Individual nucleotides, in addition to providing the building blocks of nucleic acids, carry out a variety of biological functions. Many nucleotides are molecules built on nucleotides that transport chemical energy in the form of phosphate groups or electrons from one reaction system to another. Others carry metabolites such as acetyl groups between reactions. Still other nucleotides in cyclic form are important in cell regulation. The nitrogenous bases are pyrimidines and purines containing carbon and nitrogen atoms.
Pyrimidines contain one ring and purines contain two rings.
The nitrogenous bases link covalently to ribose or deoxyribose five-carbon sugars. A unit consisting of only the base and sugar without phosphates is called a nucleoside and is named according to its nitrogenous base. To number the phosphate groups, nucleosides with added phosphates. This arrangement creates a backbone chain of alternating sugar and phosphate groups. The nucleotides of RNA chains contain the sugar ribose, and one of the four bases A, U, C, and G. Thymine and uracil differ only in a methyl group linked to the ring in thymine but absent in Uracil.