A nucleotide is composed of a nitrogenous base, pentose sugar and phosphate group.

Pentose Sugar : Two types of sugar are present a given below : $(a)$ Ribose (in case of $RNA$) $(b)$ Deoxyribose (in case of $DNA$).

Nitrogenous Base : It is a nitrogen containing organic molecule having similar physical properties of a base.

There are two types of nitrogenous bases : $(a)$ Purines (Adenine and Guanine).$(b)$ Pyrimidines (Cytosine, Uracil and Thymine) Out of the Pyrimidins, Cytosine is common for both $DNA$ and $RNA$, while Thymine is present in $DNA$ and Uracil is present in $RNA.$

A nitrogenous base is linked to the pentose sugar through a $N$-glycosidic linkage to form a nucleoside, such as adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine and uridine or deoxythymidine.

When a phosphate group is linked to $5^{\prime}-\mathrm{OH}$ of a nucleoside through phosphoester linkage, a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed.

Two nucleotides are linked through $3^{\prime}-5$ ' phosphodiester linkage to form a dinucleotide.

– More nucleotides can be joined in such a manner to form a polynucleotide chain.

A polymer thus formed has at one end a free phosphate moiety at $5^{\prime}$-end of ribose sugar, which is referred to as $5^{\prime}$-end of polynucleotide chain.

Similarly, at the other end of the polymer the ribose has a free $3^{\prime}-$ $OH$ group which is referred to as $3$ '-end of the polynucleotide chain.

The backbone in a polynucleotide chain is formed due to sugar and phosphate.

The nitrogenous bases linked to sugar moiety project from the backbone.

In RNA, every nucleotide residue has an additional -$OH$ group present at $2^{\prime}$-Position in the ribose.

Also, in $RNA$ the uracil is found at the place of thymine ($5$-methyl uracil, another chemical name for thymine).