Question

Researchers design experiments so that only one thing is different between the treatments that are being compared. In the Hershey-Chase experiment, what was this single difference?

Short answer

In the Hershey-Chase experiment, the single difference between the treatments was the element used to radiolabel the bacteriophages. Treatment 1 used phosphorus-32 (P-32) to label the DNA, while Treatment 2 used sulfur-35 (S-35) to label the protein coats.

Step-by-step solution

Understanding the Hershey-Chase Experiment

The Hershey-Chase experiment was conducted by Alfred Hershey and Martha Chase in 1952 to determine whether genetic material was stored and transmitted via DNA or proteins. They conducted their experiment using bacteriophage (a virus that infects and replicates within bacteria) T2 that infects the bacterium Escherichia coli. They wanted to find out whether it's the DNA or the protein of the virus that enters the bacterial cell and directs the production of new bacteriophages.

Preparing the Treatments

To determine whether DNA or proteins contain the genetic material, Hershey and Chase prepared two separate groups of bacteriophages. They radiolabeled one group with phosphorus-32 (P-32) and the other with sulfur-35 (S-35). This was because DNA contains phosphorus but not sulfur, while proteins contain sulfur but not phosphorus. So, they would be able to trace whichever component enters the bacterial cell.

Treatment 1: Radiolabeled DNA

In the first treatment, the bacteriophages had their DNA radiolabeled with P-32. This allowed Hershey and Chase to track the DNA within the experiment.

Treatment 2: Radiolabeled Protein

In the second treatment, the bacteriophages had their protein coats radiolabeled with S-35. This allowed Hershey and Chase to track the protein coats within the experiment.

Finding the Single Difference

The single difference between the treatments in the Hershey-Chase experiment was the element used to radiolabel the bacteriophages. Treatment 1 used P-32 to label the DNA, while Treatment 2 used S-35 to label the protein coats. This labeling allowed the researchers to determine whether it was DNA or proteins that entered the bacterial cells and contained the genetic material needed for replication.

Key concepts

Genetic Material

The Hershey-Chase experiment is a hallmark study in biochemistry and genetics, setting the stage for our current understanding that DNA is the molecule that carries genetic information. Before their experiment, scientists debated whether proteins or DNA was responsible for heredity. Proteins were considered strong candidates due to their complexity and abundance in cells. Hershey and Chase changed that notion by proving that DNA, not proteins, is the genetic material.

Genetic material is responsible for heredity; it is the blueprint that carries instructions for the creation and functioning of living organisms. It needs to be accurately replicated and passed on to progeny, which is exactly what DNA does. Hershey and Chase's clever design highlighted that DNA injected by bacteriophages into bacteria contains all the necessary instructions to produce more viruses, thus acting as the source of genetic continuity.

DNA

DNA, or deoxyribonucleic acid, is the substance that constitutes the genetic material of most living organisms (RNA is the genetic material in some viruses). It's composed of two long strands forming a double helix, with each strand made up of a sugar-phosphate backbone and nitrogenous bases (adenine, thymine, guanine, and cytosine).

The sequencing of these bases encodes all genetic information, making DNA the molecular code for hereditary traits. The Hershey-Chase experiment underscored DNA's role, as they found the phosphorus-32, indicative of DNA, within bacteria cells that had been infected with treated bacteriophages, confirming DNA carried the genetic information for virus replication.

Radiolabeling

Radiolabeling is a technique used to track the presence, movement, and interaction of specific molecules in biochemical experiments, by attaching a radioactive isotope to the molecule of interest. This method provides a sensitive means to detect minuscule amounts of a substance through its emitted radiation.

In the context of the Hershey-Chase experiment, radiolabeling was essential. By incorporating phosphorus-32 in DNA and sulfur-35 in protein coats, Hershey and Chase could distinguish which of these molecules entered the E. coli bacteria upon infection. Because phosphorus is a component of DNA but not proteins, while sulfur is found in proteins but not DNA, the distinction based on radioactivity clearly identified the genetic material.

Bacteriophage

A bacteriophage, commonly referred to as a phage, is a type of virus that infects and replicates within bacteria. They are an invaluable tool in molecular biology and played a pivotal role in the Hershey-Chase experiment.

Phages like the T2 bacteriophage used by Hershey and Chase attach to a bacterial cell, inject their genetic material, and subsequently use the cell machinery to produce new virus particles. Crucially, their simple structure – consisting of a protein coat and DNA or RNA – makes them ideal models to study the molecular components responsible for carrying genetic instructions. In this landmark experiment, the phage's ability to inject only DNA into the host bacteria was pivotal in identifying DNA as the genetic material.

Biochemistry

Biochemistry is the branch of science that explores the chemical processes within and related to living organisms. By combining biology and chemistry, it provides insights into how biological molecules like proteins, nucleic acids, carbohydrates, and lipids function and interact in cellular processes.

The Hershey-Chase experiment is a quintessential biochemistry study that integrates the use of chemical isotopes, understanding of biologically significant molecules, and experimental inquiry to solve a biological question. It reflects the methodical approach of isolating variables—in this case, using radiolabeling to distinguish between different types of molecules—to draw conclusions about fundamental aspects of life, like the basis of genetic inheritance.