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8.1 Identifying DNA as the Genetic Material
KEY CONCEPT
DNA was identified as the genetic material through a
series of experiments.
8.1 Identifying DNA as the Genetic Material
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.
8.1 Identifying DNA as the Genetic Material
1902 Chromosome Theory of Heredity – Walter Sutton
• After studying cell division, Walter Sutton proposes that
chromosomes are the location of Mendel’s “factors”
Anaphase
8.1 Identifying DNA as the Genetic Material
Chromosomes Structure
Chromosomes are composed of:
• One DNA molecule
• Thousands of protein molecules, called histones
8.1 Identifying DNA as the Genetic Material
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.
8.1 Identifying DNA as the Genetic Material
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
8.1 Identifying DNA as the Genetic Material
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”?
8.1 Identifying DNA as the Genetic Material
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”
8.1 Identifying DNA as the Genetic Material
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.
8.1 Identifying DNA as the Genetic Material
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.
8.1 Identifying DNA as the Genetic Material
Alfred Hershey and Martha Chase
American biologists
who studied viruses
that infect bacteria –
bacteriophage.
8.1 Identifying DNA as the Genetic Material
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
8.1 Identifying DNA as the Genetic Material
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
8.1 Identifying DNA as the Genetic Material
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.
8.1 Identifying DNA as the Genetic Material
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
8.1 Identifying DNA as the Genetic Material