A nucleotide is composed of a nitrogenous base, pentose sugar and a phosphate group.
Part # 1. A 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 pyrimidines, cytosine is common for both DNA and RNA, while thymine is present in DNA. Uracil is present in RNA in place of thymine.
Part # 2. A Pentose Sugar:
Two types of sugars are present in RNA and DNA respectively.
Part # 3. A Phosphate Group:
A nucleoside is formed, when a nitrogenous base is linked to a pentose sugar through a N-glycosidic linkage.
For example, Adenine (N-glycosidic linkage) + Ribose sugar → Adenosine
Adenine + Deoxyribose sugar → Deoxyadenosine
Guanine + Ribose → Guanosine
Guanine + Deoxyribose → Deoxyguanosine.
A nucleotide (or deoxy nucleotide) is formed, depending upon the type of sugar present, when a phosphate group is linked to 5′ OH of a nucleoside through a phosphoester linkage. Two nucleotides when linked through a 3′ —5′ phosphodiester linkage, form a dinucleotide. In a similar fashion, more nucleotides may join to form a polynucleotide chain.
Of the two ends of the polymer chain formed:
(a) One end has a free phosphate moiety at 5′ end of ribose sugar. This is marked as 5′ end of polynucleotide.
(b) The other end has a free hydroxyl (—OH) group at the 3′ end of the ribose sugar. This is marked as 3′ end of the polynucleotide chain. Thus, the sugar and phosphates form the backbone in a polymer chain and the nitrogenous bases linked to sugar moiety project from this backbone.
In the structure of RNA, there is an additional – OH group at 2’ position in the ribose of every nucleotide residue.