DNA Structure and Function
Since only four types of nucleotides (adenine, guanine, cytosine and thymine) make up DNA, many scientists were skeptical that this was the hereditary molecule of life. Most believed proteins, with their building blocks of 20 amino acids, provided the complexity necessary to carry the genetic information.
In 1952, Alfred Hershey and Martha Chase designed an experiment to determine whether DNA or protein was the genetic material. They used a bacteriophage (a virus that infects bacteria) called T2. This bacteriophage was only composed of proteins and DNA. A short time after infecting a bacterial cell, the cell would lyse (break open) and release new T2 particles. T2 reprogrammed the cell to make more phage, but did it use DNA or proteins?
Hershey and Chase infected bacteria with T2 phage in growth medium containing radioactive sulfur (which labels proteins) and radioactive phosphate (which labels DNA). The resulting radioactive phages were then incubated with nonradioactive bacteria and allowed to infect these cells. After a short time, the mixture was placed in a blender to shake loose any phage particles remaining outside of or attached to the bacteria. The mixture was then centrifuged: bacterial cells would form a pellet at the bottom of the tube while any phage outside the cell would remain in the liquid portion (the supernatant). The radioactivity in each sample was measured.
A possible alternative to the blender technique used in this experiment would be:
|A||add a competitive inhibitor to the bacteria before adding phage.|
|B||break apart the cell membranes with a strong detergent.|
|C||incubate bacteria with a mutant phage that could not bind to the cells.|
|D||shake the cell/phage mixture vigorously.|
Answer D is the only choice that makes any sense. All the other answers defeat the purpose of the experiment: a competitive inhibitor would prevent the phage from entering the cells at all; breaking apart the cell membrane would cause the contents of the cell to be released, where they would be found in the supernatant, along with the components of the phage that did not infect the cell; a mutant phage that could not bind to the cell would probably not infect the cell.