A nucleoside consists of a nitrogenous base covalently attached to a sugar (ribose or deoxyribose) but without the phosphate group. A nucleotide consists of a nitrogenous base, a sugar (ribose or deoxyribose) and one to three phosphate groups.
Nucleoside = Sugar + Base
Nucleotide = Sugar + Base + Phosphate
edit Biological Function
Nucleotides are building blocks of nucleic acids (DNA and RNA). A nucleic acid contains a chain of nucleotides linked together with covalent bonds to form a sugar-phosphate backbone with protruding nitrogenous bases. For example, DNA contains two such chains spiraling round each other in the famous double helix shape. The two chains in the double helix are held together along their length by hydrogen bonds that form between the bases on one chain and the bases on the other.
The biological functions of nucleotides are:
- Data storage - as part of DNA/RNA
- Energy Currency - ATP
- Cellular communication (cAMP; ATP allosteric regulator)
- Co-enzyme catalysis
This video explains the difference between nucleotides, nucleosides and nucleic acids.
Nucleotidases are hydrolytic enzymes which break down nucleotides (such as the thymine nucleotide) into nucleosides (such as thymidine) and phosphate.