Thymine Definition

Thymine is a pyrimidine base having the formula C₅H₆N₂O₂ that pairs complementary with adenine via 2 hydrogen bonds in the DNA molecule.

What is Thymine?

Nucleotides form the fundamental monomers that comprise the polymer of nucleic acids like DNA. Each nucleotide is formed of 3 sub-units: pentose sugar, phosphoric acid, and a nitrogenous base.

Nitrogenous bases include There are five nucleobases that serve as fundamental units of the genetic code: thymine, adenine, cytosine, guanine, and uracil. These nucleobases are grouped into purines and pyrimidines.

Thymine like other pyrimidines has a heterocyclic aromatic ring and has a molar mass of 126.115 g/mol. It has a melting point of 316- 317 °C. It forms the component of occurring as a component of a nucleoside or a nucleotide.

It forms the part of nucleobases in the case of DNA but it is absent in RNA and uracil is present instead. Uracil and thymine both pair complementary with adenine.

Thymine vs Uracil

Thymine, cytosine, and uracil are pyrimidine nucleobases. Thymine is distinct from other pyrimidine bases in having in 5^th position a methyl group and 2 keto groups at positions 2 and 4 in the heterocyclic ring. Thymine pairs with adenine with 2 hydrogen bonds.

Uracil has a similar structure to thymine and it only differs in terms of a methyl group at the 5^th position in the heterocyclic ring. One of the possible explanations why DNA has thymine instead of uracil can be due to stability.

Cytosine another pyrimidine gets readily deaminated into uracil by losing an amine group. This defect is sensed and corrected by repair systems in DNA before a mutation could occur.

If uracil was present as a pyrimidine base in DNA, the repair system may fail to differentiate between deaminated cytosine turned uracil and the original uracil.

Cytosine in the 2^nd position has a keto group and an amine group at position 4 in the pyrimidine ring and the formula C₄H₅N₃O. Cytosine complementary bonds with guanine via 3 hydrogen bonds in both RNA and DNA.

Biosynthesis of Thymine

Thymine biosynthesis, like other pyrimidines, is initiated by the formation of carbamoyl phosphate. It forms in a reaction catalyzed by carbamoyl phosphate synthetase that involves ATP, glutamine, water molecules, and bicarbonate.

In a series of steps, the carbamoyl phosphate is first converted into carbamoyl aspartate that then yields dihydroorotate. It undergoes oxidation to yield orotate that then gets converted into orotidine-5-monophosphate (OMP).

OMP on decarboxylation generates uridine monophosphate (UMP). Finally, down the pathway, uridine triphosphate (UTP) and uridine diphosphate (UDP) are produced.

Uridine in a reaction catalyzed by ribonucleotide reductase gets reduced to deoxyuridine that is methylated to form thymidine in presence of thymidylate synthase.

Thymine on being attached to a deoxyribose sugar is known as deoxythymidine. Further, when 3 phosphoric acid groups are attached to it, then it becomes deoxythymidine triphosphate (dTTP), one of the monomeric nucleotide units.

Thymine on degradation gets converted to β-aminoisobutyrate that then enters the Citric acid cycle. Thymine may also be recycled in a salvage pathway. Thymine on reaction deoxyribose-1-phosphate and enzyme thymidine phosphorylase gets first converted into thymidine. That is then converted in presence of enzyme nucleoside kinase to thymidine monophosphate.

Thymine Dimers

Mutations are alterations in the nucleotide sequences that may cause a change in the functioning of the gene. Thymine dimers are common mutations in the DNA that involve adjacent thymines.

Thymine dimers formed in presence of UV light and can result in kinks in the DNA structure. Such mutations are repaired by base excision repair by utilizing the undamaged DNA strand as a template to replace the mutated region of the other strand.

Thymine Function

Thymine is one of the primary nucleobases in DNA that comprise the genetic code. The nucleotide sequence codes for a particular protein having a specific amino acid sequence.