DNA and RNA Structure and Functions

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 3I – 5I 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 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 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

 

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