Exam 2-Experiments

Friedrich Miescher

He isolated an unknown compound, which was precipitated by acid. He named the unknown, phosphorus rich compound that came from the nucleus, nuclein

Frederick Griffith

Found that heat killed smooth strain plus rough strain kills the mouse. Proving that baterial transformation exists showing that harmless can pick up a “transforming principle“ from dead virulent bacteria and become deadly themselves

Genetic traits can be transferred horizontally between different strains of bacteria.

Avery, Macleod, and McCarty Experiments

They continued from Griffith’s experiments to find the transforming principle. Found that the transforming principle is DNA through the process of elimination. Because destroying DNA prevented the genetic trait from transferring, they proved that DNA is the substances responsible for heredity.

Hershey-Chase Experiment

Thought to be too simple of a molecule to be the genetic code, this experiment found that DNA was in fact the molecule driving heritability. They used two bacteriophages, one with their protein labeled with radioactive sulfur, and one with their DNA labeled with radioactive phosphorus. They found that DNA holds the genetic information.

Meselson-Stahl experiments

Tested and found that DNA is semi-conservative. They mixed N14 (lighter) and N15 (heavier) DNA into CsCl gradient and allow cells to replicate twice. After one round of replication, in light nitrogen, all DNA showed an intermediate density. After the second round of replication, the DNA separated into two distinct bands- The top band was N14 but the intermediate band was N14N15. 

Meselson-Weigle experiments

They demonstrated that recombination involves breaking and ligating DNA. They had two bacterial DNA’s, one with heavy isotopes, one with light isotopes. They mixed them together, and discovered a chunk of heavy was on the light strand and a chunk of light was on the heavy strand, so recombination must have occurred. 

Delbrück-Luria experiments

Begin with a bacterial strain which is susceptible to a specific virus but can undergo a mutation to become resistant. Add virus. If mutations are caused by the addition of the virus then all tubes should develop the same amount of mutants, if mutations occur at random, then mutant frequencies will vary based on time virus was added. The result was that the number of mutant bacteria which were resistant to the virus varied from plate to plate, which indicates that mutations occur at random. This proved bacterial genetic mutations arise randomly BEFORE exposure to a selective environmental pressure.