The Hershey and Chase Experiment | Discovery of DNA as the genetic material

Summary

In the 1940s and 50s, the scientific community debated the nature of hereditary material. Avery, MacCleod, and McCarty's 1944 experiment suggested DNA was the key, not proteins. Hershey and Chase's 1952 experiment using bacteriophages confirmed DNA as the hereditary material through a clever labeling method with radioactive isotopes. Their findings, along with Watson and Crick's DNA double helix model, solidified DNA's role in heredity.

Takeaways

• 🧬 The 1940s and 50s scientific community debated the nature of hereditary material.
• 🔬 Avery, MacCleod, and McCarty's 1944 experiment suggested DNA, not proteins, was responsible for heredity.
• 🌟 Hershey and Chase's 1952 experiments further investigated whether DNA or protein was the hereditary material.
• 🦠 They used bacteriophages, viruses that infect bacteria, as a simple model organism for their experiments.
• 🌀 The bacteriophage injects its genetic material into bacteria to replicate, suggesting the material contains hereditary information.
• 🏷 Hershey and Chase labeled DNA with radioactive phosphate and proteins with radioactive sulfur to track their fate.
• 🧴 They used a Waring blender to separate bacteria from attached viruses, then centrifuged the samples to isolate components.
• 📉 Sulfur-labeled protein experiments showed most radioactivity remained outside the bacteria, indicating proteins were not inherited.
• 📈 Phosphate-labeled DNA experiments revealed that DNA was injected into bacteria and was passed on to new virus generations.
• 🔑 The Hershey-Chase experiment provided critical evidence supporting DNA as the hereditary material.
• 🎓 The experiment is celebrated for its elegant design and significant contribution to understanding DNA's role in heredity.

Q & A

What was the debate in the 1940s and 50s about the nature of hereditary material?

The debate was about whether proteins or DNA were responsible for passing traits from one generation to the next.

What did Avery, MacLeod, and McCarty's experiment in 1944 suggest about the hereditary material?

Their experiment suggested that DNA, not proteins, was the hereditary material, contradicting the prevailing belief at the time.

Why was the scientific community initially skeptical of Avery, MacLeod, and McCarty's findings?

Their findings contradicted the widely accepted belief that proteins were the hereditary material, and significant evidence is required for a new scientific idea to be accepted.

What was the significance of the Hershey and Chase experiment in 1952?

The experiment provided crucial evidence that DNA, and not protein, was the hereditary material by using bacteriophages to demonstrate which component was injected into bacteria.

Why did Hershey and Chase choose bacteriophages as their model organism?

Bacteriophages were chosen because they are simple organisms that infect bacteria, allowing for a clear demonstration of which substance (DNA or protein) was injected into the bacteria.

How did Hershey and Chase use radioactive tags in their experiments?

They used radioactive phosphate to label DNA and radioactive sulfur to label proteins, allowing them to track which component was injected into the bacteria.

What method did Hershey and Chase use to separate bacteria from the viruses attached to their outsides?

They used a Waring blender to physically separate the bacteria from the viruses and then centrifuged the sample to further separate the two based on size.

What did the experiment with sulfur-labeled protein show about the role of protein in heredity?

It showed that most of the radioactivity remained with the virus and did not enter the bacteria, indicating that protein was not being inherited.

What was the outcome of the experiment with DNA labeled with radioactive phosphate?

Most of the radioactive DNA was found inside the bacteria, and much of this labeled DNA was inherited by the new virus generation, supporting the idea that DNA is the hereditary material.

How did the Hershey and Chase experiment contribute to the acceptance of DNA as the hereditary material?

Their experiment added to the evidence supporting DNA as the hereditary material, which, combined with the subsequent publication of the DNA double helix model by Watson and Crick, helped convince the remaining skeptics.

Why is the Hershey-Chase experiment considered a classic in biology?

It is considered a classic because it addressed a fundamental question in biology with a well-chosen model organism and an elegant experimental design, leading to a clear distinction between competing hypotheses.

