Chapter 16, Problem 1IQ

Hershey and Chase devised an experiment using radioactive isotopes to determine whether it was a phage’s DNA or protein that entered the bacteria and served as the genetic material of T2 phage.

1. a. How did they label phage protein?
2. b. How did they label phage DNA?

After infecting separate samples of E. coli with the differently labeled T2 cells, they blended and centrifuged the samples to isolate the bacterial cells from the lighter viral particles.

1. c. Where was the radioactivity found in the samples with labeled phage protein?
2. d. Where was the radioactivity found in the samples with labeled phage DNA?
3. e. What did Hershey and Chase conclude from these results?

Summary Introduction

To determine: In what manner Hershey and Chase labeled phage protein.

Introduction: DNA is the organic molecule that carries information from one generation to another. Several experiments are to be conducted to prove that DNA is the genetic material. One such experiment is the Hershey–Chase experiment, which is commonly known as the blender experiment.

Explanation of Solution

In the year 1952, Alfred Hershey and Martha Chase performed a series of experiment to confirm if DNA was the genetic material inherited by the offspring from parents. DNA and protein are important macromolecules that can carry genetic information. Hershey–Chase took T2 phage in the experiment because it contained a single DNA surrounded by a protein coat. They labeled two bacteriophages with different radioactive substances. They labeled phage protein with the radioactive sulfur (S³⁵). Radioactive sulfur tags only proteins, as protein contains sulfur.

Summary Introduction

To determine: In what manner Hershey and Chase labeled phage DNA.

Introduction: Deoxyribonucleic acid (DNA) is a macromolecule that transfers genetic information from parents to offspring. The structure of DNA is made of nucleotides, sugar, and phosphates appearing as a double-helical polymer.

Explanation of Solution

Alfred Hershey and Martha Chase labeled two bacteriophages with different radioactive substances. They labeled phage DNA with radioactive phosphorus (P³²). Radioactive phosphorus tags only the DNA part, as DNA’s backbone has the phosphorus group.

Summary Introduction

To determine: Where the radioactivity was found in the samples with labeled phage protein.

Introduction: Bacteriophages are a group of viruses that infect bacteria and replicate inside the bacteria. T2 phage is called as Enterobacteria phage T2. It infects Escherichia coli.

Explanation of Solution

Alfred Hershey and Martha Chase labeled two bacteriophages with different radioactive substances. One was labeled with the radioactive sulfur, and the other with radioactive phosphorous. They allowed these two batches of radioactive-labeled bacteriophage to infect the non-radioactive bacterial cell separately in the test tube.

After centrifugation, the culture got separated into a pellet and a supernatant form. The supernatant (liquid) showed the presence of radioactive sulfur. This means that the protein part of bacteriophage was left behind, as it did not enter the bacterial cell.

Summary Introduction

To determine: Where the radioactivity was found in the samples with labeled phage DNA.

Introduction: DNA or deoxyribonucleic acid carries hereditary information from one generation to another. After several experiments, scientists have found evidence which claims that DNA is the hereditary material that passes on the genetic information from one generation to the next.

Explanation of Solution

After centrifugation, the culture got separated into a pellet and a supernatant form. The pellet showed the presence of radioactive phosphorus and the DNA of the bacteriophage was responsible for the infection of bacteria, as it entered into the bacterial cell.

Summary Introduction

To determine: What did Hershey and Chase concluded from these results.

Introduction: Bacteriophages are viruses that infect the bacteria and take up their translation machinery for their life cycle. This property was exploited to prove that DNA is a genetic material along with the help of radioactive isotopes.

Explanation of Solution

The bacterial cells that were infected by the bacteriophages consisting radioactive phosphorus showed the presence of radioactive phosphorus. This means that during infection of the bacterial cell, DNA is the part of bacteriophage that got transferred to the bacterial cell and not protein. Thus, they concluded that DNA is the genetic material which is passed on from one generation to the next generation, not the protein.