MANOJ KUMAR (SHELFORD)

Wednesday, June 5, 2013

Griffith and Avery’s experiment for the evidence that DNA is the genetic material.


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:
www.shelfordtutorials.co.nr
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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