Nucleic Acids – Definition, Function, Structure and Types

Nucleic Acids: Just like we save our important data in a pen drive, similarly, the genetic information is saved within cells in the form of DNA and RNA. This genetic information is transferred from parents to offspring. Nucleic acids are polymers made up of monomeric units of nucleotides. The nucleotide is composed of a nitrogenous base, pentose sugar and phosphoric acid. DNA is the genetic material present in all prokaryotes, eukaryotes and certain viruses, but RNA is the genetic material present only in a few retroviruses and viroids. 

What are Nucleic Acids?

Nucleic acids are macromolecules present in all living cells, either in the state or in combination with other substances. These are long chains that are formed by end-to-end polymerisation of a large number of units called nucleotides joined by phosphodiester bonds. “Nucleic acid” is the term used to describe specific large molecules in the cell.

Nucleic Acid Properties

i) A nucleic acid is composed of nucleotides. 
ii) These constitute the genetic material of all living organisms.
iii) Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are two types of nucleic acids present in the living cell.
iv) Both DNA and RNA were discovered by Friedrich Miescher in \(1969.\)
v) There are three chemically distinct components in a nucleotide. One of them is a heterocyclic base, i.e. nitrogenous base, the second is a monosaccharide pentose sugar, and the third is phosphoric acid or phosphate group. 
vi) The nitrogenous bases are composed of one or two heterocyclic rings having nitrogen atoms present in the ring. These are adenine (A), guanine (G), uracil (U), cytosine (C) and thymine or 5-methyluracil (T).
vii) Adenine and guanine are substituted purines (2 heterocyclic rings), while uracil, cytosine and thymine are substituted pyrimidines (1 heterocyclic ring). 
viii) DNA has A, T, G and C, while RNA has A, U, G and C nitrogenous bases.
ix) Polynucleotides contain either beta-ribose sugar (in RNA) or beta \({\rm{2′}}\) deoxyribose sugar (in DNA). 
x) Nucleosides: Base \(+\) Sugar
xi) Nucleotides: Base \(+\) Sugar \(+\) Phosphate
xii) Phosphodiester bonds, i.e. sugar and phosphate residues, make the backbone of the strands of DNA.
xiii) They are acidic in nature and negatively charged due to the presence of phosphate groups.

Nucleic Acids Structure

The nucleotide is the small unitary structure of nucleic acids, which are joined by phosphodiester bonds. Each nucleotide comprises – 1) A nitrogen base, 2) a pentose sugar, and 3) a phosphoric acid.

A nucleoside is made up of a pentose sugar bonded with the nitrogenous base by N-glycosidic linkage.

Nitrogen base \({\rm{ + }}\) Pentose sugar \(=\) Nucleoside

1) Nitrogen Bases: The nitrogenous components present in the nucleotide are called nitrogen bases. The five nitrogenous bases are derived from two types of heterocyclic bases such as – purine and pyrimidine.

a) Purines are double ring bases. The two principal purine bases found in DNA and RNA are adenine (A) and guanine (G).

b) Pyrimidines are single ring bases. The pyrimidine bases present in DNA molecules are cytosine (C) and thymine (T), and in the RNA molecule, the pyrimidine bases are cytosine (C) and uracil (U).

Fig: Nitrogen Bases

2) Pentose Sugar: The sugar molecule or a monosaccharide with five carbon atoms is known as pentose sugar. The pentose sugar in nucleic acid is an aldose sugar. The nucleic acid, which contains a ribose sugar, is RNA, and the nucleic acid, which contains a Beta-\({\rm{2′}}\)-deoxyribose sugar, is called DNA. These two sugars differ in their chemical nature. The molecular formula of ribose sugar is \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{10}}}}{{\rm{O}}_{\rm{5}}}{\rm{,}}\) while that of Beta-\({\rm{2′}}\)-deoxyribose sugar is \({{\rm{C}}_{\rm{5}}}{{\rm{H}}_{{\rm{10}}}}{{\rm{O}}_{\rm{4}}}{\rm{.}}\) These sugars form a bond with the phosphate groups with OH groups of \({\rm{5′}}\) and \({\rm{3′}}\) carbon, while it forms bonds with OH groups of \({\rm{1′}}\) carbon with nitrogenous bases.

Fig: Deoxyribose and Ribose Sugar

3) Phosphoric Acid: The formula is \({{\rm{H}}_{\rm{3}}}{\rm{P}}{{\rm{O}}_{\rm{4}}}{\rm{.}}\) The phosphoric acid contains three reactive -\({\rm{OH}}\) groups. Out of these, two are involved in forming a sugar-phosphate backbone with the help of phosphodiester linkages.

Fig: Structure of Nucleic Acids

Bonds Between Different Units of Nucleotides

A nucleotide unit shows the following types of linkage or bond in its components:
(a) N–glycosidic linkage: A nitrogenous base is linked to the pentose sugar through a N– glycosidic linkage to form a nucleoside. Purine nucleosides have \(1′ – 9’\) glycosidic linkage (carbon \(1’\) of sugar and \(9’\) nitrogen of \({\rm{A/G}}\)). Pyrimidine nucleosides have \(1′ – 1’\) linkage (sugar carbon \(1’\) and \(1’\) nitrogen of \({\rm{T/C}}\)).
(b) Phosphoester linkage: When a phosphate group is linked to \({\rm{5’\;–\;OH}}\) of a pentose sugar of nucleoside through phosphoester linkage, a corresponding nucleotide is formed. Two nucleotides are linked through \({\rm{3’\;–\;5′}}\) phosphodiester linkage to form a dinucleotide.

