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Biology
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12–1, 12.2 DNA
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12–1 DNA
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Griffith and Transformation
Griffith and Transformation
In 1928, British scientist Fredrick Griffith was trying to learn how certain types of bacteria caused pneumonia.
He isolated two different strains of pneumonia bacteria from mice and grew them in his lab.
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12–1 DNA
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Griffith and Transformation
Griffith made two observations:
(1) The disease-causing strain of bacteria grew into smooth colonies on culture plates.
(2) The harmless strain grew into colonies with rough edges.
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12–1 DNA
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Griffith and Transformation
Griffith's Experiments
Griffith set up four individual experiments.
Experiment 1: Mice were injected with the disease-causing strain of bacteria. The mice developed pneumonia and died.
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12–1 DNA
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Griffith and Transformation
Experiment 2: Mice were injected with the harmless strain of bacteria. These mice didn’t get sick.
Harmless bacteria (rough colonies)
Lives
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12–1 DNA
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Griffith and Transformation
Experiment 3: Griffith heated the disease-causing bacteria. He then injected the heat-killed bacteria into the mice. The mice survived.
Heat-killed disease-causing bacteria (smooth colonies)
Lives
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12–1 DNA
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Griffith and Transformation
Experiment 4: Griffith mixed his heat-killed, disease-causing bacteria with live, harmless bacteria and injected the mixture into the mice. The mice developed pneumonia and died.
Live disease-causing bacteria (smooth colonies)
Dies of pneumonia
Heat-killed disease-causing bacteria (smooth colonies)
Harmless bacteria (rough colonies)
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12–1 DNA
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Griffith and Transformation
Griffith concluded that the heat-killed bacteria passed their disease-causing ability to the harmless strain.
Live disease-causing bacteria (smooth colonies)
Heat-killed disease-causing bacteria (smooth colonies)
Harmless bacteria (rough colonies)
Dies of pneumonia
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12–1 DNA
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Griffith and Transformation
Transformation
Griffith called this process transformation because one strain of bacteria (the harmless strain) had changed permanently into another (the disease-causing strain).
Griffith hypothesized that a factor must contain information that could change harmless bacteria into disease-causing ones.
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12–1 DNA
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Avery and DNA
Avery and DNA
Oswald Avery repeated Griffith’s work to determine which molecule was most important for transformation.
Avery and his colleagues made an extract from the heat-killed bacteria that they treated with enzymes.
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12–1 DNA
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Avery and DNA
The enzymes destroyed proteins, lipids, carbohydrates, and other molecules, including the nucleic acid RNA.
Transformation still occurred.
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12–1 DNA
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Avery and DNA
Avery and other scientists repeated the experiment using enzymes that would break down DNA.
When DNA was destroyed, transformation did not occur. Therefore, they concluded that DNA was the transforming factor.
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12–1 DNA
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Avery and DNA
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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12–1 DNA
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The Hershey-Chase Experiment
The Hershey-Chase Experiment
Alfred Hershey and Martha Chase studied viruses—nonliving particles smaller than a cell that can infect living organisms.
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12–1 DNA
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The Hershey-Chase Experiment
Bacteriophages
A virus that infects bacteria is known as a bacteriophage.
Bacteriophages are composed of a DNA or RNA core and a protein coat.
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12–1 DNA
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The Hershey-Chase Experiment
They grew viruses in cultures containing radioactive isotopes of phosphorus-32 (32P) and sulfur-35 (35S).
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12–1 DNA
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The Hershey-Chase Experiment
If 35S was found in the bacteria, it would mean that the viruses’ protein had been injected into the bacteria.
Bacteriophage with suffur-35 in protein coat
Phage infects bacterium
No radioactivity inside bacterium
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12–1 DNA
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The Hershey-Chase Experiment
If 32P was found in the bacteria, then it was the DNA that had been injected.
Bacteriophage with phosphorus-32 in DNA
Phage infects bacterium
Radioactivity inside bacterium
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12–1 DNA
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The Hershey-Chase Experiment
Nearly all the radioactivity in the bacteria was from phosphorus (32P).
Hershey and Chase concluded that the genetic material of the bacteriophage was DNA, not protein.
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12–1 DNA
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The Components and Structure of DNA
The Components and Structure of DNA
DNA is made up of nucleotides.
A nucleotide is a monomer of nucleic acids made up of:
• Deoxyribose – 5-carbon Sugar
• Phosphate Group
• Nitrogenous Base
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12–1 DNA
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The Components and Structure of DNA
There are four kinds of bases in in DNA:
• adenine
• guanine
• cytosine
• thymine
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12–1 DNA
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The Components and Structure of DNA
Chargaff's Rules
Erwin Chargaff discovered that:
• The percentages of guanine [G] and cytosine [C] bases are almost equal in any sample of DNA.
• The percentages of adenine [A] and thymine [T] bases are almost equal in any sample of DNA.
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12–1 DNA
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The Components and Structure of DNA
X-Ray Evidence
Rosalind Franklin used X-ray diffraction to get information about the structure of DNA.
She aimed an X-ray beam at concentrated DNA samples and recorded the scattering pattern of the X-rays on film.
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12–1 DNA
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The Components and Structure of DNA
The Double Helix
Using clues from Franklin’s pattern, James Watson and Francis Crick built a model that explained how DNA carried information and could be copied.
Watson and Crick's model of DNA was a double helix, in which two strands were wound around each other.
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12–1 DNA
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The Components and Structure of DNA
DNA Double Helix
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12–1 DNA
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The Components and Structure of DNA
Watson and Crick discovered that hydrogen bonds can form only between certain base pairs—adenine and thymine, and guanine and cytosine.
This principle is called base pairing.
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12–1
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12–1
Avery and other scientists discovered that
a. DNA is found in a protein coat.
b. DNA stores and transmits genetic information from one generation to the next.
c. transformation does not affect bacteria.
d. proteins transmit genetic information from one generation to the next.
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12–1
The Hershey-Chase experiment was based on the fact that
a. DNA has both sulfur and phosphorus in its structure.
b. protein has both sulfur and phosphorus in its structure.
c. both DNA and protein have no phosphorus or sulfur in their structure.
d. DNA has only phosphorus, while protein has only sulfur in its structure.
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12–1
DNA is a long molecule made of monomers called
a. nucleotides.
b. purines.
c. pyrimidines.
d. sugars.
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12–1
Chargaff's rules state that the number of guanine nucleotides must equal the number of
a. cytosine nucleotides.
b. adenine nucleotides.
c. thymine nucleotides.
d. thymine plus adenine nucleotides.
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12–1
In DNA, the following base pairs occur:
a. A with C, and G with T.
b. A with T, and C with G.
c. A with G, and C with T.
d. A with T, and C with T.
END OF SECTION
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