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12–1 DNA
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Many scientists worked to discover that DNA was the
genetic material and what its structure looked like
1. Griffith. Experimented with bacteria and
mice. Found out bacteria could be
transformed—changed by taking in other
DNA.
2. Avery and friends. Found DNA was the
genetic material and passed on. DNA
was the transforming substance.
3. Hershey & Chase. Confirmed that DNA (not
proteins) was the genetic material.
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EXPERIMENT
Bacteria of the “S” (smooth) strain of Streptococcus pneumoniae are pathogenic because they
have a capsule that protects them from an animal’s defense system. Bacteria of the “R” (rough) strain lack a capsule
and are nonpathogenic. Frederick Griffith injected mice with the two strains as shown below:
Living S
(control) cells
Living R
(control) cells
Heat-killed
(control) S cells
Mixture of heat-killed S cells
and living R cells
RESULTS
Mouse dies
Mouse healthy
Mouse healthy
Mouse dies
Living S cells
are found in
blood sample.
CONCLUSION
Griffith concluded that the living R bacteria had been transformed into pathogenic S bacteria by an
unknown, heritable substance from the dead S cells.
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Hershey and Chase’s Experiment
Phage
Head
(proteins)
Tail
Tail fiber
100 nm
DNA
Bacterial
cell
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12–1 DNA
The Components and Structure of DNA
What is the overall structure of the DNA
molecule?
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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 a five-carbon sugar called deoxyribose, a
phosphate group, and a nitrogenous base.
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12–1 DNA
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
The Components and Structure of DNA
The backbone of a DNA chain is formed by sugar
and phosphate groups of each nucleotide.
The nucleotides can be joined together in any order,
as long as Chargaff’s Rules are followed.
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12–1 DNA
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
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
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
The Components and Structure of
DNA
DNA Double Helix
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12–1 DNA
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 DNA
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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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DNA is a long molecule made of monomers
called
a. nucleotides.
b. purines.
c. pyrimidines.
d. sugars.
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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.
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END OF SECTION