Transcript Chapter 12 DNA & RNA

DNA: History of
discovery of its
Structure & Function
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DNA Structure & Function
• How do genes work?
• What are they made of?
• How do they determine
characteristics of organisms?
• In the middle of the 1900s, questions
like these were on the minds of
biologists everywhere.
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Griffith and Transformation
• 1928: British scientist Frederick
Griffith was trying to figure out how
bacteria make people sick.
• Griffith wanted to learn how certain
types of bacteria produce a serious
lung infection known as
pneumonia.
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Frederick Griffith
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What did he do?
• isolated two slightly different strains of
pneumonia bacteria from mice
• Both strains grew very well, but only one of
the strains caused pneumonia.
• disease-causing strain of bacteria = smooth
colonies
• harmless strain = colonies with rough edges
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Griffith's Experiments
• injected mice with the S bacteria 
pneumonia and died.
• mice injected with R bacteria  totally
healthy.
• Griffith wondered if the disease-causing
bacteria might produce a poison.
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• Heat killed S bacteria into mice  Healthy
mice!
– The mice survived, suggesting that the cause of
pneumonia was not a chemical poison released
by the disease-causing bacteria.
• Heat killed S bacteria + R bacteria in mice 
DEAD mice!
– He found their lungs filled with the deadly S
bacteria.
– Some factor from the dead bacteria had
“transformed” the harmless bacteria into
disease-causing ones.
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Transformation
• process in which one strain of bacteria is
changed by a gene or genes from another
strain of bacteria
• Griffith hypothesized since the ability to cause
disease was inherited by the transformed
bacteria's offspring, the transforming factor
might be a gene (genetic material)
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• Although Griffith made a great discovery,
he did not follow through and figure out
what actually transformed the bacteria.
• Avery came along and did that.
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Avery and DNA
• 1944, a group of scientists led by Canadian
biologist Oswald Avery at the Rockefeller
Institute in New York decided to repeat
Griffith's work.
• to determine which molecule was the genetic
material responsible for transformation.
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• Oswald Avery
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Avery’s experiment
• Made an extract from the heat killed bacteria
• Used enzymes that destroy the lipids,
carbohydrates, proteins, and RNA.
– Transformation still occurred.
• Used DNA destroying enzyme.
– Transformation did NOT occur.
DNA must be the genetic material.
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.
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• Scientists are skeptical.
• Scientists don’t always believe things
with proof from only one experiment…
• So other scientists set out to prove
what the genetic/hereditary
information of an organism is.
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Hershey & Chase
• Alfred Hershey and Martha
Chase
• 1952, American scientists
• Studied viruses (non living
particles)
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Martha Chase and Alfred Hershey
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The Hershey-Chase Experiment
• Used bacteriophages (virus that infects
bacteria)
• composed of a DNA or RNA core and a
protein coat
• When a bacteriophage attacks a
bacterium, it injects its genetic
information into the bacterium.
• Those genes take over the cell, producing
many new viruses.
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Bacteriophage
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Nucleic Acid or protein??
• They wanted to find out which part of the
phage (DNA or protein), produced new
phages.
• Grew bacteriophages with radioactive
markers
– phosphorus-32 (32P) only in DNA
– sulfur-35 (35S) only in Protein
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• mixed the marked viruses with bacteria
• waited a few minutes for the viruses to
inject their genetic material.
• separated the viruses from the bacteria
and tested the bacteria for radioactivity.
• Nearly all the radioactivity in the bacteria
was from phosphorus (32P), the marker
found in DNA.
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• Hershey and Chase
concluded that the genetic
material of the
bacteriophage was DNA,
not protein.
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The
Components
and Structure
of DNA
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DNA components
• DNA is a long molecule made up of units
called nucleotides.
– Nucleotides are made up of three basic
components:
• 5-carbon sugar (deoxyribose)
• a phosphate group
• a nitrogenous base
• 4 kind of nitrogen bases found in DNA
• adenine (A), guanine (G) = double ringed (purines)
• cytosine (C), thymine (T) = single ringed (pyrimidines)
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DNA structure
• backbone of a DNA chain = sugar and
phosphate groups of each nucleotide.
• The nitrogenous bases stick out sideways
from the chain.
• The nucleotides can be joined together in
any order
– any sequence of bases is possible.
– With 4 bases – millions of different
combinations are possible
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DNA
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Erwin Chargaff
• American biochemist, 1940s
• discovered that the percentages of
guanine [G] and cytosine [C] bases are
almost equal in any sample of DNA
– Later found that [A] = [T]
• Scientists had NO idea what this was…
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Chargaff's Rules
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Franklin & Wilkins
• British scientists, 1952
• used a technique called X-ray diffraction to get
information about the structure of the DNA
molecule
• X-shaped pattern shows that the strands in
DNA are twisted around each other like the coils
of a spring
– a shape known as a helix
– the X suggests that there are two strands in the
structure
– Other clues suggest that the nitrogenous bases are
near the center of the molecule
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Franklin
Wilkins
X-shaped DNA
(from X-ray
Diffraction
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How did they (and others) do it???
X-ray crystallography
• Showed that the DNA molecule is a helix
Watson & Crick
• Francis Crick, a British physicist &
James Watson, an American biologist
• trying to understand the structure of DNA
– by building three-dimensional models of the
molecule
• 1953, they are shown a picture of Franklin’s
x-ray and immediately knew the structure
• Watson and Crick's model of DNA was a
double helix, in which two strands were
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wound around each other.
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Double Helix
• Twisted ladder or spiral staircase
• realized that the double helix accounted for
many of the features in Franklin's X-ray
pattern
• did not explain what forces held the two
strands together.
• discovered that hydrogen bonds could form
between certain nitrogenous bases and
provide just enough force to hold the two
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strands together
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Base Pairing
• hydrogen bonds can form only between
certain base pairs
• adenine (A) with thymine (T)
• guanine (G) with cytosine (C)
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