Difference Between Nucleotide and Nucleoside

The key difference between nucleotide and nucleoside is that the nucleotide contains a phosphate group while the nucleoside lacks a phosphate group.

Nucleosides and nucleotides are a similar type of molecules that differ by a slight structural change. Both nucleotide and nucleoside are composed of same two components; a pentose sugar and a nitrogenous base. Additionally, nucleotide has one or more phosphate group. Hence, by adding a phosphate group, a nucleoside can be converted into a nucleotide by the enzymes called kinase. Nucleotide is the building block of nucleic acids. On the other hand, nucleosides are good anticancer and antiviral substances.

CONTENTS

1. Overview and Key Difference
2. What is a Nucleotide
3. What is a Nucleoside
4. Similarities Between Nucleotide and Nucleoside
5. Side by Side Comparison – Nucleotide vs Nucleoside in Tabular Form
6. Summary

What is a Nucleotide?

Nucleotide is the building block of two crucial macromolecules (nucleic acids) in living organisms called DNA and RNA. They are the genetic material of an organism and are responsible for passing genetic characteristics from one generation to the next generation. Furthermore, they are important in controlling and maintaining cellular functions. Other than these two macromolecules, there are several other important nucleotides. For example, ATP (Adenosine tri phosphate) and GTP are two important energy molecules. NADP (Nicotinamide adenine dinucleotide phosphate) and FAD (flavin adenine dinucleotide) are nucleotides which act as cofactors. Nucleotides like CAM (cyclic adenosine monophosphate) are essential for cell signalling pathways.

Structure

A nucleotide has three components namely a pentose sugar molecule, a nitrogenous base and phosphate group/s. Nucleotides differ from each other based on the type of pentose sugar molecule, a nitrogenous base and the number of phosphate groups. For example, deoxyribonucleotide has a deoxyribose sugar while ribonucleotide has a ribose sugar. There are mainly two groups of nitrogenous bases such as purines and pyrimidines.

Structurally, pyrimidines are smaller heterocyclic, aromatic, six-membered rings containing nitrogen atoms at 1 and 3 positions. Cytosine, thymine and uracil are pyrimidine bases. On the other hand, purine bases are much larger than pyrimidines. Other than the heterocyclic aromatic ring, they have an imidazole ring fused to that. Adenine and guanine are the two purine bases. When constructing the DNA and RNA molecules, complementary bases form hydrogen bonds between them. That is adenine: thiamine/uracil and guanine: cytosine are complementary to each other. Phosphate groups link with the –OH group of 5 carbon of the sugar molecule.

In the nucleotides of DNA and RNA, normally there is a one phosphate group. However, in ATP, there are three phosphate groups. The linkages between phosphate groups are high energy bonds. Primarily, there are eight basic types of nucleotides in DNA and RNA. And other nucleotides can be derivatives of these eight types. Nucleotides can be linked with each other to form a polymer such as DNA and RNA. This linkage occurs between the phosphate group of one nucleotide with a hydroxyl group of the sugar molecule of the second nucleotide. It is the phosphodiester bond that joins nucleotides and forms DNA and RNA.

What is a Nucleoside?

Nucleoside is a nucleobase attached to a sugar molecule usually a pentose sugar; ribose or deoxyribose.  This linkage refers to as a beta-glycosidic bond. The significant feature of the nucleoside is that, if a nucleoside links with a phosphate group, eventually it becomes a nucleotide or a nucleoside monophosphate, which is the basic unit of nucleic acids.

This reaction catalyzes by the enzymes called kinases. Hence, if a nucleic acid digests with a nucleotidase enzyme, nucleosides can be formed. Nucleosides are good anticancer agents, and also they have antiviral properties as well. Examples of nucleosides include cytidine, uridine, adenosine, guanosine, thymidine and inosine.

What are the Similarities Between Nucleotide and Nucleoside?

• Both nucleotide and nucleoside have a pentose sugar and a nitrogenous base.
• When a phosphate group links with a nucleoside, eventually it becomes a nucleotide.

What is the Difference Between Nucleotide and Nucleoside?

The key difference between nucleotide and nucleoside is that the nucleotide has a phosphate group while nucleoside lacks that. Other parts like the sugar molecules and nitrogenous bases are common to both, nucleotide and nucleoside. Normally, in living cells, nucleotides are the functional units, not nucleosides. This is because nucleotides are the building blocks of nucleic acids and certain nucleotides serve as the energy currency of the cell. However, nucleosides are also important in medicine since they have anticancer and antiviral properties. So, there is a difference between nucleotide and nucleoside in their fuctions too.

Summary – Nucleotide vs Nucleoside

Nucleotide and nucleoside are important molecules. The key difference between nucleotide and nucleoside is the presence and absence of a phosphate group/s. Nucleotide has three components namely a pentose sugar, a nitrogenous base and a phosphate group while nucleoside has two components namely pentose sugar and a nitrogenous base. It lacks a phosphate group. Furthermore, nucleosides are good anticancer and antiviral substances while nucleotides are building blocks of DNA and RNA and some are energy molecules. However, malfunction nucleotides can cause fatal cancers too.

Reference:

1.“Nucleoside.” Wikipedia, Wikimedia Foundation, 18 Aug. 2018. Available here  

Image Courtesy:

1.”Nucleotide nucleoside general vi”By Yikrazuul, Translator: Phan Thành Đăng – Own work, (CC BY-SA 4.0) via Commons Wikimedia 
2.”Aristeromycin”By DMacks (talk) – Own work, (Public Domain) via Commons Wikimedia