Nucleotides are phosphorylated forms of nucleosides. Phosphorylation (addition of phosphate group) occurs generally in the 5΄OH group of the ribose or deoxy ribose sugar.
A nucleotide with one phosphate group is called as monophosphate of the respective nucleoside. For example, the monophosphate of adenosine is called as Adenosine Mono Phosphate. If two phosphate groups are attached to the sugar moiety at the 5΄OH group, it is called as diphosphate. eg. Cytidine Diphosphate.A nucleotide containing three phosphate groups is called as triphosphate eg. Adenosine Triphosphate (Fig. 7.5). The corresponding nucleosides and nucleotides are listed in Table 1 (Bases, their nucleosides and nucleotides).
· Nucleotides are the energy currency of the cells (ATP).
· They actively take part in metabolism as hydrogen donors, phosphate group donors and methyl group donors.
· They form the structural components of some co-enzymes (NAD and FAD).
· Nucleotides like cAMP (cyclic AMP) and cGMP (cyclic GMP) act as second messengers involved in hormonal signaling pathways.
Table 7.1 Bases, their nucleosides and nucleotides
Oligonucleotides are polymers which yield two to ten residues of mononucleotides on hydrolysis. Two nucleotides join together to form dinucleotides.Example for biologically important dinucleotides are NAD and FAD, which act as co-enzymes.
Polynucleotides are polymers that yield more than ten nucleotides on hydrolysis.
Polynucleotides have directions. If the first nucleotide of the sequence has 5΄ triphosphate free and 3΄OH group bonded to the next nucleotide, then the direction of the polynucleotide is 5΄to 3΄. If the first nucleotide in the sequence has 3΄OH group free, then it is said to be from 3΄to 5΄direction.