Describe the structure of a polynucleotide chain of $DNA$ or $RNA$.

(N/A) nucleotide is composed of a nitrogenous base,a pentose sugar,and a phosphate group.
Pentose Sugar: Two types of sugar are present as given below: $(a)$ Ribose (in case of $RNA$) $(b)$ Deoxyribose (in case of $DNA$).
Nitrogenous Base: It is a nitrogen-containing organic molecule having physical properties similar to a base.
There are two types of nitrogenous bases: $(a)$ Purines (Adenine and Guanine). $(b)$ Pyrimidines (Cytosine,Uracil,and Thymine). Out of the pyrimidines,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 an $N$-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 the $5^{\prime}-OH$ of a nucleoside through a phosphoester linkage,a corresponding nucleotide (or deoxynucleotide depending upon the type of sugar present) is formed.
Two nucleotides are linked through a $3^{\prime}-5^{\prime}$ 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 the $5^{\prime}$-end of the ribose sugar,which is referred to as the $5^{\prime}$-end of the polynucleotide chain.
Similarly,at the other end of the polymer,the ribose has a free $3^{\prime}-OH$ group which is referred to as the $3^{\prime}$-end of the polynucleotide chain.
The backbone in a polynucleotide chain is formed due to sugar and phosphate.
The nitrogenous bases linked to the sugar moiety project from the backbone.
In $RNA$,every nucleotide residue has an additional $-OH$ group present at the $2^{\prime}$-position in the ribose.
Also,in $RNA$,the uracil is found in place of thymine ($5$-methyl uracil,another chemical name for thymine).