14. DNA Synthesis
The Hershey-Chase Experiment
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>> In 1953, Alfred Hershey and Martha Chase confirmed DNA's preeminent role in genetics by demonstrating that DNA is the genetic material of a virus called phage T2. The phage, which infects E coli, consists of a head, sheath, tail, and base plate made of different proteins. DNA is packaged within the head of the virus. When T2 comes in contact with E coli, the phage attaches to the bacterium by its tail. Next, the phage injects genetic material into the cell. The genetic material directs bacterial enzymes to produce viral offspring. When the life cycle is complete, 100 to 200 progeny phages have been assembled inside each bacterium. The bacterium breaks open, or lyses, and the phages are released. Since T2 consists of only DNA and protein, Hershey and Chase reasoned that the genetic material must be one of the two components, so they designed an experiment to determine which it is. For their experiment, the scientists prepared T2 phages that either had radioactive DNA or radioactive proteins. To make the components radioactive, Hershey and Chase infected E coli with T and grew the bacterial cells in two different culture media, one containing the radioactive isotope phosphorus-32, and the other containing the radioactive isotope sulfur-35. All the phages released from the bacteria grown in the phosphorus-32 medium had radioactively labeled DNA, while all the phages released from the bacteria grown in the sulfur-35 medium had radioactively labeled proteins. In the next phase of their experiment, Hershey and Chase infected E coli with the two types of labeled T2. They then used a blender and a centrifuge to separate the protein parts of the phage, called the phage ghosts, from the rest of the infected cell. The scientists discovered that the labeled DNA appeared in the host cell, but not the phage ghosts, while the labeled proteins appeared in the phage ghosts, but not the host cell. In addition, the progeny of the T2 with labeled DNA were radioactive, while the progeny of the T2 with labeled proteins were not radioactive. Because genetic material is passed on from parent to offspring, Hershey and Chase concluded that DNA had to be the genetic material of T2. Their results helped convince the scientific community that DNA was the hereditary material.
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