Unit – I B

DNA as Genetic Material (Contd.)

Avery, Macleod and McCarty Experiment:

  • In 1944  Oswald Avery, Collin Macleod and Maclyn McCarty performed the same experiment as that by Griffith but their aim was definite to locate the factor responsible for a transformation of non-lethal R-type bacteria into lethal S – Type bacteria. They used a test tube assay instead of mice.
  • They purified DNA, RNA, proteins and other materials from heat-killed S – type bacteria using corresponding dissolving enzymes. Then they mixed purified content with R – type to see which one could transform living R – type into s – type.
  • Only those mixed with DNA were transformed into S – type bacteria. When DNA was treated with Deoxyribonuclease, the DNA was digested and dissolved, there was no transformation of R-type bacteria into S – Type bacteria.
  • This confirmed that  “transforming principle” is DNA. But scientist community at that time was not convinced.

Hershey – Chase Experiment:

  • Alfred Hershey and Martha Chase (1952) experimentally proved that DNA is the only genetic material. They worked with viruses that infect bacteria called bacteriophages.
  • The bacteriophage attaches to the bacteria and its genetic material then enters the bacterial cell. The bacterial cell treats the viral genetic material as if it was its own and subsequently produces more virus particles.
  • Hershey and Chase performed an experiment to discover whether it was protein or DNA from the viruses that entered the bacteria.

  • Step – 1:

  • They used the fact that DNA contains phosphorus but not sulphur, while protein contains sulphur but not phosphorous.
  • They grew some viruses on a medium that contained radioactive phosphorus and some others on the medium that contained radioactive sulfur.

    Observations:

  • Viruses grown in the presence of radioactive phosphorus contained radioactive DNA but not radioactive protein. Similarly, viruses grown on radioactive sulfur contained radioactive protein but not radioactive DNA.

    Step – 2:

  • Radioactive phages were allowed to attach to E. coli bacteria. As the infection proceeded, the viral coats were removed from the bacteria by agitating them in a blender and the virus particles were separated from the bacteria by spinning them in a centrifuge.

    Observations:

  • Bacteria which was infected with viruses that had radioactive DNA were radioactive, indicating that DNA was the material that passed from the virus to the bacteria.
  • Bacteria that were infected with viruses that had radioactive proteins were not radioactive. This indicates that proteins did not enter the bacteria from the viruses.

    Conclusion:

  • Therefore DNA is the genetic material that is passed from virus to bacteria.

Frankel-Conrat and Singer Experiment:

  • H. Frankel-Conrat and B. Singer performed experiment with tobacco mosaic virus (TMV) and demonstrated that in some cases RNA acts as a genetic material.



  • Tobacco mosaic virus (TMV) does not contain any DNA. It consists of RNA surrounded by hollow cylinder of protein subunits.
  • They found that the virus could be broken into component parts and they could again be reassembled or reconstituted to form a functional virus.
  • When these reassembled viruses were used to infect tobacco leaves, the progeny viruses produced were always found to be phenotypically and genotypically identical to the parent strain from which the RNA had been obtained.

Back                               Next Topic