Like DNA, RNA polymers are make up of chains of nucleotides. These nucleotides have three parts: 1) a five carbon ribose sugar, 2) a phosphate molecule and 3) one of four nitrogenous bases: adenine, guanine, cytosine or uracil. RNA nucleotides form polymers of alternating ribose and phosphate units linked by a phosphodiester bridge between the #3 and #5 carbons of neighboring ribose molecules. RNA nucleotides differ from DNA nucleotides by the presence of a hydroxyl group linked to the #2 carbon of the sugar.