Medicine & USMLE


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


Uracil, abbreviated as the letter U, is a nitrogenous base that serves as a building block for nucleotides found in RNA. Uracil’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 ring are two carbonyls attached at the 2nd and 4th position. Uracil is usually only found in RNA, and not in DNA.

Key Points

  • Uracil (U)
    • 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
        • Uridine is a nucleoside with uracil as its base
      • Present in only RNA
        • Thymine is its methylated counterpart in DNA
        • Incorporated during transcription of RNA in the nucleus
    • Structure
      • Chemical Formula = C4H4N2O2
      • Pyrimidine
        • Contains 1 ring (six-membered)
      • Functional Groups
        • Carbonyl (–C=O)
          • Located at 2nd position
        • Carbonyl (–C=O)
          • Located at 4th position
          • Important in differentiating vs. cytosine
        • Note: no methyl group
          • Important in differentiating vs. Thymine