Medicine & USMLE


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


Thymine, abbreviated as the letter T, is a nitrogenous base that serves as a building block for nucleotides found in DNA. Thymine’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 three functional groups: two carbonyls at the 2nd and 4th position and a methyl group at the 5th position. Thymine is usually only found in DNA, and not in RNA.

Key Points

  • Thymine (T)
    • 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
        • Thymidine is a nucleoside with thymine as its base
      • Present in only DNA
        • Uracil is its unmethylated counterpart in RNA
    • Structure
      • Chemical Formula = C5H6N2O2
      • 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
        • Methyl (–CH3)
          • Methyl at 5th position
          • Important in differentiating vs. cytosine and uracil