How It Was Proved That Deoxyribonucleic Acid (DNA) Is The Heredity Material

Deoxyribonucleic Acid (DNA)

Deoxyribonucleic acid is a molecule that carries most of the genetic instructions used in the development, functioning and reproduction of all known living organisms and many viruses.

DNA as Hereditary Material

Work of Griffith: The first evidence of hereditary nature of DNA was provided by a British microbiologist Frederick Griffith who made some unexpected observations while experimenting with pathogenic bacteria. His observations are:

• When he injected mice with a virulent strain of Streptococus pneumoniae bacteria (then known as Pneumococcus), the mice died of blood poisoning.
• When he injected similar mice with a mutant strain of S. pneumoniae that lacked the virulent polysaccharide coat, the mice showed no ill effects. This shows that coat was apparently necessary for virulence.

S forms: The normal pathogenic form of this bacterium is referred to as the S form because it forms smooth colonies on a culture disk.

R forms: The mutant forms, which lack an enzyme needed to manufacture the polysaccharide coat, is called the R form because it forms rough colonies.

Decisive Experiment

Griffith Experiment of Transformation

Purpose of Experiment: To determine whether the polysaccharide coat that lacked in mice which showed no ill effect, had a toxic effect.

Actual Experiment: Griffith injected heat killed dead bacteria of the virulent S strain into the mice, the mice remained perfectly healthy.

Control Experiment: As a control experiment (a test where the person conducting the test only changes one variable at a time in order to isolate the results. An experiment where all subjects involved in the experiment are treated exactly the same except for one deviation is an example of a control experiment.), 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.

Result: Unexpectedly, the mice developed the disease symptoms and many of them died.

Further Observations: The blood of the dead mice was found to contain high levels of live, virulent streptococcus type S bacteria, which had surface proteins characteristic of the live (previously R) strain. Somehow, the information specifying the polysaccharide coat had passed from the dead, virulent S bacteria to the live, coatless R bacteria in the mixture, transforming permanently the coatless R bacteria into the virulent S variety.

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

Work of Avery, Macleod AND McCarty

Determining that DNA is Heredity Material

Transforming Principle

In 1944, in a classic series of experiments Oswald Avery along with Colin Macleod and Maclyn McCarty repeated Griffith's experiments and 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, eventually achieving 99.98% purity. Despite removal of nearly all the protein, the transforming activity was not reduced. Moreover the properties of transforming principle resembled there of DNA. The protein digesting enzymes or RNA digesting enzymes did not affect the transforming activity, but the DNA digesting enzyme DNase destroyed all the transforming activity.

Hershey and Chase Experiment

Hershey and Chase Experiment To Determine DNA is Heredity Material

In 1952 by Alfred Hershey and Martha Chase experimented with bacteriophages T₂ and provided additional evidence supporting Avery's conclusion.

³²P Labeled Viruses: In some experiments they labeled viruses with radio (radioactive) isotope ³²P, which was incorporated into the newly synthesized DNA of growing phage.

³⁵S Labeled Viruses: In other experiments the viruses were grown on a medium containing ³⁵S an isotope (radioactive) of sulphur which is incorporated into the amino acids of newly synthesized protein coats.

Exposure of Bacteria to Labeled Viruses: Now one bacterial culture was exposed to ³²P labeled viruses and other culture was exposed to ³⁵S labeled viruses.

Removal of Protein Coats: The bacterial cells were agitated violently in a blender to remove the protein coats of the infecting viruses from the surfaces of the bacteria.

Test for the Presence of Label: When bacteria were tested for the presence of label it was found that bacteria had lost nearly all of ³⁵S label. However, 32P label was present in bacteria of the other group.

Reason: It is because ³⁵S was only present in protein coat while ³²P was present in DNA that had transferred to the interior of the bacteria.

Confirmation: It was confirmed when viruses were released from the infected bacteria. The viruses released from ³²P culture were labeled while viruses released from ³⁵S culture were unlabelled.

DNA — the Hereditary Material

Thus it was proved that the hereditary information injected into the bacteria that specified the new generation of viruses was DNA and not protein.