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
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| 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
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| 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.
