DNA as a Genetic Material
Introduction
Deoxyribonucleic acid(DNA) the principal genetic material of living organisms.
DNA is chemically referred to as a nucleic acid, it is a large molecule that contains carbon, oxygen, hydrogen, nitrogen and phosphorus.
It has a double helix structure with complimentary strands that are antiparallel.
Prior to the discovery of the structure of DNA, various experiments were carried out by different scientists to ascertain what exactly was the genetic material.
I would be discussing the Transformation
experiment in this article;
THE TRANSFORMATION EXPERIMENT
This experiment was carried out by Fredrick Griffith in 1928. He used two different strains of pneumonia causing bacteria; Diplococcus pneumoniae.
The smooth strain (S)has a glistering appearance due to the presence of a polysaccharide capsule, it is the virulent strain
The rough strain (R) has a dull, rough appearance due to the absence of the polysaccharide, it is the non-virulent strain.
In his experiment, Griffith observed that the injection of the R strain into mice caused no illness or harm to the mice but when the mice were injected with the S strain they suffered pneumonia and died. He then observed that the injection of heat killed S strain caused no harm to the mice but the mixture of the heat killed S strain and the live R strain led to high mortality in the mice and from the post mortem carried out on the dead mice, live R and S strain was discovered in the dead mice
Griffith then concluded that the presence of the heat killed S strain somehow caused a transformation of the R strain so as to restore the capacity of capsule formation that they had lost earlier due to genetic mutation. This was known as "Griffith's Effect"
A summary of Griffith's experiment:
16 years later, the identification of this transforming principle was done by Oswald Avery, Colin Macleod and Maclyn McCarty.
They isolated the DNA from the heat killed S strain then added the DNA to a live R strain. After a period of time, they placed this mixture on an agar surface and allowed it grow to form colonies. Both the R strain and S strain colonies grew and they both retained their characters.
To prove that the genetic material is indeed DNA;
They took an extract from the mixture of the heat killed S strain and the live R strain then added enzymes such as trypsin, chymo-trypsin which reacts with protein but found out it had no effect on the transforming activity.
They added ribonuclease which depolymerizes RNA and also lipase which reacts with lipids but they also had no effect on the transforming activity.
But on the addition of enzymes that contain DNA depolymerizing activity, the transforming activity was affected.
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In conclusion, this experiment points to the fact that DNA is the genetic material and also to some of the functions and nature of DNA.
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