DNA as a Genetic Material - Dr. P. Saranraj, Assistant Professor, Department of Microbiology, Sacred Heart College (Autonomous), Tirupattur, Vellore District, Tamil Nadu, India.

1. Dr. P. Saranraj M.Sc., M.Phil., Ph.D., NET
Assistant Professor
Department of Microbiology
Sacred Heart College (Autonomous)
Tirupattur, Vellore District.
Mobile: 9994146964
E.mail: microsaranraj@gmail.com

2.  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 Streptococcus pneumoniae
 Secrete a polysaccharide capsule
 Produce smooth colonies on solid media
 Virulent
◦ R  Rough Streptococcus pneumoniae
 Unable to secrete a capsule
 Produce colonies with a rough appearance
 Avirulent

4. 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.

5. 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.

6.  Oswald Avery, MacLeod and McCarty 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
constituents in purified extracts of virulent
pneumococci (S cells), using enzymes that would
hydrolyze DNA, RNA, or protein.

7.  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.

9.  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.

11. a) When E. coli was infected with a T2
Bacteriophage containing 35S protein,
most of the radioactivity remained
outside the host cell.
b) When a T2 phage containing 32P 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.

12.  Hershey and Chase made the virus’s DNA
radioactive with 32P or they labeled the its
protein coat with 35S.
 They mixed radioactive bacteriophage with
Escherichia coli and incubated the mixture for
a few minutes.
 The suspension was then agitated violently in
a blender to shear off any adsorbed
bacteriophage particles.

13.  After centrifugation, radioactivity in the
supernatant (where the virus remained)
versus the bacterial cells in the pellet was
determined.
 They found that most radioactive protein was
released into the supernatant, whereas 32P
DNA remained within the bacteria. Since
genetic material was injected and T2 progeny
were produced, DNA must have been
carrying the genetic information for T2.