DNA and RNA
Biology
Ch. 12
• 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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http://upload.wikimedia.org/wikipedia/commons/b/b8/Fred_Griffith_and_%22Bobby%2
2_1936.jpg
Griffith and Transformation
Griffith and Transformation
• (1) The diseasecausing strain of
bacteria grew into
smooth colonies on
culture plates.
• (2) The harmless
strain grew into
colonies with rough
edges.
http://www.bio.miami.edu/~cmallery/150/gene/sf11x1a.jpg
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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Griffith and Transformation
• Experiment 2: Mice
were injected with the
harmless strain of
bacteria. These mice
didn’t get sick.
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Griffith and Transformation
• Experiment 3: Griffith
heated the diseasecausing bacteria. He
then injected the
heat-killed bacteria
into the mice. The
mice survived.
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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.
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Griffith and Transformation
• Griffith concluded that
the heat-killed
bacteria passed their
disease-causing
ability to the harmless
strain.
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Griffith and Transformation
• Transformation
– 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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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.
http://www.genoscope.cns.fr/externe/HistoireBM/avery.jpg
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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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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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Avery and DNA
• What did scientists discover about the
relationship between genes and DNA?
• The nucleic acid DNA stores
and transmits the genetic
information from one generation
of an organism to the next.
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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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http://www.ndpteachers.org/perit/Chase%20&%20Hershey%201953.JPG
The Hershey-Chase Experiment
• Bacteriophages
Head
DNA
Tail
Tail fiber
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300,000
– A virus that infects
bacteria
– Composed of a DNA
or RNA core and a
protein coat.
The Hershey-Chase Experiment
• The virus attaches to
the surface of the cell
and injects its genetic
information into it.
• Takes control of cell
to make more viruses
• When the cell splits
open, hundreds of
new viruses burst out.
http://www.canlilarbilimi.net/k/baktrfjm/images/bacteriophage_jpg.jpg
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The Hershey-Chase Experiment
• If Hershey and Chase could determine
which part of the virus entered an infected
cell, they would learn whether genes were
made of protein or DNA.
• They grew viruses in cultures containing
radioactive isotopes of phosphorus-32
(32P) and sulfur-35 (35S).
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The Hershey-Chase Experiment
Phage
Radioactive
protein
Bacterium
Empty
protein shell
Radioactivity
in liquid
Phage
DNA
DNA
Batch 1
Radioactive
protein
Centrifuge
Pellet
1 Mix radioactively
labeled phages with
bacteria. The phages
infect the bacterial
cells.
2 Agitate in a blender to
separate phages outside
the bacteria from the cells
and their contents.
3 Centrifuge the mixture
so bacteria form a pellet
at the bottom of the test
tube.
4 Measure the
radioactivity in
the pellet and
the liquid.
Radioactive
DNA
Batch 2
Radioactive
DNA
Centrifuge
Pellet
Figure 10.1B
Radioactivity
in pellet
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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The Structure of DNA
• Nucleotides are the
monomeric units that
make up DNA
 3 main parts:
 5 carbon sugar—
deoxyribose
 Phosphate group
 Nitrogenous base
The Structure of DNA
• There are four kinds
of bases in in DNA:
•
•
•
•
adenine
guanine
cytosine
thymine
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The 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.
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The 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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• X-Ray Evidence
– Rosalind Franklin
used X-ray
diffraction to get
information about
the structure of
DNA.
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http://www.chem-is-try.org/wpcontent/uploads/2009/03/rosalind-franklin.jpg
The Structure of DNA
The Structure of DNA
• Using clues from
Franklin’s pattern,
James Watson and
Francis Crick built a
model that explained
how DNA carried
information and could
be copied.
http://history.nih.gov/exhibits/nirenberg/images/photos/03_watCrk_pu.jpg
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The Structure of DNA
• Watson and Crick's
model of DNA was a
double helix, in which
two strands were
wound around each
other.
http://gregortimlin.files.wordpress.com/2009/05/watsonjames-crickfrancis.jpg
The Structure of DNA
Double Helix
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The 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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DNA Structure
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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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 phosphorus, while protein has sulfur in
its structure.
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DNA is a long molecule made of monomers
called
a)
b)
c)
d)
nucleotides.
purines.
pyrimidines.
sugars.
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Chargaff's rules state that the number of
guanine nucleotides must equal the number of
•
•
•
•
cytosine nucleotides.
adenine nucleotides.
thymine nucleotides.
thymine plus adenine nucleotides.
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In DNA, the following base pairs occur:
a)
b)
c)
d)
A with C, and G with T.
A with T, and C with G.
A with G, and C with T.
A with T, and C with T.
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