question_answer 76)

  Give an account of Hershey and Chase experiment. What did it conclusively prove? If both DNA and proteins contained phosphorus and sulphur do you think the result would have been the same?

Answer:

                    Hershey and Chase conducted experiments on bacteriophage to prove that DNA is the genetic material.     (i) Some bacteriophage virus were grown on a medium that contained radioactive phosphorus (32P) and some in another medium with radioactive sulphur (35S). (ii) Viruses grown in the presence of radioactive phosphorus (32P) contained radioactive DNA. (iii) Similar viruses grown in presence of radioactive sulphur (35S) contained radioactive protein. (iv) Both the radioactive virus types were allowed to infect E. coil separately. (v) Soon after infection, the bacterial cells were gently agitated in blender to remove viralcoats from the bacteria. (vi) The culture was also centrifuged to separate the viral particle from the bacterial cell. Observations and Conclusions (i) Only radioactive32Pwas found to be associated with the bacterial cell, whereasradioactive35S was only found in surrounding medium and not in the bacterial cell. (ii) This indicates that only DNA and not protein coat entered the bacterial cell. (iii) This proves that DNA is the genetic material which is passed from virus to bacteria and not protein. If both DNA and proteins contained phosphorus and sulphur, the result might change. In case (i) Radioactive35S and + Bacteriophage32Plabeled protein capsule No radioactive 35Sand32PDetected in cells + Radioactivity (35Sand 32P) detected in supernatant In case (ii) Radioactive35S and 32Plabeled DNA ? Bacteriophage Radioactive32P and35S Detected in cells - No radioactivity detected in supernatant.