Hershey and Chase Experiment: DNA is the Molecule of Heredity

Summary

In this Bogobiology episode, the Hershey and Chase Experiment of 1952 is explored, which confirmed DNA as the molecule of heredity. The video discusses the historical context of genetic material discovery, building on Griffith's transformation principle and Avery's DNA identification. Hershey and Chase utilized T2 bacteriophages, marking proteins with sulfur-35 and nucleic acids with phosphorus-32, to demonstrate that DNA, not protein, is the genetic material. Their findings were pivotal, leading to Hershey's 1969 Nobel Prize, though Chase was notably unrecognized.

Takeaways

• 🔬 The Hershey and Chase Experiment in 1952 proved that DNA was the molecule of heredity, building upon previous work by Griffith and Avery, MacLeod, and McCarty.
• 🧬 DNA was considered the molecule of heredity because it was found to be the material that entered bacterial cells during the infection process by bacteriophages.
• 🌟 The experiment utilized T2 bacteriophages, which are simple viruses that infect E. coli bacteria and are composed of nucleic acids and proteins.
• 🛡️ The protein coat of the bacteriophage, known as the 'ghost,' was shown to remain outside the bacterial cell while the DNA entered, indicating the DNA's role in heredity.
• 📈 The use of radioactive isotopes, sulfur-35 for proteins and phosphorus-32 for nucleic acids, allowed Hershey and Chase to track the genetic material during the infection process.
• 🔧 The experiment involved three main steps: infection, blending (using a high-speed blender to separate the protein coats from the bacteria), and centrifugation to separate the components.
• 🧪 The results showed that radioactivity was more concentrated in the pellet (bacterial cells) when DNA was marked, suggesting that DNA was the genetic material being transferred.
• 🏆 Dr. Alfred Hershey was awarded the 1969 Nobel Prize in Medicine for this work, but Dr. Martha Chase was not included, despite her significant contributions.
• ⚖️ The scientific community initially disputed Avery's findings, but Hershey and Chase's experiment provided conclusive evidence that DNA, not proteins, was the molecule of inheritance.
• 🌐 While some scientists remained skeptical about the implications for more complex organisms, the experiment's findings were ultimately accepted by the broader scientific community.

Q & A

What was the main objective of the Hershey and Chase experiment?

The main objective of the Hershey and Chase experiment was to determine conclusively whether DNA or proteins were the molecules responsible for genetic inheritance.

What were the four major groups of biomolecules that scientists considered as potential carriers of genetic information?

The four major groups of biomolecules considered were carbohydrates, lipids, proteins, and nucleic acids.

Why did many scientists initially believe that proteins were the molecules of heredity?

Many scientists believed proteins were the molecules of heredity because they are large, structurally complex, and exhibit a great deal of variety.

What was the 'transformation principle' that Griffith discovered?

The 'transformation principle' was a substance that Griffith discovered could be transferred between bacteria, changing their properties, which we now understand as DNA.

How did Avery, MacLeod, and McCarty contribute to the understanding of the genetic material?

Avery, MacLeod, and McCarty performed experiments that isolated the transformation principle and found it to be chemically similar to DNA, demonstrating that DNA was essential for bacterial transformation.

Why did Hershey and Chase choose to use T2 bacteriophages in their experiment?

Hershey and Chase chose T2 bacteriophages because they reproduce quickly and are composed of only two of the 'Big Four' biomolecules: nucleic acids and proteins.

What was the significance of the 'protein ghost' in the Hershey and Chase experiment?

The 'protein ghost' was the empty protein shell left outside the host cell after the bacteriophage injected its genetic material. It was significant because it allowed Hershey and Chase to separate and study the roles of proteins and nucleic acids in inheritance.

How did Hershey and Chase use radioactive isotopes to label the proteins and nucleic acids in their experiment?

Hershey and Chase replaced sulfur in proteins with sulfur-35 and phosphorus in nucleic acids with phosphorus-32. These isotopes were unique to each biomolecule, allowing them to track which component was responsible for genetic inheritance.

What were the three major steps of the Hershey and Chase experiment?

The three major steps were infection, where bacteriophages infected host cells; blending, where cells were mixed to separate the protein coats from the bacteria; and centrifugation, where the mixture was spun to separate the heavier bacteria from the lighter virus particles.

What did the results of the Hershey and Chase experiment indicate about the molecule of inheritance?

The results indicated that the pellet, which contained the bacterial cells and the injected radioactive material, was more radioactive in the nucleic acid group, providing compelling evidence that nucleic acids, not proteins, were the molecules of inheritance.

Why was there initial skepticism about the applicability of Hershey and Chase's findings to more complex organisms?

There was initial skepticism because the experiment was conducted using bacteriophages, which are simple viruses. Some scientists doubted whether the results could be generalized to more complex organisms.