10.1 Flashcards
Note 1 —-»
Today, scientists routinely manipulate DNA in the laboratory and use it to change the heritable traits of cells. Early in the 20th century, however, the molecular basis for inheritance was a mystery. Biologists did know that genes were located on chromosomes. The two chemical components of chromosomes—DNA and protein—were therefore the leading candidates to be the genetic material. Until the 1940s, the case for proteins seemed stronger than that for DNA because proteins appeared to be more structurally complex: Proteins were known to be made from 20 different amino acid building blocks, whereas DNA was known to be made from just four kinds of nucleotides. It seemed logical that the more complex molecule would serve as the hereditary material. Biologists finally established the role of DNA in heredity through experiments with bacteria and the viruses that infect them. We can trace the discovery of the genetic role of DNA to 1928. British medical officer Frederick Griffith was trying to develop a vaccine against pneumonia by studying two strains (varieties) of a bacterium: a harmless strain and a pathogenic (disease-causing) strain that causes the disease in mammals. Griffith was surprised to find that when he killed the pathogenic bacteria and then mixed the bacterial remains with living harmless bacteria, some living bacterial cells became pathogenic. Furthermore, all of the descendants of the transformed bacteria inherited the newly acquired ability to cause disease. Clearly, some chemical component of the dead bacteria caused a heritable change in live bacteria.
Note 2 —-»
In 1952, American biologists Alfred Hershey and Martha Chase performed a very convincing set of experiments that showed DNA to be the genetic material of T2, a virus that infects the bacterium Escherichia coli (E. coli), a microbe normally found in the intestines of mammals (including humans). Viruses that exclusively infect bacteria are called bacteriophages (“bacteria-eaters”), or phages for short.
Bacteriophage
A virus that infects the bacteria.
Note 3 —-»
Hershey and Chase knew that T2 could reprogram its host cell to produce new phages, but they did not know what component of the virus conferred this capability. At the time, it was known that the structure of phage T2 consists solely of two types of molecules: DNA (blue in the figure) and protein (gold). The researchers took advantage of this fact to devise an elegantly simple experiment that determined which of these molecules the phage transferred to E. coli during infection. Hershey and Chase found that when the bacteria had been infected with T2 phages containing labeled protein, the radioactivity ended up mainly in the solution within the centrifuge tube, which contained phages but not bacteria. This result suggested that the phage protein did not enter the cells. But when the bacteria had been infected with phages whose DNA was tagged, most of the radioactivity was in the pellet of bacterial cells at the bottom of the centrifuge tube. Furthermore, when these bacteria were returned to a liquid growth medium, they soon lysed, or broke open, releasing new phages that contained some radioactive phosphorus in their DNA. After the virus attaches to the host bacterial cell, it injects its DNA into the host. Notice that virtually all of the viral protein (yellow) is left outside of the bacterium (which is why the radioactive protein did not show up in the host cells during the experiment). Once injected into the bacteria, the viral DNA causes the bacterial cells to produce new phage proteins and DNA molecules—indeed, complete new phages—which soon cause the cell to lyse, releasing the newly produced phages. These phages may then attach to other host bacterial cells.
Note 4 —-»
It is the viral DNA that contains the instructions for making phages.
What structural features of viruses like phage T2 made them ideally suited for the Hershey-Chase experiment?
Phage T2 has a very simple structure consisting of just two “ingredients”— DNA and protein—making it easier to identify which component served as the genetic material.
