Difference Between Nucleotide and Nucleoside, Practice Problems and FAQs

Have you ever seen how long chains of flowers are made for decorating the house during festivals? Flowers are added one after another and weaved together using a thread to form a long chain. Did you know that the DNA in your cell is assembled in a similar way? It is made up of repeating units of a compound known as nucleotide and is therefore considered as a polynucleotide chain. The bond between the nucleotides is the thread that joins them together.

But not just DNA, even RNA is composed in a similar way but the nucleotides in DNA and RNA are different in many respects. In fact, not all the nucleotides in a DNA or RNA are identical to each other, they vary in their composition. The variable component of a nucleotide is the nucleoside which is nothing but the nucleotide devoid of its phosphate group. In this article we will discuss the nucleotides and their nucleosides and how they differ from each other.

Table of Content:

• Nucleotide
• Nomenclature of nucleosides and nucleotides
• Difference between nucleoside and nucleotide
• Practice Problems
• FAQs

Nucleotide

The monomeric subunits of nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are known as nucleotides. Nucleotides are composed of -

• A Nitrogenous base
• A Pentose sugar
• Phosphate group

The presence of a phosphate group (PO₄^3-) makes them acidic at physiological pH. Nucleotides can be mone, di or triphosphates having one, two or three phosphate groups attached to the 5’OH group of the pentose sugar.

Fig: Phosphate group

An N-glycosidic linkage helps the nitrogenous base to bind to the 1’ carbon of the pentose sugar. A phosophoester bond helps the phosphate group to attach to the 5’ hydroxyl (-OH) group of the pentose sugar.

Fig: Structure of a nucleotide

A phosphodiester bond connects the 5’ phosphate group of one nucleotide to the 3’ hydroxyl group r of the adjacent nucleotide in a polynucleotide chain.

Fig: Phosphodiester bond

Pentose sugar

Pentose sugar is a 5 carbon atom containing monosaccharide. Based on the pentose sugar, nucleotides can be of two types - ribonucleotides and deoxyribonucleotides. Ribonucleotides have ribose sugar (having the 2’hydroxyl (-OH) group) whereas deoxyribonucleotides have deoxyribose sugar (oxygen is missing at the 2’ carbon position and only 2’ hydrogen (H) is present). Ribonucleotides are the monomeric unit of RNA (Ribonucleic acid) and deoxyribonucleotides are the monomeric units of DNA (Deoxyribonucleic acid).

Fig: Examples of pentose sugar found in nucleic acids

Nitrogenous bases

These are nitrogen containing compounds having heterocyclic ring structures. Based on the heterocyclic rings present, nitrogenous bases can be:

• Purines (double ring structure) - Adenine and Guanine
• Pyrimidines (single hexagonal ring) - Cytosine, Thymine and Uracil

Fig: Different types of nitrogenous bases

Deoxyribonucleotides in DNA can have either adenine, guanine, cytosine or thymine as nitrogenous bases. Ribonucleotides in RNA have either adenine, guanine, cytosine or uracil as nitrogenous bases. In RNA, thymine is replaced by uracil.

Fig: Nitrogenous bases - Purines

Fig: Nitrogenous bases - Pyrimidines

Nucleoside

A nucleotide without the phosphate group, that is, a pentose sugar having a nitrogenous base attached to its 1’ carbon is known as a nucleoside. A N-glycosidic bond helps the 1’ carbon of the pentose sugar to bind to the 1^st nitrogen of the pyrimidines and 9^th nitrogen of the purines.

Fig: Nucleoside

Addition of a phosphate group to the 5’OH group of the pentose sugar in a nucleoside, converts it into a nucleotide.

