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.

Outlines

00:00

🔬 Hershey and Chase Experiment: DNA as the Molecule of Heredity

This paragraph introduces the Hershey and Chase Experiment, a pivotal study in 1952 that confirmed DNA as the molecule of heredity. It sets the stage by outlining the four major biomolecules and the historical debate over which molecule was responsible for passing genetic information. The paragraph discusses previous experiments by Griffith and Avery, MacLeod, and McCarty, which laid the groundwork for Hershey and Chase's research. It also explains the choice of the T2 bacteriophage as a model system due to its simplicity and the ease of distinguishing between its protein and nucleic acid components. The experiment involved tagging proteins with sulfur-35 and nucleic acids with phosphorus-32 to track their roles in genetic inheritance.

05:01

🧪 Hershey and Chase's Experimental Design and Results

This paragraph details the experimental steps of Hershey and Chase, including infection, blending, and centrifugation, which were used to determine whether proteins or nucleic acids were responsible for genetic inheritance. The blending step, humorously referred to as the 'Blender Experiment,' was crucial for separating the protein coats from the bacteria. The centrifugation step allowed for the separation of heavier bacterial cells from the lighter virus particles. The results showed that the radioactive label from the nucleic acids, not the proteins, ended up in the bacterial cells, strongly suggesting that DNA was the genetic material. Despite initial skepticism, the scientific community eventually accepted these findings, leading to Hershey's Nobel Prize in 1969. However, it also highlights the exclusion of Martha Chase from the recognition, noting the importance of acknowledging all contributors to scientific discoveries.

Keywords

💡Hershey and Chase Experiment

The Hershey and Chase Experiment refers to a groundbreaking study conducted in 1952 by Alfred Hershey and Martha Chase, which demonstrated that DNA, not protein, was the genetic material responsible for heredity. This experiment was pivotal in the field of molecular biology, as it provided conclusive evidence that DNA carries genetic information. In the video, this experiment is discussed as the culmination of a series of investigations into the nature of genetic material, building upon the work of Griffith and Avery, MacLeod, and McCarty.

💡Bacteriophage

A bacteriophage, often shortened to phage, is a type of virus that infects bacteria. In the Hershey and Chase Experiment, the T2 bacteriophage was used to study the nature of genetic material. The video explains how these phages were utilized to show that DNA, and not protein, was injected into bacterial cells during infection, thus proving DNA's role in heredity.

💡Protein coat

The protein coat, also known as the capsid, is the external protein structure of a virus, including bacteriophages. In the context of the Hershey and Chase Experiment, the protein coat was labeled with sulfur 35 to track its role in the infection process. The video describes how the protein coat was separated from the nucleic acids during the experiment, and how this separation helped to demonstrate that it was not the protein coat but the nucleic acids that were responsible for genetic inheritance.

💡Nucleic acids

Nucleic acids are biological macromolecules, including DNA and RNA, that carry genetic information. In the video, the Hershey and Chase Experiment is highlighted for its use of radioactive phosphorus 32 to label the nucleic acids of the T2 bacteriophage. The experiment showed that it was the nucleic acids, not the protein, that entered the bacterial cells and were responsible for the transmission of genetic information.

💡Radioactive isotope

A radioactive isotope is a version of an element that has an unstable nucleus and emits radiation as it decays. In the video, sulfur 35 and phosphorus 32 are mentioned as radioactive isotopes used to label the protein and nucleic acids, respectively, in the Hershey and Chase Experiment. This labeling allowed the researchers to track which component of the bacteriophage was responsible for genetic inheritance.

💡Transformation principle

The transformation principle refers to the concept that some substance can change the properties of bacteria, as demonstrated by Frederick Griffith in his 1928 experiment. The video explains how Griffith's discovery led to further investigations by Avery, MacLeod, and McCarty, and ultimately to the Hershey and Chase Experiment, which identified DNA as the transformation principle.

💡Bacterial transformation

Bacterial transformation is the process by which bacteria are genetically altered, usually by the introduction of foreign DNA. The video discusses how Griffith's experiments with pneumococcus bacteria showed that a harmless strain could be transformed into a virulent strain through the acquisition of the transformation principle, which was later identified as DNA.

💡Pellet and supernatant

In the context of the Hershey and Chase Experiment, the terms 'pellet' and 'supernatant' refer to the separated components of the bacterial culture after centrifugation. The pellet, which is heavier, contains the bacterial cells and any associated genetic material, while the supernatant contains the lighter, suspended particles. The video describes how the radioactivity levels in these components provided evidence for the role of DNA in heredity.

