A nucleotide is made of a phosphate, a molecule of ribose or 2'-deoxyribose, and another molecule called a nucleobase (a pyrimidine or purine). 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 DNA base pairing, adenine pairs with thymine, and guanine pairs with cytosine, while in RNA adenine pairs with uracil.
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 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. Cytodine can be phosphorylated with phosphoric acid groups, creating Cytodine monophosphate (CMP), Cytodine diphosphate (CDP) or Cytodine triphosphate (CTP).
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. Uridine can be phosphorylated with phosphoric acid groups, creating Uridine emonophosphate (UMP), Uridine diphosphate (UDP) or Uridine triphosphate (UTP).
Adenine is derived from the two-ring parent molecule purine. Adenine is comprised a six-carbon pyrimidine ring fused with a five-carbon imidazole ring with an additional amine attached to the six-carbon ring. The adenine nucleoside is called adenosine and lacks only a phosphate to form a nucleotide. Adenosine can be phosphorylated with phosphoric acid groups, creating Adenosine monophosphate (AMP), Adenosine diphosphate (ADP) or Adenosine triphosphate (ATP).
Guanine, a two-ring molecular structure, and is derived from the two-ring parent molecule purine. Guanine is comprised a six-carbon pyrimidine ring fused with a five-carbon imidazole ring with a carbonyl and amine attached to the six-carbon ring. Being unsaturated, the bicyclic molecule is planar. The guanine nucleoside is called guanosine and lacks only a phosphate to form a nucleotide. Guanosine can be phosphorylated with phosphoric acid groups, creating Guanosine monophosphate (GMP), Guanosine diphosphate (GDP) or Guanosine triphosphate (GTP).