Many nucleotides join together to form a chain of polynucleotides and form the structure of DNA and RNA. Each nucleotide has three components : nitrogenous base, pentose sugar (ribose in RNA and de-oxyribose in DNA) and a phosphate group. There are two types of nitrogenous bases—purines (adenine and guanine) and pyrimidines (cytosine, uracil and thymine). Cytosine is found in both DNA and RNA, whereas thymine is found in DNA. Uracil is found in RNA in place of thymine.
Nitrogenous base by binding to pentose sugar, forms nucleotides, such as - adenosine or deoxyadenosine, guanosine or deoxy guanosine, cytidine or deoxycytidine and uridine and deoxy thymidine. When the phosphate group gets attached to the nucleoside 5' hydroxyl group by phosphoester bond, then the corresponding nucleotide is formed. It happens that two nucleotides are joined by 3'-5' phosphoester bond to form a dinucleotides. In this way, several nucleotides join together to form a polynucleotide chain.