Outlines

00:00

🧬 DNA: The Molecule of Heredity

In the 1940s and 50s, the scientific community was engaged in a debate about the nature of hereditary material. The experiment by Avery, MacLeod, and McCarty in 1944 suggested that DNA, not proteins, was responsible for heredity. However, their findings were met with skepticism. To provide more substantial evidence, Alfred Hershey and Martha Chase conducted experiments in 1952 using bacteriophages, viruses that infect bacteria. They aimed to determine whether DNA or protein was the hereditary material. Their experiments involved labeling DNA with radioactive phosphate and protein with radioactive sulfur. After infecting bacteria with the labeled viruses, they used a Waring blender to separate bacteria from viruses and centrifugation to distinguish between the two. The results showed that the radioactive DNA was injected into the bacteria and was passed on to new virus generations, while the protein was not inherited. This experiment added significant evidence supporting DNA as the hereditary material, which was further reinforced by Watson and Crick's publication of the DNA double helix model a year later.

Keywords

💡Heredity

Heredity refers to the passing of genetic information from one generation to the next, which determines the traits and characteristics of offspring. In the video, the central debate revolves around the nature of the hereditary material, questioning whether it is proteins or DNA that carries these genetic instructions. The script discusses how Avery, MacLeod, and McCarty's experiment suggested DNA as the hereditary material, challenging the prevailing belief.

💡DNA

Deoxyribonucleic acid (DNA) is a molecule that carries the genetic instructions for the development, functioning, growth, and reproduction of all known living organisms. The video script highlights DNA's role as the hereditary material, as demonstrated by the Hershey-Chase experiment, which showed that DNA is injected into bacteria by bacteriophages, suggesting it contains the genetic information necessary for replication.

💡Proteins

Proteins are large biomolecules composed of amino acids and are essential for the structure, function, and regulation of the body's tissues and organs. In the script, proteins were initially thought to be the hereditary material, but experiments by Avery and later by Hershey and Chase indicated that DNA, not proteins, is responsible for heredity.

💡Bacteriophage

A bacteriophage, or phage, is a virus that infects bacteria. In the video, Hershey and Chase used bacteriophages as a model organism to study the nature of the hereditary material. The script describes how bacteriophages attach to bacteria and inject their genetic material, which is crucial for understanding the mechanism of heredity.

💡Radioactive Tags

Radioactive tags are isotopes that emit radiation and can be used to track and detect the presence and movement of specific molecules. In the script, Hershey and Chase used radioactive phosphate to label DNA and radioactive sulfur to label proteins in their experiments. This method allowed them to determine which component of the bacteriophage was injected into the bacteria.

💡Waring Blender

A Waring blender is a high-speed blender used for mixing and emulsifying foods. In the context of the video, it was used by Hershey and Chase to physically separate bacteria from the viruses attached to their outsides, which was a crucial step in their experimental process to study the hereditary material.

💡Centrifugation

Centrifugation is a process that uses centrifugal force to separate substances of different densities. In the video script, Hershey and Chase used centrifugation to separate the heavier bacteria from the lighter viruses after infection, allowing them to analyze the distribution of radioactive labels and thus infer the nature of the hereditary material.

💡Hershey-Chase Experiment

The Hershey-Chase experiment was a series of experiments conducted by Alfred Hershey and Martha Chase that demonstrated DNA as the genetic material. The video script details how they used bacteriophages labeled with radioactive isotopes to show that DNA, not protein, was injected into bacteria, supporting the idea that DNA carries genetic information.

💡Model Organism

A model organism is a species that is extensively studied to understand particular biological phenomena, often because it is easy to manipulate and observe. In the video, bacteriophages were chosen as a model organism due to their simplicity, which allowed Hershey and Chase to effectively investigate the transmission of hereditary material.

💡Double Helix

The double helix refers to the structure of DNA, which consists of two intertwined strands. The video script mentions that after the Hershey-Chase experiment, Watson and Crick published their model of the DNA double helix, which provided a mechanism for how DNA replicates and passes genetic information.

Highlights

Debate in the 1940s and 50s about the nature of hereditary material.

Avery, MacCleod, and McCarty's 1944 experiment suggesting DNA, not proteins, was responsible for heredity.

Hershey and Chase's 1952 experiments to determine if DNA or protein was the hereditary material.

Choice of bacteriophage as a simple model organism for the experiment.

Bacteriophage's method of infecting bacteria and the potential hereditary material it injects.

The use of radioactive tags to label DNA and proteins in the viruses.

Radioactive phosphate for DNA and radioactive sulfur for protein to differentiate the hereditary material.

Use of a Waring blender to separate bacteria from viruses for analysis.