💡Centrifugation

Centrifugation is a process that uses centrifugal force to separate substances of different densities. In the Hershey and Chase Experiment, centrifugation was used to separate the bacterial cells (pellet) from the virus particles (supernatant) after infection. The video explains how this process was crucial in demonstrating that the radioactive label associated with DNA, and not protein, was found in the pellet, indicating that DNA had entered the bacterial cells.

💡Genetic material

Genetic material refers to the molecules that carry genetic information in an organism, which are DNA and RNA. The video's main theme revolves around the identification of DNA as the genetic material, a discovery made through the Hershey and Chase Experiment. This experiment resolved a long-standing debate about whether proteins or nucleic acids were responsible for passing genetic information from one generation to the next.

💡Nobel Prize in Medicine

The Nobel Prize in Medicine is an annual award given to individuals who have made significant contributions to the field of medicine. The video mentions that Alfred Hershey was awarded the Nobel Prize in 1969 for his work on the Hershey and Chase Experiment. However, it also points out the historical oversight of not acknowledging Martha Chase's contributions to the discovery.

Highlights

Hershey and Chase Experiment proved DNA was the molecule of heredity in 1952.

Scientists in the early 20th century debated which biomolecule passed genetic information.

Proteins were a leading candidate for the molecule of heredity due to their complexity and variety.

Hershey and Chase's experiment was the third major investigation into the composition of genes.

Frederick Griffith's experiment demonstrated bacterial transformation through an unknown substance.

Avery, MacLeod, and McCarty's experiments suggested DNA was the transformation principle.

Hershey and Chase used T2 bacteriophages to test whether proteins or nucleic acids were responsible for inheritance.

Bacteriophages reproduce by injecting their genetic material into a host cell, leaving an empty protein shell.

The experiment involved marking proteins with sulfur 35 and nucleic acids with phosphorus 32 to track them.

The infection step allowed bacteriophages to infect host cells to see the effect of radioactive markers.

The blending step used a high-speed blender to separate the protein shell from the bacteria.

Centrifugation helped separate heavier bacteria from lighter virus particles.

Radioactivity levels in the pellet and supernatant indicated that nucleic acids were the molecule of inheritance.

The experiment provided evidence that nucleic acids, not proteins, were responsible for genetic inheritance.

Some scientists remained skeptical, thinking the results might not apply to more complex organisms.

The scientific community ultimately accepted Hershey and Chase's findings.

Alfred Hershey received the 1969 Nobel Prize in Medicine for this work, but Martha Chase was excluded.

Hershey did not acknowledge Chase’s role in the discovery in his Nobel acceptance speech.

Transcripts

Welcome back to Bogobiology!

In this video, we’ll be discussing the classic Hershey and Chase Experiment and how it proved

conclusively that DNA was the molecule of heredity in 1952.

Remember that there are four major groups of biomolecules; carbohydrates, lipids, proteins

and nucleic acids.

In the first half of the 20th century, scientists still weren’t entirely in agreement about

which biomolecule passed genetic information from parent to offspring.

Many believed it to be proteins because they are so large, structurally complex, and have

so much variety.

Dr. Alfred Hershey and Dr. Martha Chase’s now famous experiment was actually the third

major investigation into what genes are made of.

They learned from the previous work of Frederick Griffith in 1928 and Oswald Avery, Colin MacLeod

and Maclyn McCarty in 1944.

It’s important to know how each experiment built upon those that came before in order

to understand the experimental design.

First, Frederick Griffith proved that some sort of substance could be transferred between

bacteria, which could change its properties in a process we now call bacterial transformation.

He called this mysterious substance the “transformation

principle” and demonstrated its power in a famous experiment involving mice and pneumococcus

bacteria.

Harmless R-strain bacteria could acquire material from dangerous R-strain bacteria and become

deadly.

However, at the time, Griffith didn’t know what kind of molecule the transformation principle

was.

Second, Avery, MacLeod and McCarty performed a series of experiments to isolate the transformation

principle, and analyzed it extensively.

They found that the mysterious substance was most chemically similar to DNA.

Then, they methodically destroyed proteins, RNA and DNA and discovered that disabling

DNA prevents bacterial transformation from occurring.

However, the scientific community disputed Avery’s results, believing that trace amounts

of transformation principle could have contaminated their samples of DNA, and that proteins could

still be the molecules of heredity.

If you’d like more details about Griffith’s or Avery’s experiments, I’ve left links

to tutorials on their work in the video description, as well as the links to the guided notes for

each video.

