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.
If the bacterial pellet were incubated in an appropriate growth medium, what would most likely occur?
|A||No phage would be produced as the protein coat was removed.|
|B||Bacteria would survive as there is no proof they were infected with phage.|
|C||Phage should be produced and kill the cells.|
|D||The radioactive phosphate would kill the cells immediately.|
The experiment verified that the DNA of the phage entered the cell. Therefore, the bacteria were indeed infected. It is not likely the protein coats are necessary for further functioning of the phage as they never entered the cell. It is also unlikely the radioactive phosphate would immediately kill the cells (after all, phage were propagated in bacteria grown in medium containing the isotope). The most likely answer is that the phage would reproduce as normal.