Question

Describe one experiment played a role in understanding DNA. Why was this experiment significant?

Answer 1

Ans. Hershey and Chase Experiment: In 1952, Alfred Hershey and Martha Chase carried out an experiment to check if protein or DNA was the genetic material. They chose T2 bacteriophage and its host Escherichia coli for their experiment. T2 bacteriophage has simple structural organization- a linear dsDNA surrounded by a protein coat. It is an intracellular parasite of E. coli. They cultured T2 phage in two different culture media. One medium had radiolabeled sulfur (S³⁵), thus the virus cultured on it had their outer protein coat labelled. Since DNA does not contain sulfur atoms, it remained unlabeled with S³⁵. Virus culture on radiolabeled phosphorous (P³²) had their phosphate atoms in DNA molecules labelled with P³². Since, proteins generally do not contain phosphorous atoms; they are not labelled with P³².

The E. coli suspension was incubated with S³⁵ labelled T2 phage inoculum. After incubation period, the suspension was blended in laboratory blender to remove any viral particle and subunits attached on the surface of E. coli. The blended suspension was then centrifuged to sediment E. coli (mass of bacteria is much greater than viral protein coat) and obtain the viral protein coats or DNA (whichever does not enter the cell) in the supernatant. When examined for radioactivity, the S³⁵ labelled proteins coats were present outside E. coli in the supernatant. The same experiment repeated with P³² labelled T2 phage showed radiolabeled DNA inside E. coli.

T2 phage culture in radiolabled S35 medium T2 phage cultured in radiolabled p32 medium T2 phage with p32 radiolabeled DNA T2 phage with s35 radiolabeled protein coat Mixed and incubated with E.coli suspenion separatel Suspension blended to remove viral particles from the cell surface of E.coli The blended suspension centrifuged-E.coil pelleted at the bottom + free viral particles in supernatant Both the Supernatant and pellet radiographed to detect the presence of radiolabled S35 or P32 in it S9 present in supernatant p32 present in pellet (inside E.coli coner Conclusion Conclusion Protein does not enter into the cell, 丨::: Hence it is not the genetic material DNA enters into the cell, Hence, it is the genetic material Fig: Schematic Representation of Hershey- Chase Experiment: DNA is the Genetic Material

Significance of the experiment: The experiment proves DNA acts as genetic material and settled the quest of genetic material between DNA and protein forever.