DNA and RNA Structure and Functions

Objective

•       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 3^I – 5^I phosphodiester bridge 

Nucleotides

—  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 C^I

—  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 5^l to 3^l direction while other in 3^l to 5^l direction

—  Width of a double helix is 20A^o (2nm)

—  Each turn of the helix is 34A^o (3.4nm) with 10 base pairs of nucleotides

—  Each pair placed at a distance of 3.4A^o

—  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 µm 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	Features	B-DNA	A-DNA	Z-DNA
1	Helical diameter (nm)	2.37	2.55	1.84
2	Distance per each complete turn (nm)	3.4	3.2	4.5
3	Each base pair distance (nm)	0.34	0.29	0.31
4	No of base pair per complete turn	10	11	12
5	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

Summary

•       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