Medicine & USMLE


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


Guanine, abbreviated as the letter G, is a nitrogenous base that serves as a building block for nucleotides found in both DNA and RNA. Guanine’s chemical structure contains two fused rings, meaning that it belongs to the purine family of double-ringed, nitrogen-containing bases. Guanine’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 second position, and a carbonyl group, located at the sixth position. Guanine is also the heaviest nucleobase by molecular weight.

Key Points

  • Guanine (G)
    • 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
        • Guanosine is a nucleoside with guanine as its base
        • Guanosine monophosphate (GMP), guanosine diphosphate (GDP), and guanosine triphosphate (GTP) are all nucleotides with guanine as their base
      • Component of DNA and RNA
    • Structure
      • Chemical Formula = C5H5N5O
      • Purine
        • Contains 2 fused rings
        • Six-membered ring fused to a five-membered ring
      • Functional Groups
        • Amino (–NH2)
          • Located at 2nd position
        • Carbonyl (–C=O)
          • Located at 6th position
          • Important in differentiating vs. Adenine
      • Heaviest of the 5 nitrogenous bases seen in DNA and RNA