Dna structure and analysis

Which molecule can be called as Genetic Material?
 It must have 4 crucial characteristics
1. Replication
2. Storage of information
3. Expression of information
4. Variation by Mutation

Protein as Genetic Material
• Early 19th century both protein and nucleic acid were major candidates
for the role of genetic material
• Until 1944 observations favoured protein as genetic material
For two major reasons:
I. Proteins are diverse and abundant in the cell
II. Much more known about the chemistry of Protein than Nucleic Acid

DNA Study
• DNA was 1st studied in 1869 by a Swiss Chemist Friedrick Miescher
• He isolated cell nuclei and derived an acidic substance (now known to
contain DNA), that he called nuclein
• In 1910 Levene observed that DNA contain approximately equal amount of 4
similar molecules called Nucleotides
• In 1940 Erwin Chargaff showed that Levene’s proposal was incorrect
• when he Studied that most organisms do not contain precisely equal
proportions of the four nucleotides

DNA as Genetic Material
• In 1944 Oswald Avery, Colin MacLeod, and Maclyn McCarty
studied transforming principle” in bacteria
• The first direct experimental proof that DNA, and not protein, is the
biomolecule responsible for heredity
• Frederick Griffith, a medical officer
• He performed experiments with several different strains of the
bacterium Streptococcus pneumoniae

Grifﬁth concluded that the heatkilled IIIS bacteria somehow converted live avirulent IIR cells into
virulent IIIS cells
(Transformation)

Transformation Early Studies
After Frederick Grifﬁth many biologists studied Transformation
Dawson and his co-workers showed that transformation could occur
in vitro
 Therefore Mice was not necessary

The Avery, MacLeod, and McCarty Experiment
• In 1944, after 10 years of work, Avery, MacLeod, and McCarty
published their results
• They reported that they had obtained the transforming principle in a
puriﬁed state and it was DNA

The Hershey–Chase Experiment
2nd Evidence (1952)
 They studied bacterium E.coli and Bacteriophage T2
 The virus consists of protein coat surrounding a core of DNA

Direct Evidence in Eukaryotes (Recombinant DNA
Studies)

RNA as Genetic Material
• In Some Viruses RNA core is present rather than DNA
• So in these Viruses RNA serves as genetic material

Understanding DNA Structure
• DNA is Nucleic Acid
• Building blocks of Nucleic Acid are Nucleotides
Nucleotide
Nitrogenous Base
Pentose Sugar
Phosphate Group

Understanding Nucleotide Structure
Nitrogenous
Bases
Purines
Adenine
Guanine
Pyrimedines
Cytosine
Thymine
Uracil

Understanding Nucleotide Structure
Nucleoside: If a molecule is composed of a purine or pyrimidine base
and a ribose or deoxyribose sugar, the chemical unit is called a
nucleoside
Nucleotide: If a phosphate group is added to the nucleoside, the
molecule is now called a nucleotide

Names of the Nucleosides and Nucleotides of RNA and
DNA.

Types of Nucleosides
• Nucleotides are also define by the term nucleoside nonophosphate
(NMP)
• NDP nucleoside diphosphate
• NTP nucleoside triphosphate
Examp:
adenosine triphosphate (ATP) and guanosine triphosphate (GTP)

Structures of nucleoside diphosphates and triphosphates. Deoxythymidine diphosphate
and adenosine triphosphate

Polynucleotides
 The 2 mononucleotides are linked with a phosphate group & two
sugars by the help of Phosphodiester bond
 Short chains consisting of up to approximately 30 nucleotides linked
together are called oligonucleotides
 Chains consisting of large number of nucleotides are called
Polynucleotides

Structure of DNA
• From 1940 to 1953, many scientists were interested in solving the
structure of DNA
• In 1953, James Watson and Francis Crick proposed that the structure
of DNA is in the form of a double helix

The Watson–Crick Model
Major Features of Model
 Two long polynucleotide chains are coiled around a central axis,
forming a right-handed double helix
 Both the chains are antiparallel, their C-5´to-C-3´ orientations run in
opposite directions
 Nitrogenous bases of opposite chains are paired as a result of
hydrogen bonding
 In DNA, only A pairs with T and G pairs with C occur
 The double helix has a diameter of 20 Å (2.0 nm

Differences b/w DNA and RNA
DNA
• Double Stranded
• Pentose Sugar Deoxyribose
• Thymine is present
• Genetic material of all the
individuals except Viruses
RNA
• Single Stranded
• Oxyribose sugar is present
• Uracil is present
• Genetic material of some viruses

Types of RNA
rRNA:
 Constitutes about 80 percent of all RNA in an E. coli cell
 Important structural components of ribosomes, which function as
nonspeciﬁc workbenches where proteins are synthesized
mRNA:
 Carries genetic information from the DNA to the ribosome
 About 5 percent of RNA is mRNA in E. coli
tRNA:
 Carries amino acids to the ribosome during translation

Analytical Techniques Which can be Used for
Analysis of DNA and RNA
Absorption of Ultraviolet Lights:
 Nucleic acids absorb ultraviolet (UV) light most strongly at
wavelengths of 254 to 260 nm
 In aqueous solution, peak absorption by DNA and RNA occurs at 260
nm
 Thus, UV light can be used in the localization, isolation, and
characterization of molecules that contain nitrogenous bases

Denaturation and Renaturation of Nucleic Acids
 Heat or other stresses can cause a complex molecule like DNA to denature
 DNA lose its function due to the unfolding of its three-dimensional
structure
 The hydrogen bonds of the duplex (double-stranded) structure break
 The helix unwinds, and the strands separate
 The viscosity of the DNA decreases
 UV absorption Increases
 If DNA that has been denatured thermally is cooled slowly, random
collisions between complementary strands will result in their reassociation
 H bonds will reform

Dna structure and analysis

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