The document discusses the establishment of DNA as the genetic material through historical experiments, primarily by Griffith, Avery, and Hershey and Chase. Initial assumptions had favored proteins until these experiments demonstrated that DNA is responsible for carrying genetic information. Key evidence includes Griffith's transformation experiment and subsequent findings that DNA is the molecule responsible for genetic transmission in organisms.

1. Dr. S. Sathish
Assistant Professor
Department of Biochemistry
APCAS, Kalavai
sathishapcasbio@gmail.com
“DNA as a Genetic Material”
Experimental Evidences

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It is well known fact that transmission of traits
(genes) takes place from one generation to other.
Gregor Johann Mendel (1866 ) on the basis of his
hybridization experiments on Sweet pea gave the
idea that transmission of traits over generations
take place through Factor or Determiner or Gene
which carries information for expression of trait
or phenotype.
But which macromolecule act as a genetic
material was not conformed from his experiment
Genetic material

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Genetic material must be
capable of
• Self replication (Make its copy)
• Storage of information for expression of
trait (gene)
• Controls the expression of traits (genes)
• Must be stable
• Change in controlled way (undergo
mutation)

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Topic of Discussion, till 1944 was which chromosomal
component DNA or Protein carries hereditary
information or is the genetic material
Untill 1940 Proteins were considered as genetic material
because
 Proteins are polymer of 20 amino acids and present in
larger quantity, encode more and variety of
information.
 DNA is polymer of only 4 different
deoxyribonucleotides (ATP, CTP, GTP & TTP) and is
present in smaller quantity
Most Geneticists focused on “transmission
genetics” and passively accepted proteins as the genetic
material
Is the Genetic Material Protein or DNA?

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But On the basis of certain experiments conducted from
time to time, it was ultimately demonstrated that DNA
carries genetic information and not the proteins
Here are some direct evidences which prove DNA as Genetic
Material
Direct evidences come from:
 Griffith’s (1928) experiment on Bacterial
Transformation
 Avery, Macleod and McCarty’s (1944 )
experiment on Transformation
 Hershey and Chase (1952 ) experiment on
T4 - Bacteriophage

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In 1928, Frederick Griffith’s experimented with
Streptococcus pneumoniae bacteria in mice.
Frederick Griffith used two strains of Streptococcus
pneumoniae (Gram +ve cocci arranged in chains)
◦ S - Smooth strain of S. pneumoniae (TYPE-IIIS)
 Secrete a polysaccharide capsule
 Produce smooth colonies on solid media
 Virulent – Causes Pneumonia – mice died
◦ R – Rough strain S. pneumoniae (TYPE-IIR)
 Unable to secrete a polysaccharide capsule
 Produce colonies with a rough appearance
 Avirulent - No Pneumonia – mice Alive

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a) Mice died of pneumonia when injected with pathogenic
strains of Smooth Streptococcus pneumoniae, which
have a capsule and form smooth-looking colonies.
b) Mice survived when injected with a non- pathogenic
strain of Rough Streptococcus pneumoniae, which
lacks a capsule and forms rough colonies.
c) Injection with heat-killed strains of Smooth
Streptococcus pneumoniae had no effect.
d) Injection with a live Rough strain and a heat-killed
Smooth strain gave the mice pneumonia, and live Smooth
strain of Streptococcus pneumoniae could be isolated from
the dead mice.

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 Griffith called this agent the transforming
principle, but did not know what it was or how it
worked.
 Griffith called this change of non-virulent bacteria
into virulent pathogens Transformation (Transfer
of genetic material from heat-killed S-Strain
from the surrounding to R-Strain and convert
the Avirulent R-Strain to Virulent S-Strain).

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 Oswald Avery, Colin Macleod and Maclyn McCarty’s set
out to discover which constituent in the heat - killed
virulent Pneumococci (Streptococcus pneumoniae) was
responsible for Griffith’s transformation.
 They had shown that only DNA extracts from Smooth
cells caused transformation of Rough cells (R cells) to
Smooth cells (S cells).
 These Investigators selectively destroyed macromolecule
constituents in purified extracts of virulent pneumococci
(S- cells), using enzymes that would
hydrolyze DNA, RNA, or protein.

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 They then exposed non-virulent pneumococcal strains
(R strains) to the treated extracts.
 Transformation of the non-virulent bacteria was
blocked only if the DNA was destroyed, suggesting that
DNA was carrying the information required for
transformation.
 The publication of these studies by Avery, C. M.
MacLeod, and M. J. McCarty in 1944 provided the first
evidence that Griffith’s transforming principle was DNA
and therefore that DNA carried genetic information.

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 In 1952, Alfred Hershey and Martha Chase performed
several experiments indicating that DNA was the genetic
material in a bacterial virus called T2 Bacteriophage.
 They used radioisotopes to distinguish DNA from proteins
 32P labels DNA specifically
 35S labels protein specifically
They infected non-radioactive
Escherichia coli with radioactively-
labeled T2 Bacteriophage.

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35
S – Protein coat
32
P – in DNA
32
P-DNA in bacteria
Proteins contain sulphur in cysteine
and methionine but not phosphorous
DNA contain
phosphorous but not
sulphur

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 When E. coli was infected with a T2 Bacteriophage
containing 35
S protein, most of the radioactivity
remained outside the host cell.
 When a T2 phage containing 32
P DNA was mixed
with the host bacterium, the radioactive DNA was
injected into the cell and phages were produced.
Thus, DNA was carrying the virus’s genetic
information.
 32
P labelled phage DNA entered into a bacteria to
produce progeny phage that are also labelled. This
biological experiment confirms that DNA is the
transforming substance (genetic material), and shows
that transformation is inherited.

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