DNA Discovery: Videos & Practice Problems

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

The discovery of DNA as the source of genetic material was a significant milestone in molecular biology, evolving through a series of pivotal experiments and insights. Initially, in the early 1900s, proteins were believed to be the carriers of genetic information due to their complexity, stemming from the existence of 20 different amino acids compared to only four DNA bases. This misconception persisted until groundbreaking experiments began to reveal the true nature of genetic material.

One of the key figures in this discovery was Oswald Avery, who, in the 1940s, conducted experiments demonstrating that DNA was the genetic material in bacteria. Avery's experiment involved injecting mice with two types of bacteria: one non-infectious and one dead but infectious. Surprisingly, the mice became infected, indicating that the nucleic acids were responsible for the infectivity. This experiment was crucial in shifting the scientific consensus towards recognizing DNA as the genetic material.

Further supporting this conclusion were the Hershey-Chase experiments conducted in the 1950s. In these experiments, bacteriophages (viruses that infect bacteria) were labeled with different radioactive isotopes—one for protein and another for DNA. Upon infecting bacteria, it was observed that only the DNA entered the bacterial cells, confirming that DNA, not protein, was the genetic material responsible for infection.

The final piece of the puzzle came from the work of James Watson and Francis Crick, who elucidated the double helix structure of DNA. This discovery was pivotal as it provided a clear mechanism for how genetic information is stored, replicated, and inherited. The double helix model explained the complementary base pairing and the stability of the DNA molecule, which was essential for its role in heredity.

In summary, the journey to understanding DNA as the genetic material involved critical experiments that debunked the protein-centric view of genetics. Avery's work with bacterial transformation, the Hershey-Chase experiments with bacteriophages, and Watson and Crick's structural insights collectively established DNA as the fundamental molecule of heredity, reshaping our understanding of biology.

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)

Answer: (D)

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

Answer: (B)

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

• A. Protein
• B. RNA
• C. DNA

Answer: (A)

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.