2.21.1 Chemical Structure of Nucleic Acids

Types of Nucleic Acids:

Monomers:

• The monomers of nucleic acids are called nucleotides.
• Nucleotides of DNA are called deoxyribonucleotides.
• Nucleotides of RNA are known as ribonucleotides.

Composition of a Nucleotide

Each nucleotide consists of three components:

• Pentose sugar
• Phosphate group
• Nitrogen-containing ring structure called a base

Detailed Composition:

• In deoxyribonucleotides: Deoxyribose
• In ribonucleotides: Ribose

• Phosphoric acid is a common component of both nucleotides.
• It provides acidic properties to DNA and RNA.

Nitrogenous Bases: - The nitrogen-containing ring structures are called bases because of the unshared pair of electrons on nitrogen atoms, which can thus acquire a proton.

Formation of a Nucleotide

• First, a nitrogenous base is linked with the 1' carbon of pentose sugar.
• Such a combination is called a nucleoside.

Step 2: Nucleotide Formation - When a phosphoric acid is linked with the 5' carbon of pentose sugar of a nucleoside, the nucleotide is formed.

Types of Nucleotides Based on Number of Phosphates:

Structure of a Nucleotide

General Structure:

Base - Sugar - Phosphate

Detailed Structure:

Nucleotides in DNA and RNA

• Sugar: Deoxyribose
• Bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C)
• Phosphate: Phosphoric acid

• Sugar: Ribose
• Bases: Adenine (A), Uracil (U), Guanine (G), Cytosine (C)
• Phosphate: Phosphoric acid

Importance of Nucleotides

• Building blocks of nucleic acids: Nucleotides are the monomers of DNA and RNA.
• Energy currency: ATP (adenosine triphosphate) is a nucleotide that serves as the energy currency of the cell.
• Coenzymes: NAD, FAD, and coenzyme A are nucleotide derivatives.
• Signaling molecules: cAMP (cyclic AMP) is a nucleotide derivative that acts as a second messenger.
• Genetic information: The sequence of nucleotides in DNA stores genetic information.

Intermediate Level

Chemical Structure of Nucleic Acids

• Linear, unbranched polymers.
• Monomers: Nucleotides.

Nucleotides: - Sugar + Phosphate + Base.

Nucleosides: - Sugar + Base (no phosphate).

Types of Nucleic Acids:

Composition of a Nucleotide

Formation of Nucleotide

Step 1: Nucleoside Formation Base + Sugar → Nucleoside

Step 2: Nucleotide Formation Nucleoside + Phosphate → Nucleotide

Types of Nucleotides

Importance of Nucleotides

Important Points

• 1. Nucleic acids were first reported (in 1869) by a Swiss physician when he isolated a new compound from the nuclei of pus cells (white blood cells).
• 2. This compound was neither a protein nor a lipid nor a carbohydrate; therefore, it was a novel type of biological molecule.
• 3. He named this molecule as nuclein, because it was located in the nucleus.
• 4. The basic structure and chemical nature of nuclein was determined (in 1920) and was renamed as nucleic acid because of its acidic nature.
• 5. Now it has been cleared that nucleic acids are of two types, i.e.:
• - Deoxyribonucleic acid (DNA)
• - Ribonucleic acid (RNA)
• 6. Both nucleic acids are linear, unbranched polymers.
• 7. The monomers of the nucleic acid are called nucleotides.
• 8. Composition of a nucleotide: Nucleotides of DNA are called deoxyribonucleotides and of RNA are known as ribonucleotides.
• 9. Each nucleotide consists of:
• - Pentose sugar
• - A phosphate
• - A nitrogen containing ring structure called base
• 10. The pentose sugar in deoxyribonucleotides is deoxyribose and in ribonucleotides is ribose.
• 11. Phosphoric acid is a common component of both nucleotides which provides acidic properties to DNA and RNA.
• 12. The nitrogen containing ring structures are called bases because of unshared pair of electron on nitrogen atoms, which can thus acquire a proton.
• 13. During the formation of a nucleotide, first nitrogenous base is linked with 1' carbon of pentose sugar. Such combination is called nucleoside.
• 14. When a phosphoric acid is linked with 5' carbon of pentose sugar of a nucleoside, the nucleotide is formed.
• 15. A nucleotide with one phosphoric acid is called nucleoside monophosphate, with two phosphoric acids is called nucleoside diphosphate, and with three phosphoric acids is called nucleoside triphosphate.

