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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.
Uracil is also involved in the biosynthesis of polysaccharides and the transportation of sugars containing aldehydes. [22] Uracil is important for the detoxification of many carcinogens, for instance those found in tobacco smoke. [23] Uracil is also required to detoxify many drugs such as cannabinoids (THC) [24] and morphine (opioids). [25]
They function as the fundamental units of the genetic code, with the bases A, G, C, and T being found in DNA while A, G, C, and U are found in RNA. Thymine and uracil are distinguished by merely the presence or absence of a methyl group on the fifth carbon (C5) of these heterocyclic six-membered rings.
5-Methylcytosine is a methylated form of the DNA base cytosine (C) that regulates gene transcription and takes several other biological roles. [1] When cytosine is methylated, the DNA maintains the same sequence, but the expression of methylated genes can be altered (the study of this is part of the field of epigenetics). 5-Methylcytosine is incorporated in the nucleoside 5-methylcytidine.
Methylation of cytosine to form 5-methylcytosine occurs at the same 5 position on the pyrimidine ring where the DNA base thymine's methyl group is located; the same position distinguishes thymine from the analogous RNA base uracil, which has no methyl group. Spontaneous deamination of 5-methylcytosine converts it to thymine. This results in a T ...
Deoxythymidine is non-toxic and is part of one of the four nucleosides in DNA. It is a naturally occurring compound that exists in all living organisms and DNA viruses. Instead of thymidine, RNA contains uridine (uracil joined to ribose). Uracil is chemically very similar to thymine, which is also known as 5-methyluracil.
Cytosine can also be methylated into 5-methylcytosine by an enzyme called DNA methyltransferase or be methylated and hydroxylated to make 5-hydroxymethylcytosine. The difference in rates of deamination of cytosine and 5-methylcytosine (to uracil and thymine) forms the basis of bisulfite sequencing. [8]
In enzymology, a tRNA (uracil-5-)-methyltransferase (EC 2.1.1.35) is an enzyme that catalyzes the chemical reaction S-adenosyl-L-methionine + tRNA containing uridine at position 54 ⇌ {\displaystyle \rightleftharpoons } S-adenosyl-L-homocysteine + tRNA containing ribothymidine at position 54