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

Historical timeline of discovering DNA 1875 - 1953 Although Gregor Mendel’s experiments with pea plants in the 1870’s led to the new science of genetics, he was never able to identify the “factors of 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.

1902 Chromosome Theory of Heredity – Walter Sutton After studying cell division, Walter Sutton proposes that chromosomes are the location of Mendel’s “factors” Sutton showed that the behavior of chromosomes in meiosis and fertilization provided physical evidence of Mendel’s Law of Segregation.   Anaphase

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

Focus of Genetic Research Is the Hereditary Material in Chromosomes DNA or PROTEINS???

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.

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

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”?

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”

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.

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 assuming that proteins HAD to be the genetic material.

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

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

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 bacteriphage virus infected the bacteria they would be able to detect which radioactive element was injected

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.

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