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Historical timeline of discovering DNA

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1 KEY CONCEPT DNA was identified as the genetic material through a series of experiments.

2 Historical timeline of discovering DNA 1875 - 1953
Although Gregor Mendel’s experiments with pea plants in the 1870’s led to the the new science of genetics, he was never able to answer an important question – “What are the “factors” that control heredity?” It would be almost 100 years until the findings of different researchers proved DNA is the molecule responsible for genetic inheritance and discovered its structure.

3 1902 Chromosome Theory of Heredity – Walter Sutton
After studying cell division, Walter Sutton proposes that chromosomes are the location of Mendel’s “factors” Anaphase

4 Chromosomes Structure
Chromosomes are composed of: One DNA molecule Thousands of protein molecules, called histones

5 Frederick Griffith Epidemiologist and bacteriologist who did research to develop vaccines against bacteria that caused a global epidemic of the deadly influenza or pneumonia flu.

6 1928 Frederick Griffith’s Experiment
Griffith experimented with bacteria that cause pneumonia. Used two types of bacteria: Smooth shape (Deadly) and Rough shape (not deadly). He found that some substance in the dead S bacteria was taken up by the living R bacteria that made them deadly too

7 1928 Frederick Griffith’s Experiment
Griffith named the substance that changed the harmless bacteria into deadly bacteria the “transforming factor” “Transformation” means to change from one form to a different form What was this “Transforming Factor”?

8 Oswald Avery Bacteriologist who also did research to find cures and vaccines diseases caused by pathogenic bacteria. He and his team of fellow researchers spent a decade working to identify Griffith’s “transforming factor”

9 Avery’s Team’s Experiements
Isolated and purified Griffith’s transforming principle. Performed three quantitative chemical analyses on the transforming principle to determine what was in it. RESULTS: Qualitative tests showed DNA was present. Chemical tests showed the chemical makeup matched that of DNA. Enzyme tests showed only DNA-destroying enzymes stopped transformation.

10 Avery’s Team’s Experiments
Results identified DNA as the transforming principle Still these conclusions were questioned – “Maybe there was some protein in sample” “Maybe DNA is the genetic matter only in bacteria” Much skepticism was due to many believing the the all important proteins had to be the genetic material.

11 Alfred Hershey and Martha Chase
American biologists who studied viruses that infect bacteria – bacteriophage.

12 Alfred Hershey and Martha Chase
Viruses have genetic material but cannot reproduce on their own. They must inject their genetic matter into a host which will use the information to make more viruses

13 1952 Hershey and Chase’s Experiment
Wanted to determine whether the virus’ protein or DNA was injected into the bacteria. *DNA tagged with radioactive phosphorus *Proteins tagged with radioactive sulfur Once the bacteria was infected they checked to see if the radioactivity could be detected

14 1952 Hershey and Chase’s Experiment
Only the radioactive phosphorus was found inside the bacteria – concluded DNA was the hereditary information!! Hershey and Chase’s experiment provided conclusive evidence that confirm that DNA is THE genetic material.

15 Summary of Experiments
1928 Griffith – Discovers a “transforming factor” 1944 Avery and Team – DNA is the “transforming factor” but much skepticism from the scientific community 1952 Hershey and Chase – Finally convinced scientists that DNA, not protein, is the genetic material


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