Thymine Definition, Facts, and Functions

Thymine molecule facts
Thymine is one of the pyrimidine bases found in DNA.

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Thymine is one of the nitrogenous bases used to build nucleic acids. Along with cytosine, it is one of the two pyrimidine bases found in DNA. In RNA, it is usually replaced by uracil, but transfer RNA (tRNA) contains trace amounts of thymine.

Chemical Data: Thymine

  • IUPAC Name: 5-Methylpyrimidine-2,4(1H,3H)-dione
  • Other Names: Thymine, 5-methyluracil
  • CAS Number: 65-71-4
  • Chemical Formula: C5H6N2O2
  • Molar Mass: 126.115 g/mol
  • Density: 1.223 g/cm3
  • Appearance: White powder
  • Solubility in Water: Miscible
  • Melting Point: 316 to 317 °C (601 to 603 °F; 589 to 590 K)
  • Boiling Point: 335 °C (635 °F; 608 K) (decomposes)
  • pKa (Acidity): 9.7
  • Safety: Dust may irritate eyes and mucous membranes

Thymine is also called 5-methyluracil or it may be represented by the capital letter "T" or by its three-letter abbreviation, Thy. The molecule gets its name from its initial isolation from calf thymus glands by Albrecht Kossel and Albert Neumann in 1893. Thymine is found in both prokaryotic and eukaryotic cells, but it does not occur in RNA viruses.

Key Takeaways: Thymine

  • Thymine is one of the five bases used to build nucleic acids.
  • It is also known as 5-methyluracil or by the abbreviations T or Thy.
  • Thymine is found in DNA, where it pairs with adenine via two hydrogen bonds. In RNA, thymine is replaced by uracil.
  • Ultraviolet light exposure causes a common DNA mutation where two adjacent thymine molecules form a dimer. While the body has natural repair processes to correct the mutation, unrepaired dimers can lead to melanoma.

Chemical Structure

The chemical formula of thymine is C5H6N2O2. It forms a six-member heterocyclic ring. A heterocyclic compound contains atoms besides carbon within the ring. In thymine, the ring contains nitrogen atoms at the 1 and 3 positions. Like other purines and pyrimidines, thymine is aromatic. That is, its ring includes unsaturated chemical bonds or lone pairs. Thymine combines with the sugar deoxyribose to form thymidine. Thymidine may be phosphorylated with up to three phosphoric acid groups to form deoxythymidine monophosphate (dDMP), deoxythymidine diphosphate (dTDP), and deoxythymidine triphosphate (dTTP). In DNA, thymine forms two hydrogen bonds with adenine. The phosphate of the nucleotides forms the backbone of the DNA double helix, while the hydrogen bonds between the bases run through the center of the helix and stabilize the molecule.

Base pairings in DNA
Thymine forms two hydrogen bonds with adenine in DNA. Volodymyr Horbovyy / Getty Images

Mutation and Cancer

In the presence of ultraviolet light, two adjacent thymine molecules often mutate to form a thymine dimer. A dimer kinks the DNA molecule, affecting its function, plus the dimer cannot be correctly transcribed (replicated) or translated (used as a template to make amino acids). In a single skin cell, up to 50 or 100 dimers may form per second upon exposure to sunlight. Uncorrected lesions are the leading cause of melanoma in humans. However, most dimers are fixed by nucleotide excision repair or by photolyase reactivation.

While thymine dimers may lead to cancer, thymine may also be used as a target for cancer treatments. Introduction of the metabolic analog 5-fluorouracil (5-FU) substitutes 5-FU for thymine and prevents cancer cells from replicating DNA and dividing.

In the Universe

In 2015, researchers at Ames Laboratory successfully formed thymine, uracil, and cytosine under laboratory conditions simulating outer space using pyrimidines as a source material. Pyrimidines naturally occur in meteorites and are believed to be formed in gas clouds and red giant stars. Thymine has not been detected in meteorites, possibly because it is oxidized by hydrogen peroxide. However, the lab synthesis demonstrates the building blocks of DNA may be transported to planets by meteorites.


  • Friedberg. Errol C. (January 23, 2003). "DNA Damage and Repair." Nature. 421 (6921): 436–439. doi:10.1038/nature01408
  • Kakkar, R.; Garg, R. (2003). “Theoretical study of the effect of radiation on thymine.” Journal of Molecular Structure-TheoChem 620(2-3): 139-147.
  • Kossel, Albrecht; Neumann, Albert (1893) "Ueber das Thymin, ein Spaltungsproduct der Nucleïnsäure." (On thymine, a cleavage product of nucleic acid). Berichte der Deutschen Chemischen Gesellschaft zu Berlin 26 : 2753-2756.
  • Marlaire, Ruth (March 3, 2015). "NASA Ames Reproduces the Building Blocks of Life in Laboratory."
  • Reynisson, J.; Steenken, S. (2002). “DFT studies on the pairing abilities of the one-electron reduced or oxidized adenine-thymine base pair.” Physical Chemistry Chemical Physics 4(21): 5353-5358.
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Helmenstine, Anne Marie, Ph.D. "Thymine Definition, Facts, and Functions." ThoughtCo, Feb. 17, 2021, Helmenstine, Anne Marie, Ph.D. (2021, February 17). Thymine Definition, Facts, and Functions. Retrieved from Helmenstine, Anne Marie, Ph.D. "Thymine Definition, Facts, and Functions." ThoughtCo. (accessed May 30, 2023).