DNA Discovery - Online Tutor, Practice Problems & Exam Prep

The discovery of DNA as the genetic material involved key experiments. Oswald Avery demonstrated that DNA from dead infectious bacteria could transform non-infectious bacteria, proving DNA's role in heredity. The Hershey-Chase experiment further confirmed DNA as the genetic material in viruses by showing that only labeled DNA entered bacteria. Finally, Watson and Crick's identification of the double helix structure explained how genetic information is stored and replicated. These findings collectively shifted the understanding from proteins to DNA as the primary carrier of genetic information.

Important Scientists

Hi, in this video, we're going to be talking about DNA discovery. So for this, I'm just going to go over a few of the main people and experiments that led to the discovery of DNA as the source of genetic material. It took a really long time to actually discover DNA as the source of genetic material because, for a long time, protein was thought to be the storage molecule for genetic information. This was around the 1900s, and they thought it was protein because protein has so many more possibilities. There are 20 amino acids but only four DNA bases and because there were 20 amino acids, they thought it had all of this information to store. So, it had to be protein, but it wasn't. And so, it took some experiments to figure out that it wasn't and that DNA was instead the source.

One of these main experiments was done by Oswald Avery, and he showed that DNA was the genetic material of bacteria. This happened around the 1940s. How he did this was he injected mice with two bacteria: noninfectious and a dead infectious, but the mice became infected anyway. This actually showed that the nucleic acids were the reason for conferred infectivity. I'm going to show you an example of that in just a minute.

Another experiment, or another group of people, were involved in the Hershey-Chase experiments, which showed that DNA was the genetic material of viruses in the 1950s. How they did this was they took viruses, bacterial phages, which are viruses that infect bacteria, and they labeled them with radiation. They labeled the protein with one type of radiation and the DNA with a different type. When they infected bacteria with it, they looked at what actually infected the bacteria: was it the protein that was radioactively labeled or was it the DNA that was radioactively labeled? They found that it was actually the DNA that was being transferred into bacteria and causing the infection. This was really important in finding that genetic material was DNA and not protein.

The final experiment I want to talk about, which I'm sure you've heard of before, is that James Watson and Francis Crick discovered the DNA double helix. This was crucial in identifying DNA as a source of genetic material because with this structure, it provided a really elegant explanation of how genetic information is stored, replicated, and inherited in a way where other structures wouldn't quite make sense.

Let's go over the Avery experiment as just one of the experiments that really led to the DNA discovery. What Oswald Avery did was he injected mice with viruses. Here you have this experiment, and he obviously had four different scenarios. The first one was he infected a live non-infectious virus, and of course, the mouse lived because the virus wasn't infectious. Then he injected the mice with a live infectious virus, and of course, the mouse died because it became infected. Then he infected a different set of mice with a dead infectious virus, which would be an infectious virus if it were alive but because it's dead, it's not going to be infectious. Of course, the mouse lived. Now, you can see here that he took a fourth set of mice and he infected them with two viruses: a dead infectious, which by itself was non-effective, so the mouse lived, and a live non-infectious virus, which independently also allowed the mice to live. But together, you can see here that the mouse died. Why would two viruses that, by themselves, allowed the mice to live cause the mice to die when combined? They do this because the DNA from the dead infectious virus has the DNA to be infectious; it's just not alive but can transfer its DNA to a live non-infectious virus. Because it can transfer this DNA, it can make this non-infectious virus become an infectious one, which will kill the mouse. This is an example of the Oswald Avery experiment, which was one of the main experiments that showed that DNA was the source of genetic material and not protein. Let's move on.

Which of the following was not a crucial experiment that led to the knowledge that DNA was the source of genetic information?

• A. Oswald Avery's experiments with bacteria and mice
• B. Watson and Cricks discovery of the DNA double helix
• C. The Hershey/Chase experiments with bacteriophages
• D. Barbara McClintock's experiments with maize (corn)

Which of the following scientists discovered that DNA was the genetic material in viruses?

• A. Oswald Avery
• B. Hershey/Chase
• C. James Watson
• D. Francis Crick

Which molecule did scientists believe was the source of genetic material first (before DNA)?

• A. Protein
• B. RNA
• C. DNA

Here’s what students ask on this topic:

What was the significance of Oswald Avery's experiment in the discovery of DNA?

Oswald Avery's experiment was pivotal in demonstrating that DNA is the genetic material. Avery showed that DNA from dead infectious bacteria could transform non-infectious bacteria into infectious ones. He injected mice with different combinations of live and dead bacteria. When he combined dead infectious bacteria with live non-infectious bacteria, the mice died, indicating that the DNA from the dead bacteria had transformed the live bacteria. This experiment provided strong evidence that DNA, not protein, was responsible for heredity, shifting the scientific consensus towards DNA as the primary carrier of genetic information.

How did the Hershey-Chase experiment confirm that DNA is the genetic material?

The Hershey-Chase experiment confirmed that DNA is the genetic material by using bacteriophages, viruses that infect bacteria. They labeled the DNA of the phages with radioactive phosphorus and the protein coat with radioactive sulfur. After allowing the phages to infect bacteria, they found that only the radioactive DNA entered the bacterial cells, while the protein remained outside. This demonstrated that DNA, not protein, was the material responsible for carrying genetic information in viruses, further solidifying the role of DNA in heredity.

What role did Watson and Crick play in the discovery of DNA?

James Watson and Francis Crick played a crucial role in the discovery of DNA by identifying its double helix structure. Using data from X-ray diffraction images taken by Rosalind Franklin, they proposed a model where two strands of DNA form a helical structure, held together by complementary base pairs (adenine with thymine, and cytosine with guanine). This model explained how genetic information is stored, replicated, and inherited, providing a clear mechanism for DNA's role in heredity. Their discovery was a significant milestone in molecular biology.

Why was protein initially thought to be the genetic material instead of DNA?

Protein was initially thought to be the genetic material because it is composed of 20 different amino acids, offering a vast array of possible combinations and complexity. In contrast, DNA is made up of only four nucleotide bases, which seemed too simple to carry the vast amount of genetic information required for life. This complexity of proteins led scientists to believe that they were the storage molecules for genetic information until key experiments, like those by Avery and Hershey-Chase, demonstrated that DNA was the true genetic material.

What is the importance of the double helix structure of DNA discovered by Watson and Crick?

The double helix structure of DNA, discovered by Watson and Crick, is crucial because it explains how genetic information is stored, replicated, and inherited. The structure consists of two strands that coil around each other, with complementary base pairs (adenine with thymine, and cytosine with guanine) holding them together. This arrangement allows for the precise copying of genetic information during cell division, as each strand serves as a template for the formation of a new complementary strand. The double helix model also provided insights into how mutations occur and how genetic information is passed from one generation to the next.