The document summarizes several key experiments that helped establish DNA as the genetic material: - Griffith's experiment showed a "transforming principle" in dead bacteria could change live bacteria; - Avery, McCarty and MacLeod purified and identified this principle as DNA; - Hershey and Chase showed that DNA, not protein, was injected by viruses into infected bacteria; - Fraenkel-Conrat's experiment proved that RNA could also act as genetic material. Rosalind Franklin's X-ray crystallography work provided evidence of DNA's double-helix structure, though she did not receive full credit due to gender biases of the time.

1. DNA AS GENETIC MATERIAL
A HISTORICAL PERSPECTIVE
West Bengal State University
cbcs Botany Core viii
Dr. Riddhi Datta
Department of Botany
Dr. A.P.J. Abdul Kalam Government
College

2. GRIFFITH’S EXPERIMENT
• In 1928, British bacteriologist Frederick
Griffith conducted a series of experiments
using R and S strains of Streptococcus
pneumoniae bacteria and mice.
• Griffith wasn't trying to identify the genetic
material, but rather, trying to develop a
vaccine against pneumonia.
• The R strain was non-virulent while S strain
was virulent.
• When R strain was combined with heat-killed
S strain and injected into a mouse, the mouse
died.
• Griffith concluded that the R-strain bacteria
must have taken up a "transforming principle"
from the heat-killed S strain bacteria, which
allowed them to "transform" into smooth-
coated bacteria and become virulent.
Dr. Riddhi Datta

3. Dr. Riddhi Datta
AVERY, McCARTY AND MACLEOD’S EXPERIMENT
• In 1944, three Canadian and American
researchers, Oswald Avery, Maclyn McCarty,
and Colin MacLeod, set out to identify
Griffith's "transforming principle.“
• They began with large cultures of heat-killed
S cells and progressively purified the
transforming principle by separating out, or
enzymatically destroying the other cellular
components.
• The purified substance only gave positive
result in a chemical test known to detect
DNA.
• The elemental composition of the purified transforming principle closely resembled DNA in its ratio of nitrogen
and phosphorous.
• Protein, lipid, polysaccharide and RNA-degrading enzymes had little effect on the transforming principle, but
enzymes able to degrade DNA eliminated the transforming activity.

4. HERSHEY & CHASE’S EXPERIMENT
• Hershey and Chase knew bacteriophages attach to the
surface of a host bacterium and injected some substance
into the host. This substance gave "instructions" to the
host bacterium to start making lots and lots of phages.
• To establish whether the phage injected DNA or protein
into host bacteria, they prepared two different batches of
phage and infected bacterial cells.
• One sample was produced in the presence of radioactive
35S so that only the phage proteins will be radio-labeled.
• The other sample was produced in the presence of 32P so
that only the phage DNA will be radio-labeled.
• The infected bacterial cells (pellet) were separated from
the phage and culture medium (supernatant) by
centrifugation.
• Only 32P was identified in the bacterial pellet while 35S
remained in the supernatant. They concluded that DNA,
not protein, was injected into host cells (1952).
Dr. Riddhi Datta

5. Dr. Riddhi Datta
FRAENKEL-CONRAT’S EXPERIMENT
• They were working with tobacco mosaic virus (TMV) in 1957 to
prove that RNA can act as genetic material.
• TMV has a protein capsid and RNA inside.
• They separated the protein coat and the RNA and used them to
infect tobacco leaves.
• When only the protein coat was used, no infection occurred while
only RNA could infect tobacco leaves.
• When the separated protein coat and RNA were recombined,
entireTMV particles were reconstituted.
• Next, they selected two strains of TMV:
TMV A (causes mottling of leaves) and
TMV B (causes wrinkle patterns on leaves).
• The protein and RNA parts of each strain
were separated and chimeric TMV particles
were generated.
• The chimeric TMV produced symptoms
that were characteristics of the RNA and
not the protein.

6. The Nobel Prize in Physiology or
Medicine 1962
For discoveries concerning the molecular structure of nucleic
acids and its significance for information transfer in living
material
Dr. Riddhi Datta
DISCOVERY OF DNA STRUCTURE
Francis Crick JamesWatson MauriceWilkins
Rosalind Franklin
The dark lady of DNA

7. Dr. Riddhi Datta
DISCOVERY OF DNA STRUCTURE
• Franklin was a British biologist who used X-ray
diffraction technique to understand the structure of
DNA.
• This involved stretching out DNA molecules and
aiming powerful X-ray at them. She worked tirelessly
to obtain better patterns.
• The X-shaped pattern she obtained shows that DNA
molecules are twisted around each other in a double
helix.
• Clues also indicated that nitrogenous bases are towards
the centre of the molecule.

8. Dr. Riddhi Datta
DISCOVERY OF DNA STRUCTURE
• At the same time when Franklin was conducting
her experiments, Watson and Crick were also
trying to understand the structure of DNA.
• They made 3D models of DNA using cards but
were unable to come up with a structure that
explains all the properties of DNA.
• At this time they were shown a copy of Franklin’s
X-rays by Wilkins (who was working with
Franklin) without Franklin’s knowledge.
Immediately the structure of DNA became
apparent.
• Both the works got published at the same time, in
1953.

9. Dr. Riddhi Datta
DISCOVERY OF DNA STRUCTURE
THETRAGEDY:
• Franklin died on April 16, 1958, of ovarian cancer,
possibly caused by her extensive exposure to radiation
while doing X-ray crystallography work.
• Since Nobel prizes aren't awarded posthumously,
Franklin couldn’t share the scientific community's
highest honor for her work. Instead, the Nobel Prize
was shared by her co-worker, Wilkins along with
Watson and Crick.
• In fact, if not for Franklin's early death, she might
eventually have had two Nobel Prizes. At the time of
her death, she was working on the molecular structure
of viruses with her colleague Aaron Klug, who
received a Nobel Prize for the work in 1982.