Medicine & USMLE


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


Cytosine, abbreviated as the letter C, is a nitrogenous base that serves as a building block for nucleotides found in both DNA and RNA. Cytosine’s structure contains only one six-membered ring, meaning that it belongs to the pyrimidine family of one-ringed, nitrogen-containing bases. Branching out from this one central ring are two functional groups: a carbonyl at the 2nd position and an amino group at the 4th position. Cytosine is also the lightest nucleobase by molecular weight.

Key Points

  • Cytosine (C)
    • 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
        • Cytidine is a nucleoside with cytosine as its base
      • Component of DNA and RNA
    • Structure
      • Chemical Formula = C4H5N3O
      • Pyrimidine
        • Contains 1 ring (six-membered)
      • Functional Groups
        • Carbonyl (–C=O)
          • Located at 2nd position
        • Amino (–NH2) 
          • Located at 4th position
          • Important in differentiating vs. thymine and uracil
      • Lightest of the 5 nitrogenous bases seen in DNA and RNA