine monophosphate (AMP), adenosine diphosphate (ADP), and adenosine triphosphate (ATP) are all nucleotides with adenine as their base
      • Component of DNA and RNA
        • Including poly-A tails (a chain of adenine nucleotides) of the 3’ end of mRNA
    • Structure
      • Chemical Formula = C5H5N5
      • Purine
        • Contains 2 fused rings
        • Six-membered ring fused to a five-membered ring
      • Functional Group
        • Amino (–NH2)
          • Located at 6th position