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.
Radioactive sulfur was found almost exclusively in the supernatant fraction, while the radioactive phosphate was found in the pellet fraction. This indicated:
|A||phage protein entered the cell, but the DNA did not.|
|B||phage DNA entered the cell, but protein did not.|
|C||both protein and DNA entered the cell.|
|D||the phage did not infect the bacterial cell.|
Detection of radioactive phosphate in the pellet indicates the bacteria now contain the DNA of the bacteriophage. Radioactive sulfur in the supernatant suggests the protein of the phage did not enter the cell. Since the bacteria took up something from the phage, answer D is unlikely, as it does appear the phage infected the cell.