DNA and RNA: Structure and Types

Key Concepts

• Nucleic Acids

• Discovery of nucleic acids

• Structure of nucleic acids

• DNA – structure, types

• RNA – structure, types

Introduction

Living organisms naturally possess certain chemical substances. These naturally occurring chemical compounds are known as biomolecules. These biomolecules are essential for several biological processes. They are composed of carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur. Biomolecules are classified into four major classes, namely: 

1. Carbohydrates 

2. Proteins 

3. Lipids 

4. Nucleic Acids 

What are nucleic acids? 

Nucleic acids are important biomolecules that are mainly involved in storage and transfer of genetic information. Nucleic acids were first discovered by Swiss biochemist Friedrich Miescher in the year 1869. He discovered these substances in the nuclei of pus cells. He first named these organic compounds as ‘nuclein.’ Later, in the year 1889, Richard Altmann coined the term ‘nucleic acid.’  Since these molecules were first discovered in nucleus, and were found to possess acidic properties, they were named as nucleic acids.  

Nucleic acids are classified into two groups,  

1. Deoxyribonucleic Acid (DNA) 

2. Ribonucleic Acid (RNA) 

All living cells have both DNA and RNA. Whereas, viruses have either DNA or RNA as their genetic material. 

Discovery of Nucleic Acids: 

DNA is the primary genetic material that carries hereditary information. It contains instructions required for the growth, development and function of a living organism. These genetic information are transmitted from one generation to the next generation. In the year 1928, Frederick Griffith, a British bacteriologist identified the “transforming principle” which later led to the discovery of DNA as the genetic material. During his study on bacterial pneumonia, he performed an experimental study using Streptococcus pneumoniae. He used smooth and rough strains of S.pneumoniae for his experiment. The smooth strain has a polysaccharide coating around it and appears smooth when viewed under a microscope. Whereas the rough strain does not have the polysaccharide coating and therefore appears rough when viewed under microscope.  

Griffith’s Experiment: 

Griffith injected smooth and rough strains into mice. The mouse injected with smooth strain developed pneumonia and died. Whereas the mouse injected with rough strain stayed alive. Griffith identified that smooth strains were virulent and rough strains were non-virulent. Griffith heat-killed some smooth strain bacteria and injected those dead cells into mice. He found these mice were not infected and stayed alive. Then, he mixed some of the heat-killed smooth strains with rough strain and injected this mixture into mice. He observed that these mice developed pneumonia and died. In addition, he identified live smooth strain bacteria in dead mice.  

Conclusion of Griffith’s Experiment: 

On the basis of these experimental evidences, Griffith concluded that live rough strain bacteria would have absorbed some material from the heat-killed smooth strain bacteria. This transformation would have made rough strain bacteria virulent. 

 Later in 1944, Oswald Avery and his colleagues further studied Griffith’s experimental findings. They discovered that DNA was the genetic material that was transformed into rough strain bacteria. In 1952, two scientists Alfred Hershey and Martha Chase conducted experimental studies using bacteriophages and proved that DNA is the genetic material.  

Structure of Nucleic Acids: 

Nucleic acids are biopolymers. They are made up of repeated monomeric units known as nucleotides. Each nucleotide has three components – phosphate, sugar and nitrogen containing base.  The sugar group is of two types namely the ribose sugar (RNA) and deoxyribose sugar (DNA).  The ribose sugar is more reactive and is less stable. Whereas, the deoxyribose sugar is less reactive and is more stable. The nitrogenous bases include adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U). These nitrogenous base are categorized into two types, namely, purines and pyrimidines.  Purine bases include adenine (A) and guanine (G). Pyrimidine bases include cytosine (C), thymine (T) and uracil (U). The RNA structure contains bases – A, G, C and U (not T) whereas DNA structure contains bases – A, G, C and T (not U). 

The nucleotides join together to form a long polynucleotide chain. The sugar and phosphate form the backbone structure. The nitrogenous bases project from the sugar – phosphate backbone (Fig.No.4). The phosphate end of the chain is known as 5’ end (five prime end) and the other end is referred to as the 3’ end (three prime end). 

