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