DNA AS HEREDITARY MATERIAL


GRIFFITH EXPERIMENTS:

Work of Griffith: 

The evidence of the hereditary nature of DNA was provided by British microbiologist Frederick Griffith.

Two types of bacteria used by Griffith in his experiments:

·   S- Type Bacteria:

The normal pathogenic form of this bacterium is referred to as the S/normal/wild form because it forms

smooth colonies on a culture dish. It contains a polysaccharide coat.

·   R- Type Bacteria:

The mutant form, which lacks an enzyme needed to manufacture the polysaccharide coat, is called the

R/mutant type because it forms rough colonies.

Experiment 1

·    Griffith infected mice with a virulent strain ( S type ) of Streptococcus pneumoniae bacteria (also

Known as Pneumococcus.

The mice died of blood poisoning.

·   However, when he infected similar mice with a mutant strain ( R type ) of Streptococcus pneumoniae

that lack the virulent strain’s polysaccharide coat.

·   The mice showed no ill effects.

·   The coat was apparently necessary for virulence (he concluded).


Experiment 2

·   To determine whether the polysaccharide coat itself had a toxic effect ?

· Griffith injected dead bacteria of the virulent S strain into the mice. The mice remained perfectly

healthy.

·  As a control, he injected mice with a mixture containing dead ‘S’ bacteria of the virulent strain and

live coatless R bacteria.

· Each of which by itself did not harm the mice. Unexpectedly, the mice developed the disease

symptoms and many of them died.

Conclusion

·   The blood of the dead mice was found to contain high levels of live,virulent streptococcus types.

·   Some how, the information specifying the polysaccharide coat had passed from the dead,virulent

bacteria so the live, coatless R bacteria in the mixture, permanently transforming the coatless Rbacteria in to the virulent.

Transformation is the transfer of genetic material from one cell to another and can alter the genetic makeup of recipient cell.

WORK OF AVERY, MACLEOD AND MCCARTY (TRANSFORMING PRINCIPLE):

The agent responsible for transforming Streptococcus went undiscovered until 1944.

In a classic series of experiments, Oswald Avery along with Macleod and Mccarty characterized what they referred to as the “Transforming principle”.

·         They first prepared mixture of dead S Streptococcus and live R Streptococcus that Griffith had

used.

·         Then they removed as much of the protein as they could from their preparation by protein digesting

enzymes. Eventually achieving 99.98% purity. Despite the removal of nearly all the protein, the

transforming activity was not reduced.         

·         Moreover, the properties of transforming principle resembled those of DNA. The protein digesting

enzyme or RNA digesting enzymes did not affect the principle’s activity, but the DNA digesting enzyme  DNAase destroyed all the transforming activity.

·         So it proved that the transforming principle is DNA and not protein.

 EXPERIMENTS OF HERSHEY AND CHASE

Additional evidence supporting Avery’s conclusion was provided in 1952 by Alred Hershey and Martha Chase who experimented with bacteriophages T2.

·   In some experiments they labeled viruses with radioisotope 32p, which was incorporated into the

newly synthesized DNA of the growing phage.

·  In other experiments, the viruses were grown on a medium containing 35S, an isotope of sulfur

that is incorporated into the amino acids of newly synthesized protein coats.

·    After the labeled viruses were permitted to infect bacteria, the bacterial cells were agitated

(stimulated) violently in a blender to remove the protein coats of the infecting viruses from the surfaces of the bacteria. This procedure removed nearly the entire 35S label from the bacteria.

· However the 32P label had transferred to the interior of the bacteria and was found in viruses

subsequently released from the infected bacteria.

·  Hence, the hereditary information injected into the bacteria that specified the new generation of

viruses was DNA and not protein.