DNA and RNA Structure and Functions

DNA and RNA Structure and Functions


       At the end of this lecture, student will be able to

      Explain DNA double helix

      Discuss the salient features of DNA

      Explain the conformation of DNA double helix

      Explain structure, types and functions of RNA

Discovering the structure of DNA

James Watson and Francis Crick

Worked together at Cavendish Laboratory in Cambridge to determine the structure of DNA

Used work from Franklin, Wilkins, and Chargaff to determine the double helix shape

Watson and Crick were awarded the Nobel Prize

Rosalind Franklin passed away (1958) before the Nobel Prize was awarded in 1962

DNA Structure

  DNA is a polymer of deoxy ribonucleotides

  It is composed of monomeric units namely

                                                deoxy adenylate (dAMP)

                                                deoxy guanylate (dGMP)

                                                deoxy cytidylate (dCMP)

                                                deoxy thymidylate (dTMP)

  DNA consists of two poly deoxyribonucleotide chains twisted around each other on a common axis called a Double Helix

  The monomeric deoxy nucleotides in DNA are held together by 3I – 5I phosphodiester bridge 


  The phosphate and sugar form the backbone of the DNA molecule, whereas the bases form the “rungs”

  Each base will only bond with one other specific base

  Adenine (A)

  Thymine (T)

  Cytosine (C)

  Guanine (G)

Structure of polydeoxyribonucleotide

  The horizontal line indicate the carbon chain of sugar with base attached to CI

  Near middle of horizontal line is C3 phosphate linkage while at the other end of the line is C5 phosphate linkage

  A molecule of DNA is made up of millions of tiny subunits called Nucleotides

One Strand of DNA

       The backbone of the molecule is alternating phosphates and deoxyribose sugar

       The teeth are nitrogenous bases

DNA Structure

  Watson and Crick proposed the double helical structure of DNA in 1953. it is compared to a twisted ladder

Salient features are

  DNA is a right handed double helix

  It consist of two poly deoxy ribonucleotide chain, which are twisted around each other on a common axis

  Two strands are antiparallel i.e. one strand runs in 5l to 3l direction while other in 3l to 5l direction

  Width of a double helix is 20Ao (2nm)

  Each turn of the helix is 34Ao (3.4nm) with 10 base pairs of nucleotides

  Each pair placed at a distance of 3.4Ao

  Each strand of DNA has hydrophilic deoxyribose phosphate back bone on outside of the molecule (periphery)while the hydrophobic base are stacked inside (core)

  Two polynucleotide chain are not identical but complementary to each other due to base paring

  Two strands are held together by hydrogen bonds formed by complementary base pair

                                                                A=T has 2 hydrogen bond

                                                                C≡G has 3 hydrogen bond

   Hydrogen bond are formed between purine and pyrimidine only

  Complementary base pair in DNA helix proves chargaff’s rule i.e. DNA has equal number of adenine and thymine residue and equal number of guanine and cytosine residue

  Genetic information residues on one of the two strand known as template strand or sense strand 

   Because of this complementary base pairing, the order of the bases in one strand determines the order of the bases in the other strand

  Double-stranded DNA helix in each chromosome has a length that is thousands times the diameter of the nucleus

  For instance, in human, a 2-meter long DNA is packed in a nucleus of about 10 ┬Ám diameter

  This is made possible by a compact and marvellous packaging and organization of DNA inside in cell

  DNA molecules are huge in size

  On an average, a pair of B-DNA with a thickness of 0.34 nm has a molecular weight of 660 daltons

Hydrogen Bonds

       The bases attract each other because of hydrogen bonds

       Hydrogen bonds are weak but there are millions and millions of them in a  single molecule of DNA

       The bonds between cytosine and guanine are shown here with dotted lines

       When making hydrogen bonds, cytosine always pairs up with guanine

       Adenine always pairs up with  thymine

       Adenine is bonded to thymine here

Conformation of DNA double helix

       The double helical structure of DNA exist in at least 6 different forms i.e. A to E and Z

       Among this B, A and Z forms are important. B form of DNA is described by Watson and Crick

Sl No






Helical diameter (nm)





Distance per each complete turn (nm)





Each base pair distance (nm)





No of base pair per complete turn





Helix type

Right handed

Right handed

Right handed


Re and Denaturation of DNA strand

       The two strand of DNA are held by hydrogen bond

       Disruption of bond causes separation of two strand, this phenomenon is called as Denaturation

       Heat, acid and alkali denature DNA

       Renaturation process in which the separated complementary DNA strand can form a double helix

Functions of DNA

       Nucleic acid that contains the genetic instructions for the development and function of living things

       Main role of DNA in the cell is the long-term storage of information

RNA Structure

  RNA is a polymer of ribonucleotides held together by 3’,5’-phosphodiester bridges

  Sugar in RNA is ribose in contrast to deoxy ribose in DNA

  RNA contains the pyrimidine uracil in place of thymine (in DNA)

  RNA is usually a single stranded polynucleotide

  Chargaff’s rule is not obeyed as there is no specific relation between purine and pyrimidine contents

Types of RNA:

  mRNA – about 5-10% of cellular composition, transfers genetic information from genes to ribosomes to synthesize proteins

  tRNA – about 10-20% of cellular composition, transfers amino acid to mRNA for protein biosynthesis

  rRNA – about 50-80% of cellular composition, provides structural framework for ribosomes

Functions of RNA

  It assists DNA and acts as a messenger between DNA and ribosomes

  It helps the ribosomes to choose the right amino acid which is required in building up of new proteins in the body


       DNA is a polymer of deoxyribonucleotides

       Watson and Crick proposed the double helical structure of DNA

       The two strands are held together by hydrogen bonds formed complementary base pairs

       DNA exist in 6 different form A to E and Z

       Separation of two strand of DNA is called Denaturation

       Fusion of two strands is called Renaturation

       RNA is usually a single stranded polynucleotide


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