Experimental evidence for DNA as genetic material.docx

Experimental evidence for DNA as genetic material.docx

1. Experimental evidence for DNA as genetic
material
1.Griffith's Experiment: A Cornerstone in Genetics
Frederick Griffith's experiment in 1928 was a groundbreaking discovery that laid the
foundation for understanding the nature of genetic material. He worked with two strains of
Streptococcus pneumoniae bacteria:
●S strain: Virulent, smooth-colony forming, and encapsulated.
●R strain: Non-virulent, rough-colony forming, and unencapsulated.
The Experiment:
1.Injecting Live S Strain:
○When injected into mice, the S strain caused pneumonia and the mice died.
2.Injecting Live R Strain:
○When injected into mice, the R strain did not cause disease, and the mice
survived.
3.Injecting Heat-Killed S Strain:
○When heat-killed S strain was injected, the mice survived.
4.Injecting a Mixture of Heat-Killed S Strain and Live R Strain:
○Surprisingly, the mice injected with this mixture died.
○Moreover, live S strain bacteria were recovered from the dead mice.
The Conclusion:
Griffith concluded that a "transforming principle" was transferred from the heat-killed S
strain to the live R strain, enabling the R strain to become virulent. This transformation
allowed the R strain to produce a capsule, making it resistant to the host's immune system.
The Significance:
While Griffith's experiment didn't identify the exact nature of the "transforming principle," it
provided crucial evidence that genetic material could be transferred between bacteria. This
paved the way for subsequent research, including the Avery-MacLeod-McCarty experiment,
which identified DNA as the genetic material.
Griffith's experiment was a pivotal moment in the history of genetics, demonstrating that
genetic information can be transferred between organisms and that this information is
responsible for inheritable traits.

2. 2.The Avery-MacLeod-McCarty Experiment: DNA as the
Transforming Principle
In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty conducted a groundbreaking
experiment that identified DNA as the molecule responsible for bacterial transformation.
They built upon the work of Frederick Griffith, who had previously demonstrated that a
"transforming principle" could be transferred between bacterial strains.
The Experiment:
1.Isolation of the Transforming Principle:
○They isolated different components from heat-killed S strain bacteria,
including proteins, lipids, carbohydrates, and DNA.
2.Treatment with Enzymes:
○Each isolated component was treated with specific enzymes to degrade it:
■Proteinase: To destroy proteins.
■RNase: To destroy RNA.
■DNase: To destroy DNA.
3.Transformation Assay:
○Each treated component was then mixed with live, non-virulent R strain
bacteria.
○The mixtures were injected into mice.
Results:
●Proteinase and RNase-treated components: The R strain bacteria were still
transformed into virulent S strain bacteria.
●DNase-treated component: The R strain bacteria were not transformed.
Conclusion:
The results indicated that DNA was the "transforming principle." The degradation of DNA
prevented the transformation of the R strain bacteria, suggesting that DNA carried the genetic
information necessary for the transformation.
Significance:
The Avery-MacLeod-McCarty experiment provided strong evidence that DNA is the genetic
material. This groundbreaking discovery laid the foundation for our understanding of genetics
and molecular biology. It paved the way for further research, including the elucidation of the
structure of DNA by Watson and Crick.
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3. 3.The Hershey-Chase Experiment: DNA as the Genetic
Material
In 1952, Alfred Hershey and Martha Chase conducted a groundbreaking experiment that
definitively proved DNA, not protein, was the genetic material. They used bacteriophages,
viruses that infect bacteria, as their experimental model.
The Experiment:
1.Labeling the Phage:
○They labeled two batches of bacteriophages with radioactive isotopes:
■Batch 1: Labeled the protein coat with radioactive sulfur-35 (S-35).
Proteins contain sulfur but not phosphorus.
■Batch 2: Labeled the DNA with radioactive phosphorus-32 (P-32).
DNA contains phosphorus but not sulfur.
2.Infection of Bacteria:
○Both batches of labeled phages were allowed to infect E. coli bacteria.
3.Blending and Centrifugation:
○After infection, the bacteria-phage mixture was blended to separate the phage
particles from the bacterial cells.
○The mixture was then centrifuged to form a pellet of bacteria at the bottom of
the tube and a supernatant containing the phage particles.
4.Analysis:
○Batch 1 (S-35 labeled): Most of the radioactivity was found in the
supernatant, indicating that the protein coat remained outside the bacteria.
○Batch 2 (P-32 labeled): Most of the radioactivity was found in the bacterial
pellet, indicating that the DNA had entered the bacteria.
Conclusion:
The Hershey-Chase experiment conclusively demonstrated that DNA, not protein, is the
genetic material. The radioactive phosphorus, which labeled the DNA, was found inside the
bacterial cells, indicating that DNA is the molecule responsible for directing the replication of
new phage particles.
This experiment provided strong evidence supporting the role of DNA as the hereditary
material, solidifying its place as the foundation of molecular biology.