Nucleotides are the phosphoric esters of nucleosides. Depending on the nature of the pentose one will have ribonucleotides and deoxyribonucleotides. A ribonucleoside has 3 positions, which can be phosphorylated (2′, 3′ and 5′) while a deoxyribonucleoside can be phosphorylated only in two places (3′ and 5′). This results in the formation of nucleosides – monophosphate.
A second phosphate group can be bound to the phosphate of a nucleoside-monophosphate to form a nucleoside-di-phosphate. Likewise a third phosphate group can also be attached to the second forming nucleosides-tri-phosphate. Primary structure of DNA
In deoxyribonucleic acids, the nucleotides are joined by 3’~5′ phosphodiester bonds; in other words each phosphate group (except those present at the end of chains) esterifies to the 3′ hydroxyl group of a pentose and to the 5′ hydroxyl group of the next pentose. Therefore, the polydeoxyribonucleotide chain consists of alternating deoxyribose and phosphate residues.