DNA nucleotide
A nucleotide is made of a phosphate, a molecule of ribose or 2'-deoxyribose, and another molecule called a nitrogenous base. The initial letters of the base names that spell out the genetic code. A, T, G, and C stand for the chemicals adenine, thymine, guanine, and cytosine, respectively. In base pairing, adenine always pairs with thymine, and guanine always pairs with cytosine.
Pyrimidine ribonucleotides
Thymine
Thymine is also known as 5-methyluracil. As a pyrimidine nucleobase, thymine is a heterocyclic aromatic organic compound. Thymine may be derived by methylation of uracil at the fifth carbon. Thymine combined with deoxyribose creates the nucleoside deoxythymidine, which is also know as thymidine. Thymidine can be phosphorylated with phosphoric acid groups, creating Thymidinemonophosphate (TMP), Thymidinediphosphate (TDP) or Thymidinetriphosphate (TTP).
Cytosine
Cytosine is a pyrimidine derivative, with a heterocyclic, aromatic ring, and two substituents attached. Cytosine is inherently unstable, and can change into uracil (spontaneous deamination) which can lead to a point mutation if not repaired by the DNA repair enzymes, such as uracil glycosylase. Cytosine can also be methylated into 5-methylcytosine by an enzyme called DNA methyltransferase
Uracil
Uracil is a pyrimidine nucleobase, a heterocyclic aromatic organic compound. Uracil is a planar, unsaturated compound that has the ability to absorb light. Found in RNA, uracil base pairs with adenine through hydrogen bonding and is replaced by thymine in DNA. Uracil can base pair with any of the bases depending on how the molecule arranges itself on the helix, but readily pairs with adenine. Uracil can also bind with a ribose sugar to form a ribonucleoside, uridine. When a phosphate attaches to uridine, uridine 5'-monophosphate is produced. Uracil also recycles itself to form nucleotides by undergoing a series of phophoribosyltransferase reactions. Degradation of uracil produces substrates, aspartate, carbon dioxide, and ammonia.