Thymine in DNA Structure and Biological Role

Thymine is an organic compound of the pyrimidine family, which is a constituent of deoxyribonucleic acid (DNA). DNA, including the RNA (which is ribonucleic acid), regulates the hereditary characteristics in all living cells. Thymine, like other nitrogenous elements of nucleic acids, is found in thymidine, a corresponding nucleoside (or structural unit made up of a nitrogen compound and a sugar) in which it is chemically connected to the sugar deoxyribose. It's also a phosphate ester of thymidine, which is part of thymidylic acid, a nucleotide (which is a broader structural unit consisting of a phosphoric and nucleoside acid).

Deriving Thymine

As Thymine's alternate name (which is 5-methyl Uracil) suggests, Thymine can be derived by methylation of Uracil at the 5th carbon. It replaces thymine in RNA with Uracil in most cases. Whereas, in DNA, Thymine (T) binds to Adenine (A) with two hydrogen bonds, thereby stabilizing the structures of nucleic acid.

When thymine is mixed with deoxyribose, the nucleoside deoxythymidine is formed, which is the same as the word thymidine. Thymidine may be phosphorylated with up to three phosphoric acid groups to form dTDP, dTMP (Deoxythymidine Monophosphate), or dTTP (Deoxythymidine Triphosphate) (for both the di- and tri- phosphates, respectively).

The most common DNA mutations include two adjacent thymines or cytosines, which can form thymine dimers in the presence of ultraviolet light, creating "kinks" in the DNA molecule that prevent normal function.

Thymine could also be said to be the target for the actions of 5-fluorouracil (5-FU) in the treatment of cancer. 5-FU may be a metabolic analogue of Uracil (used in RNA synthesis) or Thymine (used in DNA synthesis) (in DNA synthesis). Substitution of this analogue inhibits the DNA synthesis in dividing cells actively.

Frequently,  the thymine bases can be oxidized to hydantoins over time after the death of an organism. The thymine chemical formula is C5H6N2O2.

Thymine Structure

The thymine structure can be represented as follows:

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Cause of Thymine Imbalance

A thymine imbalance, either a deficiency or an excess of Thymine, induces an increase in mutation during the development of bacteriophage T4. Such mutations caused by the thiamine deficiency appear to take place only at the AT base pair sites in DNA and are often AT to GC transition mutations. In the bacterium Escherichia coli, the thymine deficiency was also found to be mutagenic, and it causes AT to GC transitions.

Presence of Thymine in DNA

DNA holds 4 nitrogenous bases: Thymine, Adenine, guanine, and cytosine. Thymine is one of the nitrogenous bases, which make DNA nucleotides. DNA is made up of nucleotide chains' two strands. Each chain is made up of 4 types of nucleosides ( thymidine, adenosine, guanosine, and cytidine) linked by phosphodiester bonds.

A nucleoside can be made of sugar (deoxyribose in DNA) and a nitrogen base. The type of sugar in the DNA is constant, whereas the N - base is variable. Two nucleotides present in a chain linked by a single phosphate by Ester linkage. (2 ester bonds, therefore forms phosphodiester bond).

The base of one single strand pairs with the other strand by hydrogen bonds. Specifically, the Thymine of one strand of DNA pairs with Adenine (the other nitrogenous base) of another strand of DNA by 2 hydrogen bonds. Similarly, guanine with cytosine by 3 hydrogen bonds.

Reason Behind Thymine Usage in DNA

A key difference between the Thymine RNA and DNA is that RNA contains Uracil in place of Thymine, which also means that, in RNA, uracil pairs with Adenine. During the translation, the Thymine of DNA may still pair with RNA adenine.

Usually, nucleotides are abbreviated to just single, capitalized letters: C, G, A, T - corresponding to the 1st letter of the nucleotide name. U can be used for RNA's Uracil too.

Theoretical Aspects

NASA scientists, in March 2015, for the first time, reported that complex RNA and DNA organic compounds of life, including cytosine uracil, and Thymine, have been formed in the laboratory under the conditions of outer space, using the starting chemicals, such as pyrimidine, which is found in meteorites.

Pyrimidine, like Polycyclic Aromatic Hydrocarbons (PAHs), the other carbon-rich compound, can have been formed either in the red giants or in interstellar gas and dust clouds, according to scientists. Thymine has not been found in the meteorites that suggest the first strands of DNA had to look elsewhere to obtain this particular building block. Likely, the Thymine formed within a few meteorite parent bodies but cannot have persisted within these bodies because of an oxidation reaction with hydrogen peroxide.

Uses of Thymine

Thymine can be found in the nucleic acid DNA. In RNA, Thymine can be replaced with Uracil in most cases. Whereas, in DNA, Thymine binds to Adenine through two hydrogen bonds to assist in stabilizing the structures of nucleic acid.

In DNA, the Thymine (T) binds to Adenine (A) through two hydrogen bonds, thereby stabilizing the structures of nucleic acid. When thymine and deoxyribose are mixed, the nucleoside deoxythymidine is formed, which is synonymous with the word thymidine. This analogue substitution inhibits the synthesis of DNA in actively dividing cells.