Centrifugation to separate bacteria from viruses based on size.

Finding that protein radioactivity mostly remained outside the bacteria, indicating it was not the hereditary material.

Observation that less than 1% of protein radioactivity was found in new viruses, suggesting protein was not inherited.

Experiment with DNA-labeled virus showing most radioactive DNA was injected into bacteria.

Inheritance of labeled DNA by new virus generations, supporting DNA as the hereditary material.

Hershey and Chase's cautious conclusion that DNA had 'some function' in bacterial multiplication.

The Hershey-Chase experiment's rapid spread and impact on scientific community's view of DNA as hereditary material.

Watson and Crick's subsequent publication of the DNA double helix model, explaining DNA replication and inheritance.

The Hershey-Chase experiment's status as a classic in biology for its elegant experimental design and significant contribution to understanding heredity.

Transcripts

Turning the clock back to the 1940s and 50s, the scientific community was in the midst

of a debate about the nature of the hereditary material that is responsible for passing our

traits on to our children, to our children’s children, and so on for generations—pretty

existential question, right?. An experiment by Avery, MacCleod and McCarty in 1944 first

suggested that proteins were not responsible for heredity, as previously thought, and instead,

DNA was the culprit. However, given that their results flew in the face of the current thinking,

not everyone was convinced. In science, a great deal of evidence is needed before a

new scientific idea is accepted as fact. Therefore, in 1952, Alfred Hershey and Martha Chase carried

out experiments which addressed the same question was DNA the hereditary material? Or was

it protein? Or was it something else?

To solve this important puzzle, Hershey and Chase needed a simple model organism to solve

this complex question and a clever experiment. They chose the simplest organism possible-

a bacteriophage, which is a virus that infects bacteria. This virus looks a bit like a lunar

landing module, and attaches to the outside of the bacteria. It then injects a substance

into the bacteria to create many new copies of the virus. These newly

manufactured viruses are released when the bacteria bursts. So the substance injected

inside of the bacteria must contain the instruction manual for making new virus—it contains

the hereditary material. But what was this material that the virus injected into its

host? The answer to their question might shed light on the nature of heredity for all life

on earth!

Viruses consist of 2 things: a DNA core surrounded by a protein coat. Hershey and Chase needed

to figure out which of the two was injected into the bacteria. They ran two experiments,

and in each they labeled one of the components. For this labeling they used radioactive tags

– radioactive phosphate for the DNA, and radioactive sulfur for the protein. This was

an elegant method, because phosphate is only found in DNA, and not in the amino acids that

make up proteins, and sulfur is only found in proteins and not DNA.

After the viruses infected the bacteria, the researchers examined whether the radioactive

signal was detected inside the bacteria, or was left outside. To do so, they needed to

separate the bacteria from the viruses attached to their outsides, and did so using a Waring

blender (similar to that used for making milk shakes). Once separated, they then centrifuged

the sample - since bacteria are bigger they were centrifuged to the bottom while the smaller

viruses remained at the top of the tube.

When they prepared virus with sulfur-labeled protein and mixed it with bacteria, they found

that most of the radioactivity remained with the virus and did not enter the bacteria.

When they allowed the virus to replicate and collected the new viruses, less than 1% of

the protein radioactivity from the viral parents was found in the newly replicated virus, so

the protein was not being inherited.

On the other hand, when they performed the same experiment using virus with its DNA labeled

with phosphate, they found that most of the radioactive DNA was injected into the bacteria

from the attacking virus and much of this labeled DNA was inherited and passed on to

the new virus generation.

Hershey and Chase added to the mounting evidence pointing the finger at DNA as the hereditary

material, but they were aware that their experiments left a few unanswered questions – as experiments

often do! – and were cautious in their conclusions. From their observations, they stated that

DNA had ‘some function’ in the multiplication of the bacteria, whereas protein did not.

Word about the Hershey Chase experiment spread quickly and helped to convince many scientists

that DNA indeed was the molecule of heredity. And the remaining skeptics could not hold

out for much longer. A year later Watson and Crick published their model of the DNA double

helix which provided an elegant explanation for how DNA, with such a simple structure,

can be replicated and pass along information from parents to offspring.

The Hershey Chase experiment is considered a classic in biology because it tackled an

important question, with the use of a well-chosen model organism and elegant experimental design

in order to clearly distinguish between two competing hypotheses.