12-1: DNA
12-2: Chromosomes and DNA Replication
12-3: RNA and Protein Synthesis
12-4: Mutations
12-5: Gene Regulation
Objectives:
1) What did scientists discover about the
relationship between genes and DNA?
2) What is the overall structure of the
DNA molecule?
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1928
British scientist
Frederick Griffith
Experiment objective:
How bacteria make
people sick
• Background: Bacteria
produce a serious lung
disease known as pneumonia
• Experiment #1Procedure:
1) Isolate 2 slightly different
stains of pneumonia bacteria
from mice
2) Grow strains in different
culture plates
3) Inject mice with strains of
bacteria from culture plates
4) Observe and record results
• Griffith injected mice with the 2 strains of bacteria
pneumonia
• Results:
1) Mice injected with disease-causing strain of bacteria = mice
developed pneumonia and died
2) Mice injected with harmless strain = didn’t get sick or die
• New hypothesis: Griffith
wondered if the diseasecausing bacteria might make
a poison
• Experiment #2 procedures:
1) Heat disease-causing
bacteria
2) Inject heat-killed bacteria into
mice
3) Observe and record
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Results
•
Mice lived suggesting
pneumonia was not a
chemical poison
• Experiment #3 Procedures:
1) Mix heat-killed, disease
causing bacteria with live,
harmless bacteria
2) Inject mixture into mice
3) Observe and record
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Results
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Mice developed
pneumonia and many died
Griffith found the lungs of
the mice filled with the
disease causing bacteria
• Conclusion
• Somehow the heat-killed bacteria had passed their disease-causing
ability to the harmless strain
• Transformation
• Griffith hypothesized that when the live, harmless bacteria and the
heat-killed bacteria were mixed, some factor was transferred from
the heat-killed cells into the live cells
• That factor must contain information that could change harmless bacteria into
disease-causing ones
• Transforming factor might be a gene
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1944
Canadian biologist
Oswald Avery
He led a group of
scientists to recreate
Griffith’s experiment
• Purpose
• Determine which molecule
in the heat-killed bacteria
was most important for
transformation
• Experiment #1 Procedure
1) Made a juice from the heatkilled bacteria
2) Carefully treat the juice with
enzymes to destroy proteins,
lipids, carbohydrates, and
other molecules (RNA)
3) Observe and record
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Results
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Transformation still occurred
Conclusion
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These molecules were not
responsible for transformation
• Experiment #2 Procedure:
1) Repeat the experiment
2) Use enzymes to
breakdown DNA
3) Observe and record
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Results:
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Avery and other scientists discovered that the nucleic
acid DNA stores and transmits the genetic information
from one generation of an organism to the next.
Transformation did not
occur
Conclusion:
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DNA was the transforming
factor
• 1952
• 2 American Scientists
• Alfred Hershey and
Martha Chase
• Studied viruses
• Viruses: nonliving particles
smaller that a cell that can
infect living organisms
• Bacteriophage: one kind of
virus that infects bacteria
(a.k.a. “bacteria eater”)
• Made of a DNA or RNA core
and a protein coat
• Virus attaches to the surface of
the bacterial cell and injects its
genetic information into it
• Viral genes act to make many
new bacteriophages
• Destroys bacterium when the
bacterial cell splits open and
hundreds of new viruses burst
out
https://www.youtube.com/watc
h?v=-w4C74cu6dk
• What is a
bacteriophage?
• One kind of virus that infects bacteria
• Hypothesis: If they could determine which part of the virus entered the
infected cell, they would learn whether genes were made of protein or DNA
• Procedure:
1)
2)
3)
4)
5)
Grow viruses in cultures containing radioactive isotopes (phosphorus -32 {DNA} and
sulfur-35 {protein})
Mix marked viruses with bacteria
Wait a few minutes for the viruses to inject their genetic material
Separate the viruses from the bacteria
Test the bacteria for radioactivity
Results:
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Nearly all the radioactivity in the bacteria was from phosphorus-32
Hershey and Chase concluded that
the genetic material of the
bacteriophage was DNA, not
protein
• What part of the virus
did the Hershey-Chase
experiment show had
entered the bacteria?
• DNA
• 3 critical things genes do
1) Genes carry information
from 1 generation to the
next
2) Put information to work
by determining the
heritable characteristics
of organisms
3) Genes are easily copied
• DNA is a long molecule
made up of nucleotides
• Monomer of nucleic acids
made up of a 5-carbon
sugar, a phosphate group,
and a nitrogenous base
• 4 kinds of nitrogenous
bases
• Adenine and Guanine
• Purines
• 2 rings
• Cytosine and Thymine
• Pyrimidines
• 1 ring
• The “backbone” of a
DNA chain is formed
by sugar and
phosphate grouped
of each nucleotide
• Nitrogenous bases
stick out sideways
from the chain
• Nucleotides can be
joined together in any
order
• Erwin Chargaff
• American biochemist
• Discovered that the percentages of bases are almost equal
in almost any sample of DNA (“Chargaff’s Rules”)
• Guanine (G) and cytosine (C)
• Adenine (A) and thymine (T)
• British physicist, Francis
Crick
• American biologist, James
Watson
• Tried to understand the
structure of DNA by
building 3-dimonsional
models of the molecule
• Watson and Crick’s model
of DNA was a double
helix, in which 2 strands
were wound around each
other.
• Hydrogen bonds form
between certain nitrogenous
bases and provide just
enough force to hold the 2
strands together
• Only between certain base
pairs
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Adenine
Thymine
Guanine
Cytosine
• Base-pairing: principle that
bonds in DNA can form only
between adenine and thymine
and between guanine and
cytosine
• Answer the following
questions.
• Use complete sentences.
• You may NOT use your
book.
• You MAY use your notes.
• This is a quiz grade.
• 1) What did scientists
discover about the
relationship between
genes and DNA?
• 2) What is the overall
structure of the DNA
molecule?