Question

1. Briefly explain why it was important for Frederick Griffith to use a IIIS strain in his experiments rather than a IIS strain and what result he was able to rule out as a possible explanation for the killing of the mouse after incubation of the heat killed IIIS strain with living IIR that he would not have been able to rule out if he used a IIS instead of IIIS.

1. Briefly explain the Hershey & Chase experiment and include in your answer what macromolecule was labeled with which isotope and why, what the results were, and what was concluded

Answer 1

•In 1928, Fredrick Griffith, in a series of experiments with Streptococcus pneumoniae (bacterium responsible for pneumonia), witnessed a miraculous transformation in the bacteria. During the course of his experiment, a living organism had changed in physical form. When Streptococcus pneumoniae bacteria are grown on a culture plate, some produce smooth shiny colonies (S) while others produce rough colonies (R). This is because the S strain bacteria have a mucous (polysaccharide) coat, while R strain does not. Mice infected with the S strain (virulent) die from pneumonia infection but mice infection with the R strain do not develop pneumonia.

•S strain injected into mice, mice die.

•R strain injected into mice, mice live.

•Griffith was able to kill bacteria by heating them. He observed that heat-killed S strain bacteria injected into mice did not kill them. When he injected a mixture of heat-killed S and live R bacteria, the mice died. Moreover, he recovered living S bacteria from the dead mice.

•He concluded that the R strain bacteria had somehow been transformed by the heat-killed S strain bacteria. Some 'transforming principle', transferred from the heat-killed S strain, had enabled the R strain to synthesize a smooth polysaccharide coat and become virulent. This is due to the transfer of the genetic material.

2)

•Harshey and Chase wanted to know whether the protein coat or the DNA core of the virus entered the infected cell; this would help them determine whether genes were made up protein or of DNA.

•They used radioactive isotopes of phosphorus-32 and sulfur-35 as markers so they could trace the proteins and DNA to see what they were doing to the cell.

•They were perfect because proteins contain almost no phosphorus and DNA contains no sulfur, so the radioactive isotopes would show up in the cells if the phosphorus-32 or sulfur-35 appeared. If sulfur-35 showed up in the bacteria, it meant the viruses' protein had been injected into the bacteria; if the phosphorus-32 was found in the bacteria, the DNA had been injected into the bacteria.

•They concluded that the genetic material of the bacteriophage was DNA, NOT protein.