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Chapter: Biochemistry: Nucleic Acids

Nucleic acids

Two types of nucleic acids are present in all mammalian cells. They are DNA - deoxy ribonucleic acid and RNA- ribonucleic acid. DNA is present in the nucleus and mitochondria. RNA is present in the nucleus, risosome and cytoplasm.

Nucleic acids

Two types of nucleic acids are present in all mammalian cells. They are DNA - deoxy ribonucleic acid and RNA- ribonucleic acid. DNA is present in the nucleus and mitochondria. RNA is present in the nucleus, risosome and cytoplasm.

Nucleic acids are acidic sustances containing nitrogenous bases, pentose sugar and phosphoric acid. Both DNA and RNA are polynucleotides. They are polymers of mononucleotides.

In nucleic acids, nucleotides are joined together by phosphodiester linkages.


A nucleoside is composed of purine or pyrimidine base and a pentose sugar. Two types of pentose sugar are present in nucleoside, they are ribose and deoxy ribose (Fig. 7.3). In the case of purine nucleosides, the sugar is attached to N-9 of the purine ring, whereas in pyrimidine nucleosides, the sugar is attached to N-1 of the pyrimidine ring. The type of linkage is N-glycosidic linkage (Fig. 7.4).


Nucleotides are phosphorylated nucleosides usually one or two of hydroxyl groups of ribose (or) deoxyribose are phosphorylated. Thus a nucleotide has three structural components. They are nitrogenous base, sugar and phosphate. Phosphate is attached to ribose (or) deoxy ribose through an ester linkage (Fig.7.5).

Structure of DNA

Primary structure

Nucleotide sequence of a nucleic acid is known as its primary structure which confers individuality to the polynucleotide chain. Polynucleotide chain has direction. They are represented in 5’---> 3’ and 3’----> 5’ directions. Each polynucleotide chain has 2 ends. The 5’ end carrying a phosphate group and 3’ end carrying an unreacted hydroxyl group (Fig 7.6).

In 1953, J.D. Watson and F.H.C. Crick proposed a precise three dimensional model of DNA structure based on model building studies, base composition and X-ray diffraction studies. This model is popularly known as the DNA double helix (Fig.7.7).

The purine bases present in DNA are adenine and guanine and the pyrimidine bases present are thymine and cytosine. The purine and pyrimidine bases of DNA carry genetic information where as the sugar and phosphate groups perform the structural role.

Salient features of double helix

·           Two polynuleotide chains are coiled around a central axis in the form of a right handed double helix.

·           Each polynucleotide chain is made up of 4 types of nucleotides. They are adenylate, guanidylate, thymidylate and cytidinilate.

·           Each polynucleotide chain has direction or polarity. Further each polynucleotide chain has 5’ phosphorylated and 3’ hydroxyl ends.

·           The backbone of each strand consists of alternating sugar and phosphate. The bases project inwards and they are perpendicular to the central axis.

·           The 2 strands run in opposite direction (ie.) they are antiparallel.

·           The strands are complementary to each other. Base composition of one strand is complementary to the opposite strand. If adenine appears in one strand, thymine is found in the opposite strand and vice versa. When guanine is found in one strand, cytosine is present in the opposite strand and vice versa.

·           Bases of opposite strands are involved in pairing. Pairing occurs through hydrogen bonding and it is specific. Adenine pairs with thymine through two hydrogen bonds.

·           Guanine pairs with cytosine with three hydrogen bonds.

·           Major and minor grooves are present on the double helix. They arise because glycosidic linkages of base pairs are not opposite to each other. Protein interact with DNA through the minor and major grooves without disrupting the DNA strands.

·           According to Chargaff’s observation, the number of adenine base is equal to thymine base and the number of quanine base is equal to number of cytocine base ie. A = T and G = C. Also A + T = G + C and the ratio of A+T /G+C = nearly 1.0. The total number of purine bases = the total number of pyrimidine bases.

