6) Pyrimidine bases (uracil, thymine, cytosine). Aromatic properties. Lactim-lactam tautomerism.

Pyrimidine is a heterocyclic aromatic organic compound similar to benzene and pyridine, containing two nitrogen atoms at positions 1 and 3 of the six-member ring. The important derivatives of pyrimidine:

Uracil

Pyrimidines

• Uracil = 2,4-dioxy pyrimidine 

• Thymine = 2,4-dioxy-5-methyl pyrimidine 

• Cytosine = 2-oxy-4-amino pyrimidine 

Nucleotides

Adding one or more phosphates to the sugar portion of a nucleoside results in a nucleotide. Generally, the phosphate is in ester linkage to carbon 5' of the sugar. If more than one phosphate is present, they are generally in acid anhydride linkages to each other. If such is the case, no position designation in the name is required. If the phosphate is in any other position, however, the position must be designated. For example, 3'-5' cAMP indicates that a phosphate is in ester linkage to both the 3' and 5' hydroxyl groups of an adenosine molecule and forms a cyclic structure. 2'-GMP would indicate that a phosphate is in ester linkage to the 2' hydroxyl group of a guanosine. Somerepresentativenamesare:

The nucleosides and their mono-, di-, and triphosphates
	Base	Nucleoside	Nucleotides
DNA	Adenine (A)	Deoxyadenosine	dAMP	dADP	dATP
	Guanine (G)	Deoxyguanosine	dGMP	dGDP	dGTP
	Cytosine (C)	Deoxycytidine	dCMP	dCDP	dCTP
	Thymine (T)	Deoxythymidine	dTMP	dTDP	dTTP
RNA	Adenine (A)	Adenosine	AMP	ADP	ATP
	Guanine (G)	Guanosine	GMP	GDP	GTP
	Cytosine (C)	Cytidine	CMP	CDP	CTP
	Uracil (U)	Uridine	UMP	UDP	UTP

Purine and pyrimidine bases exist in different forms called tautomers.

(A) A tautomeric shift occurs when a proton changes its position, resulting in a rare tautomeric form. (B) Standard and anomalous base-pairing arrangements that occur if bases are in the rare tautomeric forms. Base mispairings due to tautomeric shifts were originally thought to be a major source of errors in replication, but such structures have not been detected in DNA, and most evidence now suggests that other types of anomalous pairings are responsible for replication errors.