by Microbiology Doctor-dr

What is Nucleoside?

A nucleoside is a molecule having a pentose sugar coupled to a nitrogenous base or glycosylamines. A nucleoside may alternatively be described as a nucleotide without a phosphate group connected to it.
The nucleosides contained in DNA have a 2` – deoxy – D- ribose sugar while nucleosides in RNA contain D-ribose sugar.
The fundamental difference is visible at the second position of the pentose structure, in the case of 2` – deoxy -ribose there is an absence of an alcohol group/ oxy group/ -OH group at the second position, thus the name.
In the instance of D – ribose pentose the –OH group is present at the second position.
In both kinds, the pentoses are present in their β-furanose form which is a tight five-membered ring structure.

Types of Nucleoside

The nucleosides may be split into two categories depending on the presence of the nitrogen base of the chemical.

Purine nucleosides

These nucleosides are made of two nitrogenous bases known as Adenine and Guanine.

Pyrimidine nucleosides

These nucleotides are made of three nitrogenous bases known as Thymine, Cytosine, and Uracil.

Structure of Nucleoside

The nucleosides consist of two primary heterocyclic components -

Pentose sugar

It is a five-membered ring structure often characterized as a puckered conformation.
In the case of DNA, the nucleosides lack –OH group at the second position of pentose ring and thus is designated as 2` – deoxy – D – ribose.
In RNA it is a D – ribose pentose ring structure.
The pentoses are found in β- furanose form in both kinds of sugars.
The pentose sugar is linked to the nitrogenous base at the main carbon atom (1`).
It is connected by a glycosidic bond (N-β-glycosyl bond).

Nitrogenous base

It is a cyclic carbon structure containing nitrogen and has the qualities of a base.
The bases are derivatives of two primary parent chemicals – Purines and Pyrimidines.
In RNA the nitrogenous bases are - Adenine, Uracil, Cytosine, and Guanine.
In DNA the nitrogenous bases are - Adenine, Thymine, Cytosine, and Guanine.
The bases are bonded covalently to the pentose sugar via N-β-glycosyl bond.
In the case of purines the N-9 atom links to the pentose sugar.
In the case of pyrimidines the N-1 atom links to the pentose sugar.
Although the bulk of the bases are generated from purines and pyrimidines, other minor bases are also present in the DNA.
Mostly these minor bases appear in methylated versions of purines and pyrimidines.
In the case of certain viral DNA, the bases could be hydroxyl-methylated or they might be glycosylated.
Some of the uncommon nucleotides present in DNA include 5-Methlycytidine, N6-Methlyadenosine, 7-Methlyguanosine, 4-Thiouridine.

Functions of Nucleosides

Nucleosides are a fundamental component of nucleotides. They are precursors for nucleotides. When a phosphate group is bonded to the nucleoside it produces the nucleotide which is the backbone of DNA.
Nucleoside compounds also operate as signaling molecules.
Minor bases or changed nitrogen bases in nucleosides play a function in controlling or safeguarding genetic information.
Many nucleoside analogs have been employed in the treatment of malignancies, tumors, and viral infections. This is done by certain alterations in the purine and pyrimidine bases.
Cytarabine (cytosine arabinoside) was the first medicine licensed by the US FDA for therapy against acute myeloid leukemia.
Some nucleoside analogs are also utilized in treatments of Human Immunodeficiency Virus (HIV) illness. Lamivudine is being utilized in this therapy.
Nucleoside transporters play a critical function in the movement of nucleosides across membranes.
There are two kinds of nucleoside transporters - Concentrative and Equilibrative. These transporters serve a crucial role in the transfer of antiviral and anticancer medicines across membranes.

Nucleoside - Definition, Types, Structure, Functions by Doctor-dr