Remember

• - **Nucleic acids** = DNA and RNA
• - Nucleic acids = linear, unbranched polymers of nucleotides
• - Nucleotides = monomers of nucleic acids
• - Nucleotide = sugar + phosphate + base
• - Nucleoside = sugar + base (no phosphate)
• - **DNA** = deoxyribose + A, T, G, C
• - **RNA** = ribose + A, U, G, C
• - Phosphate = provides acidic properties
• - Base = nitrogen-containing ring structure
• - Nucleoside monophosphate = 1 phosphate
• - Nucleoside diphosphate = 2 phosphates
• - Nucleoside triphosphate = 3 phosphates

Common Mistakes

• 1. Confusing nucleic acids and nucleotides – Nucleic acids are polymers; nucleotides are the monomers.
• 2. Confusing nucleosides and nucleotides – Nucleosides have sugar + base; nucleotides have sugar + base + phosphate.
• 3. Thinking DNA and RNA have the same sugar – DNA has deoxyribose; RNA has ribose.
• 4. Thinking DNA and RNA have the same bases – DNA has A, T, G, C; RNA has A, U, G, C.
• 5. Forgetting that phosphate provides acidic properties – Phosphoric acid provides acidic properties to DNA and RNA.
• 6. Confusing the 1' and 5' carbons – Base attaches to 1' carbon; phosphate attaches to 5' carbon.
• 7. Thinking all nucleotides have three phosphates – Nucleotides can have one, two, or three phosphates.
• 8. Confusing purines and pyrimidines – Purines = double ring (A, G); Pyrimidines = single ring (C, T, U).
• 9. Thinking nucleic acids are only in the nucleus – RNA is also found in the cytoplasm.
• 10. Forgetting the historical background – Nuclein was discovered in 1869; renamed nucleic acid in 1920.
• 11. Thinking nucleotides are only building blocks – Nucleotides also have other functions (energy currency, coenzymes, signaling).
• 12. Confusing the terms "nucleoside" and "nucleotide" – Nucleoside = sugar + base; Nucleotide = sugar + base + phosphate.

Quick Revision

• Nucleic acids were first reported (in 1869) by a Swiss physician when he isolated a new compound from the nuclei of pus cells (white blood cells).
• This compound was neither a protein nor lipid nor a carbohydrate; therefore, it was a novel type of biological molecule.
• He named this molecule as nuclein, because it was located in the nucleus.
• The basic structure and chemical nature of nuclein was determined (in 1920) and was renamed as nucleic acid because of its acidic nature.
• Now it has been cleared that nucleic acids are of two types i.e., deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).
• Both nucleic acids are linear un-branched polymers.
• The monomers of the nucleic acid are called nucleotides.
• Composition of a nucleotide:
• Nucleotides of DNA are called deoxyribonucleotides and of RNA are known as ribonucleotides.
• Each nucleotide consists of pentose sugar, a phosphate and a nitrogen containing ring structure called base.
• The pentose sugar in deoxyribonucleotides is deoxyribose and in ribonucleotides is ribose.
• Phosphoric acid is a common component of both nucleotides which provides acidic properties to DNA and RNA.
• The nitrogen containing ring structures are called bases because of unshared pair of electron on nitrogen atoms, which can thus acquire a proton.
• During the formation of a nucleotide, first nitrogenous base is linked with 1' carbon of pentose sugar. Such combination is called nucleoside.
• When a phosphoric acid is linked with 5' carbon of pentose sugar of a nucleoside, the nucleotide is formed.
• A nucleotide with one phosphoric acid is called nucleoside monophosphate with two phosphoric acids is called nucleoside diphosphate and with three phosphoric acids is called nucleoside triphosphate.

60-Second Revision

• **Nucleic acids** = DNA and RNA
• Nucleic acids = linear, unbranched polymers of nucleotides
• Nucleotides = monomers of nucleic acids
• Nucleotide = sugar + phosphate + base
• Nucleoside = sugar + base (no phosphate)
• **DNA** = deoxyribose + A, T, G, C
• **RNA** = ribose + A, U, G, C
• Phosphate = provides acidic properties
• Base = nitrogen-containing ring structure
• Nucleoside monophosphate = 1 phosphate
• Nucleoside diphosphate = 2 phosphates
• Nucleoside triphosphate = 3 phosphates
• Warning: Nucleic acids and nucleotides are not the same. Nucleosides and nucleotides are not the same. DNA and RNA have different sugars and bases. Phosphate provides acidic properties. Base attaches to 1' carbon; phosphate attaches to 5' carbon. Do not confuse purines and pyrimidines.