Problem 2 - Biology: The Dynamic Science

Question

The Hershey and Chase experiment showed that phage: a. \({ }^{35} \mathrm{S}\) entered bacterial cells. b. \({ }^{32} \mathrm{P}\) remained outside of bacterial cells. c. protein entered bacterial cells. d. DNA entered bacterial cells. e. DNA mutated in bacterial cells.

Step-by-Step Solution

Answer

The Hershey and Chase experiment showed that DNA entered bacterial cells.

Step 1: Understanding the Hershey-Chase Experiment

Recall that the Hershey-Chase experiment was designed to determine what component of the T2 bacteriophage - protein or DNA - enters an E. coli bacterial cell during infection. They used two isotopes to label the two components: sulfur-35 (( ^{35}S ) to label protein (since sulfur is present in proteins but not in DNA) and phosphorus-32 (( ^{32}P ) to label DNA (since phosphorus is present in DNA but not in protein).

Step 2: Analyzing Experiment Results

Hershey and Chase found that when phages with sulfur-35 labeled proteins infected bacteria, the ( ^{35}S ) was found mostly outside the cells whereas when phages with phosphorus-32 labeled DNA infected bacteria, the ( ^{32}P ) was found inside the cells. This indicated that it was the DNA, not the protein, that was the genetic material entering the bacterial cells.

Step 3: Determining the Correct Statement

Based on the experiment's findings, we can deduce that option (a) is incorrect because ( ^{35}S ), which labels protein, did not enter the bacterial cells. Option (b) is incorrect because ( ^{32}P ), which labels DNA, did indeed enter the bacterial cells. Option (c) is incorrect for the same reason as (a). Option (e) is incorrect because the experiment did not investigate DNA mutation. Therefore, option (d), which states that DNA entered bacterial cells, is correct.

Key Concepts

Bacteriophage DNA

Bacteriophages, often referred to as phages, are viruses that infect bacteria. Phages are composed of a protein shell surrounding genetic material, which is DNA in the case of the T2 bacteriophage used in the Hershey and Chase experiment. The role of bacteriophage DNA is crucial as it contains the genetic information necessary for the virus to hijack the host's cellular machinery.

In the process of infection, a bacteriophage attaches to a bacterium and injects its DNA into the host cell. The bacterial cell then begins to produce viral proteins and replicate viral DNA, leading to the creation of new phage particles. The Hershey and Chase experiment became a historical turning point by demonstrating conclusively that DNA, not protein, was the genetic material that provided the instructions for creating new phages. Understanding this key role of bacteriophage DNA has been fundamental for the field of molecular biology.

Phosphorus-32 Labeling

The element phosphorus is a key component of DNA, specifically it's present in the sugar-phosphate backbone of the DNA molecule. In the Hershey and Chase experiment, the phosphorus-32 (( ^{32}P)) isotope was used to label the DNA of bacteriophages. This labeling enables scientists to track the presence of DNA as it enters a bacterial cell upon infection by a virus.

Phosphorus-32 is a radioactive isotope and serves as a tracer that can be detected by its radiation emission. When the bacteriophages with DNA labeled with ( ^{32}P) infected the bacteria, the radiation was detected inside the bacterial cells. This was a critical evidence that supported DNA as the material carrying genetic information, as it showed that DNA enters the cell during phage replication, and not the protein component of the virus.

Sulfur-35 Labeling

Sulfur is not found in DNA, but it's an essential element of some amino acids, which are building blocks of proteins. In the Hershey and Chase experiment, sulfur-35 (( ^{35}S)) was used to label the protein coat of bacteriophages. This labeling with a radioactive isotope allowed the researchers to track the location of the protein component after phages infected bacteria.

Following infection, the absence of significant radioactive sulfur inside the bacterial cells indicated that the bacteriophage's proteins did not enter the host cells but remained outside. This was in contrast to the phosphorus-32 labeled DNA, which did enter the cells. Thus, the use of sulfur-35 labeling helped to distinguish the fate of proteins during bacteriophage infection, contributing vital evidence that proteins were not the carriers of genetic information. The sulfur-35 labeling was like leaving a 'signature' that could only be traced on the outside of the cells, echoing the conclusion that the genetic material of the phage was not protein-based.