Functions of DNA

1.        DNA is the genetic material of living organisms. It is the greatest super chip ever made by man.

2.        DNA contain all the information required for the information of an individual organism.

3.        The genetic information in DNA is converted to characteristic features of living organisms like colour of the skin and eye, height, intelligence, ability to metabolise particular substance, ability to withstand stress, susceptibility to desease and ability to produce or synthesise certain substances.

4.        DNA is the source of information for the synthesis of all cellular proteins. The segment of DNA that contain information for a protein is known as gene.

5.        DNA is transmitted from parents to offsprings and hence transmit genetic information from one generation to another

6.        The amount of DNA in any given species or cell is constant and is not affected by nutritional and metabolic states.

7.        Avery Macleod and Mc Carty in 1944 first demonstrated that DNA contained the genetic information and they referred DNA as ‘transforming factor’

8.   The  nucleotides  present  in  DNA  are  deoxy adenylic acid, deoxy quanidylic acid, deoxy cytidylic acid and deoxy thymidylic acid.

Structure of RNA

RNAs are present in the nucleus, ribosomes and cytoplasm of eukaryolic cells. They are involved in the transfer and expression of genetic information. They act as primer for DNA formation. Some act as enzymes and as coenzymes. RNA also function as genetic material for viruses.

RNAs are also polynucleotides. In RNA polymer, purine and pyrimidine nucleotides are linked together through phosphodiester linkages. The sugar present is ribose. The nitrogenous bases present in RNA are adenine and guanine (purine bases), uracil and cytosine (pyrimidine bases). The nucleotides present in RNA are adenylic acid, quanidylic acid, cytidylic acid and uridylic acid.

Types of RNA

There are mainly three types of RNAs in all prokaryotic and eukaryotic cells. They are (1) Messenger RNA (mRNA) 2) Transfer RNA (tRNA) 3) Ribosomal RNA (rRNA). They differ from each other by size, formation and stability.

1. Messenger RNA

It accounts for 1-5% of cellular RNA. They have a primary structure. They are single standed linear molecules. They consist of 1000-10,000 nucleotides. They have a free or phosphorylated 3’ and 5’ end. They have different life span ranging from few minutes to days.

mRNA molecules are capped at 5’ end by methylated guanine triphosphate. Capping protects mRNA from nuclease attack. At 3’ end a polymer of adenylate (poly A) is found as the tail. Poly A tail protects mRNA from nuclease attack.

Intrastrand base pairing among complementary bases allows folding of the linear molecule. As a result, haripin or loop like secondary structure is formed.


·           mRNA is a direct carrier of genetic information from the nucleus tothe cytoplasm.

·           It contain information required for the synthesis of protein molecules.

2. Transfer RNA

It accounts for 10-15% of total cell RNA. They are the smallest of all the RNAs. Usually they consist of 50-100 nucleotides. They are single standard molecules. They contain unusual bases such as methylated adenine, guanine, cytosine and thymine,dihydrouracil and pseudouridine. These unusual bases are important for binding 6-RNA to intra chain base pairing. Further some bases are not involved in base pairing, resulting in loops and arms formation in tRNA. These folding in the primary structure generates a secondary structure (Fig.7.8).

Secondary structure of t-RNA is in the form of a clover leaf. The important feature of the clover-leaf structure are,

1.        An acceptor arm with base sequence “CCA” 3’-OH of adenosine moiety of t-RNA.

2.        An anticodon arm which recognises codon on mRNA.

3.        TφC arm which contain unusual base cytosine.

4.        D- arm which contain many dihydrouracil residues.


It is the carrier of amino acids to the site of protein synthesis.

There is at least one t-RNA molecule to each of 20 amino acids required for protein synthesis.

3. Ribosomal RNA

This accounts for 80% of the total cellular RNA. It is present in ribosomes. In ribosomes, r-RNA is found in combination with protein. It is known as ribonucleoprotein. The length of rRNA ranges from 100-600 nucleotides. rRNA molecules have a secondary structure. Intra strand base pairing between complementary bases generate double helical segments or loops.


1.        They are required for the formation of risosomes

2.        They are involved in the initiation of protein synthesis.

Differences between DNA and RNA

Examples of nucleosides and nucleotides


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