1. Why did the amount of radioactivity in each set of samples change over time?
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Question
1. Why did the amount of radioactivity in each set of samples change over time?2. After 4 minutes in the blender, what percentage of 35S was outside the bacteria? What percentage of 32p was outside the bacteria?
3. How did the researchers know that the radioisotopes in the fluid came from outside the bacterial cells (extracellular) and not from bacteria that had broken apart?
4. The extracellular concentration of which isotope, 35S or 32p, increased the most with blending? Why do these results imply that bacteriophage viruses inject DNA into bacteria? Digging Into Data 35S remains outside cells The Hershey-Chase Experiments Virus coat proteins labeled with 35s By 1950, researchers had discovered bacteriophages, a type of virus that infects bacteria. Like all viruses, these infectious particles carry hereditary information about how to make new viruses. After a virus infects a cell, the cell starts making new virus particles. Bacteriophages inject genetic material into bacteria,cels. Thus, the viruses had not injected protein into the bactesiá. but was that material DNA, protein, or both? DNA being injected into bacterium one experiment, bacteria were infected with virus particles that had been labeled with a radioisotope of iral proteins. The viruses were dislodged tfrom the bacteria by whirling the mixture in a kitchen blender. Most of the radioactive sulfur was detected in the viruses, not in the bacterial sulfur (35s). The sulfur had labeled only v Alfred Hershey and Martha Chase found the answer to that question by exploiting the long known properties of protein (high sulfur content) and DNA (high phosphorus content). They culturedinto bacterium bacteria in a growth medium con- taining a radioisotope of sulfur (3Ss). n this medium, the protein (but not the DNA) of bacteriophages that infected the bacteria became labeled with the 35s tracer (Section 2.2). Her- shey and Chase allowed the labeled viruses to infect a fresh culture of unlabeled bacteria. They knew from electron micrographs that phages attach to bacteria by their slender tails. They reasoned it would be easy to break this precarious attachment, so they whirled the virus-bacteria mixture in a kitchen blender. The researchers then separated the bacte ria from the virus-containing fluid, and measured the 35s content of each separately. The fluid contained most of the 35s. Thus, the viruses had not injected protein into the bacteria (Figure 6.11A). Virus DNA labeled with 32p inside cells Labeled DNA being injected B In another experiment, bacteria were infected with virus particles that had been labeled with a radioisotope of phosphorus (32P). The phosphorus had labeled only viral DNA. When the viruses were dislodged from the bacterla, the radloactive phosphorus was detected mainly inside the bacterial cells. Thus, the viruses had njected DNA into the cells-evidence that DNA is the genetic material of this virus. Infected bactepia 300 Extracellular s5 60 40 Extecellular PS2 20 Running time in blendop Hershey and Chase repeated the experiment using an isotope of phosphorus, 32p, which labeled the DNA (but not the proteins) of the bacteriophage. This time, they found that the bacteria contained most of the 32p, The viruses had injected DNA into the bacteria (Figure 6.11B) C Detail of Alfred Hershey and Martha Chase's publication describing their experiments with bacteriophage Infected bacteria" refers to the percentage of bacteria that survived the blender. The micrograph on the right shows three bacteriophage particles injecting DNA into an E. col bacterium. FIGURE 6.11 Animated! The Hershey-Chase experiments. 3. How did the researchers know that the radioisotopes in the fluis The graph shown in Figure 6.11C is reproduced from Hershey
Explanation / Answer
1. When the proteins were labeled, radioactivity remained outside the cells; but when the DNA was labeled, radioactivity was found inside the cells. this causes the concentration change in each sample.
2. After the 4 minutes in the blender the percentage of S35 is 80 and P32is 35 percentage.
3. When the proteins were labeled, radioactivity remained outside the cells; but when the DNA was labeled, radioactivity was found inside the cells. Bacterial cells with radioactive phage DNA released new phages with some radioactive phosphorus. The researchers concluded that Phage DNA entered bacterial cells, but phage proteins did not. So the concentrations of which isotope, 35S or 32p, increased the most with blending.
4. Because phosphorous can be found in large quantities in DNA, but in only trace amounts in protein. The location of DNA and protein according to the radiation concentrations
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