Genome consists of long sequence of
nucleic acids that stores the information needed to construct the organism. The
genome is the only thing that defines the hereditary nature of an organism.
The genome can be functionally divided in to
genes. Each genes are nucleotide sequences that represent a particular protein.
That is the genes code for the protein (Rather the genes code for an RNA which
in turn may code for a specific protein.)
Experimental Evidence for DNA’s Role as Genetic
material:
 |
| Griffith's experiment to show that DNA is genetic material. |
The first direct evidence that the DNA transmits
genetic information comes from the experiments of Griffith. In the year
1928 Griffith performed a fantastic experiment with mouse and bacteria (Streptococcus
pneumoniae). The Streptococcus pneumoniae has two strains. One
has smooth appearance due to presence of capsule, and are virulent to cause
pneumonia, as the capsule allows the bacteria to escape destruction by the
host. The other strain appears rough due to absence of the capsule, and is
avirulent, because the host can easily destroy it. Avery first infected a mouse
with the rough strain of bacteria, the mouse survived. Then he infected other
mouse with the smooth strain, the second mouse died due to pneumonia. To the
third mouse he infected it with heat killed smooth strain. The mouse survived. To
the fourth mouse he infected with the mixture of heat killed smooth strain and
live rough strain bacteria. Here the mouse died. Thus they concluded that the
rough strain bacteria somehow transformed[1] itself to smooth strain and
killed the mouse. These transformed bacteria were recovered from the dead mouse
and then cultured. The result was culture of smooth strain, proofing that the
transformation was permanent.
Drawbacks:
·
Griffith
cannot explain the role of mouse in transformation of rough strain to smooth
strain.
·
He
also cannot make it clear, that which part of the smooth strain bacteria(DNA, RNA,
protein etc) transformed the rough strain bacteria.
Experiment of Avery:
In 1944 Avery, C. Macleod and M. Mc Carty
separated the extract of smooth, virulent bacteria into proteins, DNA,
carbohydrate fractions. The incubated the rough strain of bacteria with all of
these fractions. And they got the results. The only the bacteria incubated in
the DNA fragment of the smooth strain bacteria, transformed to smooth strain. In
other fractions there was no transformation. This experiment proved that it is
the DNA which acts as genetic material.
[1]
It is a permanent, inheritable
change produced in one strain of bacteria by a substance (DNA) isolated from
another strain of the same kind of bacteria
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