Hershey and Chase picked up where Avery and his colleagues left off, and set out to prove

once and for all whether the molecule of heredity was protein or DNA.

They chose to do their work by harnessing a simple virus called a T2 bacteriophage.

These bacteriophages have several helpful properties, including the fact that they reproduce

very quickly and are only made up of two of the "Big Four"; nucleic acids and

proteins.

Hershey and Chase had previously studied the properties of these bacteriophages and had

concluded that the protein formed a type of protective coating or “shell” around the

nucleic acids.

Bacteriophages naturally reproduce by attaching to the outside of a host cell.

In the case of the T2, they attack e coli.

Then they inject their genetic material into it, which hijacks the internal workings of

the cell, while the empty protein coat stays on the outside.

This empty shell is sometimes called a “protein ghost”.

The DNA from the bacteriophage then causes the cell to stop whatever it was doing and

begin producing identical copies of the original bacteriophage.

The host cell then explodes, releasing all of the new bacteriophages, which then go out

and infect even more cells.

Hershey and Chase harnessed the viral life cycle to prove which of the two contenders

was actually responsible for inheritance; proteins or nucleic acids.

Hershey and Chase used two groups of bacteriophages, and made a single component radioactive in

each one to keep track of what it was doing.

In one group, they tested the viral protein coat on the outside, and in the other group

they tested the nucleic acids on the inside of the virus.

They knew that whichever part was responsible for genetic inheritance would create more

radioactive bacteriophages, and the other would not.

Proteins and nucleic acids contain some of the same elements such as Carbon, Hydrogen,

Oxygen and Nitrogen.

However, they differ in that proteins can contain sulfur, and nucleic acids contain

phosphorous.

The researchers tracked the elements that were unique to each biomolecule.

In proteins, they replaced the sulfur with a radioactive isotope known as sulfur 35.

Since sulfur is not found in nucleic acids but is found in some amino acids, they were

only marking the protein coat.

In the nucleic acids, they replaced the naturally-occurring phosphorus in the nucleic acids with a radioactive

type of phosphorus called phosphorus 32.

Since there was no phosphorus found in the protein coat, the only thing they were marking

in the second group was the nucleic acids.

You might say that the sulfur and the phosphorous each got a major glow up.

Once the appropriate molecules were tagged, the experiment consisted of three major steps;

infection, blending, and centrifugation.

In the infection step, Hershey and Chase then allowed each type of bacteriophage to infect

a host cell to see whether the radioactive proteins or the radioactive nucleic acids

would make radioactive bacteriophages.

Next, in the blending step, they used a high speed blender to jiggle the e coli cells enough

to shake off the empty protein shell from the outside.

(This is why this experiment is sometimes affectionately referred to as the “Blender

Experiment”).

This created a mixture of liquid, phage parts and bacteria called a “suspension”.

Finally, in the centrifugation, they spun this mixture in a centrifuge so that the heavier

bacteria condensed and formed a pellet at the bottom of the test tube.

Since the virus particles were much smaller and lighter, they remained suspended in the

liquid or the “supernatant”.

Experiment results: Hershey and Chase measured the level of radioactivity

in the liquid vs in the pellet to see where the radioactivity had ended up.

Hershey and Chase knew that the virus had transferred its genetic material, whatever

it was, into the bacterial cells.

Because the bacterial cells were heavier, they would end up in the pellet along with

the injected radioactive material, after centrifugation.

If proteins were the molecule of inheritance, the pellet would be more radioactive than

the supernatant in the protein group.

If nucleic acids were the molecule of inheritance, the pellet would be more radioactive than

the supernatant in the nucleic acid group.

When they tested the two groups, they found that the supernatant was more radioactive

in the protein group, and the pellet was more radioactive in the nucleic acid group, indicating

that the viral DNA had entered the cell.

This provided very compelling evidence that nucleic acids, not proteins, were the molecule

of inheritance.

Because the experiment was conducted using bacteriophages, some scientists were still

skeptical of its implications.

Some believed that, while Hershey and Chase had proved DNA was the genetic material for

viruses, it might not be the genetic material for more complex organisms.

However, the scientific community did ultimately accept their findings.

Dr. Alfred Hershey would eventually receive the 1969 Nobel Prize in Medicine for this

work.

However, the Nobel Committee (frustratingly) excluded Dr. Martha Chase, and Alfred Hershey

did not acknowledge Chase’s role in discovering that DNA is the molecule of heredity in his

acceptance speech.

That wraps up our discussion of Hershey and Chase’s work.

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Thanks again for watching!