Difference Between Nucleotide and Nucleoside

Main Difference – Nucleotide vs Nucleoside

Nucleotide and nucleoside are building blocks of nucleic acids. Nucleotide contains a nitrogenous base, sugar and a phosphate group and nucleoside contains only a nitrogenous base and a phosphate group. The sugar molecule can be either deoxyribose or ribose. Phosphorylation of a nucleoside at the 5’ carbon of the sugar converts a nucleoside into a nucleotide. Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are nucleotide polymers. The main difference between nucleotide and nucleoside is that nucleotide is the precursor of both DNA and RNA whereas nucleoside is the precursor of nucleotide.

This article explains,

1. What is a Nucleotide
      – Structure, Characteristics, Function
2. What is a Nucleoside
      – Structure, Characteristics, Function
3. What is the difference between Nucleotide and Nucleoside

What is a Nucleotide

A nucleotide is a compound containing a nitrogenous base and a phosphate group attached to a pentose sugar, which can be either a ribose or a deoxyribose. One to three phosphate groups can be attached to the 5’ carbon of the pentose sugar. The nitrogenous base can be either a purine or pyrimidine. Purine bases are adenine and guanine and pyrimidine bases are cytosine, uracil, and thymine. Examples for nucleotides, corresponding to the nitrogenous base is shown in table 1.

Table 1: Nucleotides

DNA and RNA are formed by the polymerization of nucleotides. End-to-end connecting of nucleotides forms the backbone of DNA and RNA by removing a diphosphate in order to link 5′ phosphate group of the first nucleotide with the 3′ OH group of the second nucleotide via a phosphodiester bond. Hence, a single phosphate group remains in the polynucleotide chain. When ribose is the sugar in the nucleotide, the forming polynucleotide is called as RNA. On the contrary, when the pentose sugar is deoxyribose, the forming polynucleotide is called as DNA. The nitrogenous bases in RNA are adenine, guanine, cytosine and uracil. However, in DNA, uracil is replaced by thymine. DNA is a double-stranded molecule, which consists of a directionality in each of the two chains. One chain in the double-stranded structure bears a 3′ to 5′ directionality, while the other chain bears a 5′ to 3′ directionality. Two strands of the DNA is held by the hydrogen bonds, which are formed between complementary nucleotides.

Figure 1: Structure of DNA and Nucleotide

Nucleotides also serve as an energy source. ATP is a widely used energy source in many biochemical processes, while GTP serves as the energy source for protein synthesis. On the other hand, cyclic AMP is involved in the signal transduction pathways of both nervous system and endocrine system. Other than that, dideoxynucleotides are used in sequencing in order to achieve the chain termination. LNA, PNA, and morpholino are analogous for the sugar backbone in RNA, regulating the gene expression.  

What is Nucleoside

A nucleotide without any phosphate group bound to the 5′ carbon of the pentose sugar is known as a nucleoside. That means, nucleotide is composed of a nucleoside, which binds with one to three phosphate groups. Hence, a nucleoside is composed of a nitrogenous base and a pentose sugar. Nitrogenous bases and the pentose sugars of a nucleoside are same as in the nucleotide. Adenosine, thymidine, uridine, guanosine, cytidine and inosine are examples for nucleosides. The nitrogenous base is bound to the 3′ position of the pentose sugar via a beta-glycosidic bond. The relationship between a nucleoside and nucleotide is shown in figure 2. 

Figure 2: Nucleoside and nucleotide

Nucleoside analogous can be used as antiviral and anticancer agents. A diet which is rich in nucleosides would be important for optimal health.

Difference Between Nucleotide and Nucleoside

Chemical Composition

Nucleotide: Nucleotide is composed of a nitrogenous base, sugar and a phosphate group.

Nucleoside: Nucleoside is composed of only a nitrogenous base and a phosphate group.

Correspondence

Nucleotide: A nucleotide is the precursor of polynucleotides, DNA and RNA.

Nucleoside: A nucleoside is the precursor of nucleotide.

Importance

Nucleotide: Nucleotides are used in signal transduction pathways, sequencing and as an energy source.

Nucleoside: A diet which is rich in nucleosides would be important for optimal health.

Relevance in Medicine

Nucleotide: LNA, PNA, and morpholino are analogous for the sugar backbone in RNA, regulating the gene expression.  

Nucleoside: Several nucleoside analogues are used as antiviral or anticancer agents.

Examples

Nucleotide: Adenine monophosphate, adenine diphosphate, and adenine triphosphate are the examples for adenine nucleotides. 

Nucleoside: Adenosine, thymidine, uridine, guanosine, cytidine and inosine are the examples for nucleosides.

Conclusion

Nucleotide and nucleoside can serve as precursors of polynucleotides, either in DNA or RNA. A nucleoside is composed of a nitrogenous base and a pentose sugar. On the contrary, a nucleotide consists of a nitrogenous base, pentose sugar, and phosphate groups. Hence, a nucleoside can be considered as the precursor of nucleotide. Pentose sugar can be either a ribose, deoxyribose or a dideoxyribose. Nucleotides containing dideoxyribose sugars can be used to terminate the chain growth in sequencing. In DNA, adenine, guanine, cytosine and thymine are the bases found in the polynucleotide chain. But in RNA, thymine is replaced by uracil. The main difference between nucleotide and nucleoside is their correspondence with each other.

Reference:
1. Berg, Jeremy M. “A Nucleic Acid Consists of Four Kinds of Bases Linked to a Sugar-Phosphate Backbone.” Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 24 Mar. 2017.
2. Berg, Jeremy M. “Nucleotide Biosynthesis.” Biochemistry. 5th edition. U.S. National Library of Medicine, 01 Jan. 1970. Web. 24 Mar. 2017.

Image Courtesy:
1. “0322 DNA Nucleotides” By OpenStax – (CC BY 4.0) via Commons Wikimedia
2. “Nucleotides 1” By Boris (PNG), SVG by Sjef – en:Image:Nucleotides.png (Public Domain) via Commons Wikimedia