Module 7 Lesson 1 - 2

Discovery of Deoxyribonucleic Acid (DNA)

Read pages 624 - 627

The discovery of the molecule responsible for heredity took many years and great effort by many scientists. During the same time, other scientists were trying to determine the structure of DNA.

Deoxyribonucleic Acid (DNA), then known as "nucleic acid", was first isolated from the nuclei of white blood cells by Friedrich Miescher in 1869. At that time, no connection was made between this molecule and heredity. Experiments by Phoebus Levene, Frederick Griffith, Alfred Hershey, and Martha Chase finally proved that DNA and not protein was the molecule responsible for heredity. While these scientists were looking for the agent of heredity, other scientists were studying the DNA molecule more closely.

• Phoebus Levene first discovered that DNA was made of chains of four different nucleotides: adenine (A), guanine (G), cytosine (C), and thymine (T). He noted that RNA was made of four nucleotides but had the base uracil (U) instead of thymine (T).
  

• Erwin Chargaff found that adenine (A) and thymine (T) are in equal amounts in any sample of DNA, and cytosine (C) and guanine(G) are in equal amounts (but different to A and T). Chargaff's Rule of constant relationships between A and T and between C and G was accepted.
  

• The combination of Rosalind Franklin's use of x-ray photography and the work of James Watson and Francis Crick eventually produced the double-helix structure model of the DNA molecule.

Learn more about the contributions of the above-mentioned scientists by reading pages 624 to 627 in your textbook. Summarize your readings by constructing a timeline that includes scientists, experiments, and major discoveries that led to the identification of the hereditary agent and the structure of the DNA molecule.

Try This

Click on Scientists and Their Discoveries to open up this interactive activity in a new window.  Be sure to complete all four activities.  For the drag and drop activities, make sure the answer box you are dropping a term into is outlined before dropping it. 

Watch and Listen

1. Watch the following animation of the Hershey and Chase experiment provided by the publisher McGraw-Hill. After watching this animation, you should be able to explain how this experiment proves that DNA, and not protein, is the hereditary molecule.
   
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3. The following video, Introduction to Molecular Genetics:  The Search for the Genetic Code (28 minutes) reviews the same scientists and experiments that led to the discovery of DNA purpose and structure as those in your textbook readings. You may choose to watch this video in place of doing the textbook readings above, or you may use this video to review areas that are confusing for you.