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 modern experiment that confirms this experiment would be:
|A||injecting DNA into a cell which causes the cell to produce an enzyme it never had before.|
|B||fertilizing an egg with a sperm in vitro.|
|C||prescribing enzyme pills to help cystic fibrosis patients with digestion.|
|D|| consuming radioactive barium to view internal organs with X-rays.|
Bacteriophages work by making the target cell do something it could not do before. Fertilization results in a new type of cell and is the beginning of a developmental cycle, not the best analogy to the phage experiment. Consuming enzymes would not fundamentally change an organism. Although the T2 experiment used radioactivity, not all experiments involving radioactive isotopes examine the same phenomenon.