Difference between Nucleoside and Nucleotide

What Are the Types of Nucleic Acids?

Nucleic acids are of two types:
1. Deoxyribonucleic acid (DNA)
2. Ribonucleic Acid (RNA)

 Fig: Two types of Nucleic Acids- DNA & RNA

1. DNA: DNA is the main genetic material for all living organisms. It is a double-stranded nucleic acid molecule.
Occurrence: DNA is mainly found in the chromosomes in the nucleus of plant and animal cells. It is also found in mitochondria and chloroplast. In prokaryotes, it is present in the cytoplasm in circular and supercoiled chromosomes. But in eukaryotes, it is present with proteins like histones and protamine.
Structure: The most widely accepted structural model of DNA is the double-stranded double-helical model of Watson and Crick and Wilkins \((1953).\) According to the model, the structure of DNA is described below:

a) Each chain of DNA forms a right-handed helical spiral, and two chains coil around each other to form a double helix.

b) The linkage between the sugar and phosphate molecule is called the phosphodiester bond, and the bases project inside.

c) The chains run in the opposite direction that is antiparallel, i.e., one strand runs from the \(5′ \to 3’\) direction and the other runs from the \(3′ \to 5’\) direction.

d) The nitrogenous bases on one strand pair with the bases on another strand by hydrogen bonds. Adenine pairs with thymine (A-T) by \({\rm{2 H}}\) -bonds, and guanine pairs with cytosine (G-C) by \({\rm{3 H}}\) -bonds. This pairing confers stability to the helical structure.

e) The chains are complementary to each other since, for every adenine in one chain, there will be thymine in the other same way; for every guanine, there will be a cytosine in the other and so on.

f) DNA has a uniform thickness of \({\rm{2\;nm}}\) \(\left( {20\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)

g) Each turn of the double helix or the pitch of the helix is \({\rm{3}}{\rm{.4 \;nm}}\) \(\left( {34\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)

h) It has roughly \(10\) base pairs in each turn. The distance between two adjacent base pairs is approximately equal to \({\rm{0}}{\rm{.34\;nm}}\) \(\left( {3.4\,\mathop {\rm{A}}\limits^{\rm{o}} } \right).\)

i) Sugar and phosphate forms the backbone of the helix, while bases are aligned towards the axis.

Fig: Structure of DNA

2. RNA: RNA is a kind of single-stranded nucleic acid present as genetic material in few viruses like retroviruses and viroids.
Occurrence: RNA is mainly found in the cytoplasm of cells. It is also found in the nucleolus and nucleoplasm. It is mostly present in a single-stranded polynucleotide form except for few viruses where they have double-stranded RNA.

Structure: a) The single RNA strand is folded upon itself, forming structures like hairpins, either entirely or in certain regions.

b) The double-stranded but non-helical RNA occurs as the genetic material in some plant viruses.

c) Each strand of RNA is made up of many ribonucleotides joined by phosphodiester bonds.

d) The adenine pairs with uracil (A-U), and guanine pairs with cytosine (G-C).

e) RNA is divided into three types, i.e., messenger RNA, ribosomal RNA, and transfer RNA.

Fig: Types of RNAs

Difference between DNA and RNA

Functions of Nucleic Acids

The functions of nucleic acid are mentioned below:
1. Nucleic acids are considered genetic material for all living cells, i.e., transfers hereditary characters from one generation to the next.
2. Nucleic acid can also determine the phenotype of an organism.
3. Certain nucleic acids may have enzymatic activity like ribozymes.
4. Nucleic acids help in the synthesis of protein directly or indirectly.

Summary

Nucleic acid is an organic acid mainly present in the chromosomes in the nucleus, cytoplasm of cells and certain cell organelles like chloroplast and mitochondria. It mainly functions as the genetic material for all living cells. Each nucleotide comprises a nitrogen base, a pentose sugar, and a phosphoric acid.

DNA and RNA are the main two types of nucleic acids. DNA is double-stranded, and RNA is single-stranded. RNA is divided into three types, i.e., messenger RNA, ribosomal RNA, and transfer RNA. Nucleic acids help in the synthesis of protein directly or indirectly.

Frequently Asked Questions (FAQs) on Nucleic Acids

Let’s look at some of the commonly asked questions about Nucleic Acids:

Q.1. What are the components of nucleic acids?
Ans: The components of nucleotide are a nitrogen base, a pentose sugar, and a phosphoric acid.

Q.2. Name the nitrogenous bases present in DNA.
Ans: The nitrogenous bases that are present in DNA are adenine, guanine, cytosine and thymine.

Q.3. What is nucleoside?
Ans: A nitrogenous base is linked to the pentose sugar through a N – glycosidic linkage to form a nucleoside.

Q.4. What are the two types of nucleic acids?
Ans: DNA and RNA are the main two types of nucleic acids.

Q.5. What is the main function of a nucleic acid?
Ans: Nucleic acid can store and transmit genetic information to the next generation of cells.

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