Nomenclature of nucleosides and nucleotides

Nucleosides

Ribonucleosides

Deoxyribonucleosides

Nucleotides

Ribonucleotides

Deoxyribonucleotides

Differences between Nucleoside and nucleotide

Practice Problems

1. Cytidylic acid is composed of

1. Cytosine + ribose
2. Cytosine + phosphate
3. Cytidine + phosphate
4. Cytidine + ribose

Solution: Cytidylic acid is a ribonucleotide formed by association between Cytidine and phosphate. Cytidine is the nucleoside made up of ribose sugar and cytosine and it combines with a phosphate group at its 5’OH group to form cytidylic acid. Thus the correct option is c.

2. Which of the following is not found in an RNA molecule?

1. Adenylic acid
2. Deoxythymidylic acid
3. Guanylic acid
4. Uridylic acid

Solution: RNA is composed of repeating units of ribonucleotides which are made up of a ribose sugar, nitrogenous base and a phosphate group. The nitrogenous bases found in RNA are adenine, guanine, cytosine and uracil (in place of thymine). Thus, the ribonucleotides found in RNA are -

• Adenylic acid or Adenosine monophosphate (AMP)
• Guanylic acid or Guanosine monophosphate (GMP)
• Cytidylic acid or Cytidine monophosphate (CMP)
• Uridylic acid or Uridine monophosphate (UTP)

Deoxythymidylic acid is a deoxyribonucleotide found in DNA. Thus, the correct option is b.

3. Which of the following has a double ring structure?

1. Guanine
2. Cytosine
3. Uracil
4. Thymine

Answer: Purines have a double ring structure consisting of a hexagonal pyrimidine ring and an pentagonal imidazole ring, examples are adenine and guanine. Cytosine, uracil and thymine are pyrimidines with a single hexagonal ring. Thus, the correct option is a.

4. Which of the following options is correct?

1. Nucleoside = Pentose sugar + Phosphate
2. Nucleotide = Pentose sugar + Nitrogenous base
3. Nucleotide = Nucleoside + Phosphate
4. Nucleoside = Pentose sugar + Nitrogenous base + Phosphate

Solution: The monomeric subunits of nucleic acids such as DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are known as nucleotides. Nucleotides are composed of -

• A Nitrogenous base
• A Pentose sugar
• A Phosphate group

An N-glycosidic linkage helps the nitrogenous base to bind to the 1’ carbon of the pentose sugar. A phosophoester bond helps the phosphate group to attach to the 5’ hydroxyl (-OH) group of the pentose sugar.

Fig: Structure of a nucleotide

A nucleotide without the phosphate group, that is, a pentose sugar having a nitrogenous base attached to its 1’ carbon is known as a nucleoside. Thus, a nucleoside and a phosphate group together form a nucleotide and the correct option is c.

FAQs

1. What is cAMP?

Answer: cAMP or cyclic AMP is adenosine monophosphate that functions as a second messenger in the transmission of neural or hormonal messages to the cell. When certain protein or polypeptide hormones bind to the surface receptor of plasma membrane, they stimulate generation of cAMP from ATP by the action of adenylate cyclase enzyme. cAMP activates protein kinase enzymes which add phosphate groups to intracellular enzymes, thus resulting in their activation or deactivation.

2. What are higher nucleotides?

Answer: Nucleotides are mon, di- or triphosphates of nucleosides. When there are more than one phosphate groups attached to the nucleoside, they are known as higher nucleotides. For example, ADP and ATP. Linkage of additional phosphate groups involves a large amount of energy.

3. How are coenzymes formed from nucleotides?

Answer: A coenzyme is an organic carrier molecule which cooperates with a particular enzyme in catalysing a biochemical reaction. Coenzymes are formed from the nucleotides by replacing the nitrogenous base with a vitamin. For example, NAD (Nicotinamide adenine dinucleotide).

4. How are nucleotides paired in DNA double helix?

Answer: DNA is a double stranded helical molecule and the two strands of the double helix are held together by hydrogen bonds between the nitrogenous bases of the nucleotides of opposite strands. Adenine pairs with thymine using two hydrogen bonds and guanine pairs with cytosine using three hydrogen bonds.

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