Thymine () (symbol T or Thy) is one of the four in the nucleic acid of DNA that are represented by the letters G–C–A–T. The others are adenine, guanine, and cytosine. Thymine is also known as 5-methyluracil, a pyrimidine nucleobase. In RNA, thymine is replaced by the nucleobase uracil. Thymine was first isolated in 1893 by Albrecht Kossel and Albert Neumann from calf thymus glands, hence its name.[
]
Derivation
As its alternate name (5-methyluracil) suggests, thymine may be derived by
methylation of uracil at the 5th carbon. In
RNA, thymine is replaced with
uracil in most cases. In DNA, thymine (T) binds to
adenine (A) via two hydrogen bonds, thereby stabilizing the nucleic acid structures.
Thymine combined with deoxyribose creates the nucleoside deoxythymidine, which is synonymous with the term thymidine. Thymidine can be phosphorylated with up to three phosphoric acid groups, producing dTMP ( deoxy thymidine mono phosphate), dTDP, or dTTP (for the di- and tri- phosphates, respectively).
One of the common mutations of DNA involves two adjacent thymines or cytosine, which, in presence of Ultraviolet, may form thymine dimers, causing "kinks" in the DNA molecule that inhibit normal function.
Thymine could also be a target for actions of 5-fluorouracil (5-FU) in cancer treatment. 5-FU can be a metabolic analog of thymine (in DNA synthesis) or uracil (in RNA synthesis). Substitution of this analog inhibits DNA replication in actively dividing cells.
Thymine bases are frequently oxidized to over time after the death of an organism.[
]
Thymine imbalance causes mutation
During growth of bacteriophage T4, an imbalance of thymine availability, either a deficiency or an excess of thymine, causes increased
mutation.
[
]
The mutations caused by thymine deficiency appear to occur only at AT
base pair sites in DNA and are often AT to GC transition mutations.
[
]
In the bacterium
Escherichia coli, thymine deficiency was also found to cause the same mutation.
Theoretical aspects
In March 2015, NASA scientists reported that, for the first time, complex
DNA and
RNA of
life, including
uracil,
cytosine and thymine, have been formed in the laboratory under
outer space conditions, using starting chemicals, such as
pyrimidine, found in
. Pyrimidine, like polycyclic aromatic hydrocarbons (PAHs), another carbon-rich compound, may have been formed in
or in
Cosmic dust and gas clouds, according to the scientists.
Thymine has not been found in meteorites, which suggests the first strands of DNA had to look elsewhere to obtain this building block. Thymine likely formed within some meteorite parent bodies, but may not have persisted within these bodies due to an
Redox reaction with hydrogen peroxide.
Synthesis
Laboratory synthesis
Thymine was first prepared by hydrolysis of the corresponding
nucleoside obtained from natural sources. Interest in its direct chemical synthesis began in the early 1900s:
Emil Fischer published a method starting from
urea but a more practical synthesis used methylisothiourea in a condensation reaction with ethyl formyl propionate, followed by hydrolysis of the pyrimidine intermediate:
Many other preparative methods have been developed, including optimised conditions so that urea can be used directly in the reaction shown above, preferably with methyl formyl propionate.
See also
External links