DNA: 

In the year 1953 James Watson and Francis Crick proposed a model structure of DNA. This model was widely accepted and they were awarded the Nobel Prize in 1962. The following are the important features of Watson and Crick’s DNA model. 

• DNA is composed of two polydeoxyribonucleotide chains that run in opposite directions. In other words, DNA is double stranded and the strands are antiparallel.  
• The two strands of DNA are twisted around the same central axis to form a right handed coil. Ten nucleotides make up a single turn of the coil structure. 

• The nitrogenous bases from the opposite strands are linked with each other through hydrogen bonds. This forms a ladder like arrangement in the center of DNA. 
• The two strands of DNA are held together by hydrogen bonds that are formed between complementary bases. 
• Adenine (A) always pairs with thymine (T) while guanine (G) always pairs with cytosine (C). This phenomenon is known as complementary base pairing 
• The number of hydrogen bonds between A-T pair is 2 while the number of hydrogen bonds between G-C pair is 3. Hence G-C pair is comparatively more stronger. 

 

Chargaff’s Rule: 

A biochemist named Erwin Chargaff made important findings on DNA which is popularly known as Chargaff rule. It states, 

• Though the base composition of DNA may vary with species, the amount of adenine is always equal to that of thymine (A = T). Similarly, the amount of cytosine is always equal to guanine (G=C). 
• In other words, in any organism the ratio of purine to pyrimidine is 1:1. (A+G = C+T) 

Structural Forms of DNA:  

• DNA exists in different structural forms namely, A, B, C, D, E and Z forms. Among these, A, B and Z are more common.  
• B form of DNA is the major form found in cells. The model of DNA explained by Watson and Crick is B form.  

• A, B and C forms of DNA exhibit right – handed helix structure while Z form exhibit left – handed helix structure. 

RNA: 

RNA is a single stranded molecule made up of polyribonucleotide chain. It is made up of ribose sugar, phosphate and nitrogenous bases. The nitrogenous bases included in RNA structure are adenine (A), guanine (G), cytosine (C) and uracil (U). Adenine base pairs with uracil and forms 2 hydrogen bonds. There are three important types of RNA,  

1. Messenger RNA (mRNA) Ribosomal RNA (rRNA) 

3. Transfer RNA (tRNA) 

• mRNA is involved in the transfer of information from DNA to the cell machinery and aids protein synthesis. 
• rRNA are found in the ribosomes. It accounts to around 80 % of the total RNA present in a cell.  

• tRNA is the smallest RNA. It is involved in the transfer of amino acids during protein synthesis. 

DIFFERENCE BETWEEN DNA and RNA 

Summary

• Nucleic acids are important bio molecules that are mainly involved in storage and transfer of genetic information..

 • Nucleic acids were first discovered by Friedrich Miescher in the year 1869

• Richard Altmann coined the term ‘nucleic acid’ in the year 1889.

• Nucleic acids are of two types – DNA (Deoxyribonucleicacid) & RNA (Ribonucleicacid)

• In 1928 Frederick Griffith, identified the transforming principle which led to the discovery of DNA as the genetic material.

• DNA is a double stranded molecule made up of antiparallel polydeoxyribonucleotide chain.

• DNA is made up of ribose sugar, phosphate and nitrogenous bases adenine (A), guanine (G), cytosine (C) and thymine (T).

• RNA is a single stranded molecule made up of polyribonucleotide chain.

• RNA is made up of ribose sugar, phosphate and nitrogenous bases adenine (A), guanine (G), cytosine (C) and uracil (U).

• Chargaffs Rule for DNA: In any species DNA, the ratio of purine to pyrimidine is 1:1 (A+G = C+T)

• Watson and Crick’s DNA model is widely accepted

• Complementary base pairing: Adenine (A) always pairs with thymine (T) while guanine (G) always pairs with cytosine (C). In RNA thymine (T) is replaced by uracil (U).

• DNA Types : A,B,Z forms ; RNA Types : mRNA, rRNA, tRNA