DNA as a Genetic Material

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

1. DNA As a Genetic Material Dr. P. Saranraj M.Sc., M.Phil. , Ph.D., NET Assistant Professor Department of Microbiology Sacred Heart College (Autonomous) Tirupattur, Vellore District. Mobile: [phone] E.mail: [email protected]

2. In 1928, Frederick Griffith’s experimented with Streptococcus pneumoniae bacteria in mice. Frederick Griffith used t wo 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 Griffith’s Transformation Experiment

3. Mice died of pneumonia when injected with pathogenic strains of Smooth Streptococcus pneumoniae , which have a capsule and form smooth-looking colonies. Mice survived when injected with a non-pathogenic strain of Rough Streptococcus pneumoniae , which lacks a capsule and forms rough colonies. Injection with heat-killed strains of Smooth Streptococcus pneumoniae had no effect. 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.

4. 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. Conclusion of Griffith Experiment

5. 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. Oswald Avery, MacLeod and McCarty Experiment

6. 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.

7. 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 32 P labels DNA specifically 35 S labels protein specifically They infected non-radioactive Escherichia coli with radioactively-labeled T2 Bacteriophage. Alfred Hershey and Martha Chase Experiment

8. When E. coli was infected with a T2 Bacteriophage containing 35S protein, most of the radioactivity remained outside the host cell. 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.

9. 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.

10. 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.