Nucleotides: Structure and Properties

1. Base: Also referred to as heterocycles, these nitrogencontaining ring compounds are derivatives of purine or pyrimidine. The atoms within the purine/pyrimidine rings do not vary and they are given the same number in different nucleotides. A variety of chemical groups can be bonded at different positions to the ring constituents.Althoughone structure predominates for each base, tautomeric forms occur in minor amounts (Figure 2). 2. Sugar: A five-carbon sugar, usually ribose, is linked to the base. Each carbon atom is numbered and, to allow their distinction from atoms in the base, the number is followed by a prime mark: thus ribose has five carbon atoms, numbered 1’ to 5’. The sugars are locked into a five-membered furanose ring by the bond from C1’ of the sugar to the base. 3. Phosphate ester: Phosphate groups are attached to the sugar as esters, with the most common site of esterification in natural compounds being via the hydroxyl at the C5’. Typically, one, two or three phosphates are joined, producing mono-, diand triphosphates, respectively. (Note that the prefixes bis and tris are used in reference to molecules with two or three phosphates attached to different hydroxyls on the sugar.)