DNA: The Molecule of
Heredity
Chapter 11
Rosalind Franklin:
A Female Scientist Ahead of her
Time
• When we talk about Watson and Crick,
Rosalind Franklin provided crucial X-ray
diffraction photos that allowed Watson
and Crick to develop the DNA model.
• Why did Rosalind Franklin not get a
Nobel Prize?
X-ray Diffraction Studies of
DNA
Fig. 11-4
DNA X-Ray Diffraction Photo by
Rosalind Franklin
DNA Structure
Miescher Discovered DNA
• 1868
• Johann Miescher investigated the chemical
composition of the nucleus
• Isolated an organic acid that was high in
phosphorus
• He called it nuclein
• We call it DNA (deoxyribonucleic acid)
The Discovery of DNA
Fig. E11-3
Griffith Discovers
Transformation
• 1928
• Attempting to develop a vaccine
• Isolated two strains of Streptococcus
pneumoniae
– Rough strain was harmless
– Smooth strain was pathogenic
Griffith Discovers
Transformation
1 Mice injected with
live cells of harmless
strain R.
2 Mice injected with live
cells of killer strain S.
3 Mice injected with
heat-killed S cells.
4 Mice injected with
live R cells plus heatkilled S cells.
Mice live. No live R
cells in their blood.
Mice die. Live S cells in
their blood.
Mice live. No live S cells
in their blood.
Mice die. Live S cells in
their blood.
Bacterial strain(s) injected into mouse
(a)
Results
Conclusions
Transformation in Bacteria
Mouse remains
healthy
Living
R-strain
R-strain does
not cause
pneumonia.
(b)
Mouse contracts
pneumonia and
dies
S-strain causes
pneumonia.
Living
S-strain
(c)
Mouse remains
healthy
Heat-killed
S-strain
Heat-killed Sstrain does not
cause pneumonia.
(d)
Mixture of living
R-strain and
heat-killed
S-strain
Mouse contracts
pneumonia and
dies
A substance from
heat-killed S-strain
can transform the
harmless R-strain
into a deadly
S-strain.
Fig. 11-1
Transformation
• What happened in the fourth
experiment?
• The harmless R cells had been
transformed by material from the dead
S cells
• Descendents of the transformed cells
were also pathogenic
Mystery of the
Hereditary Material
• Originally believed to be an unknown
class of proteins
• Thinking was
– Heritable traits are diverse
– Molecules encoding traits must be diverse
– Proteins are made of 20 amino acids and
are structurally diverse
Oswald & Avery
• What is the transforming material?
• Cell extracts treated with proteindigesting enzymes could still transform
bacteria
• Cell extracts treated with DNA-digesting
enzymes lost their transforming ability
• Concluded that DNA, not protein,
transforms bacteria
Bacteriophages
• Viruses that infect
bacteria
• Consist of protein
and DNA
• Inject their
hereditary material
into bacteria
Hershey & Chase’s
Experiments
• Created labeled bacteriophages
– Radioactive sulfur
– Radioactive phosphorus
• Allowed labeled viruses to infect
bacteria
• Asked: Where are the radioactive labels
after infection?
Hershey and Chase Results
35S
remains
outside cells
virus particle
labeled with 35S
DNA (blue)
being injected
into bacterium
virus particle
labeled with 32P
DNA (blue)
being injected
into bacterium
35P
remains
inside cells
Structure of the
Hereditary Material
• Experiments in the 1950s showed that DNA is the
hereditary material
• Scientists raced to determine the structure of DNA
• 1953 - Watson and Crick proposed that DNA is a
double helix
Structure of Nucleotides
in DNA
• Each nucleotide consists of
– Deoxyribose (5-carbon sugar)
– Phosphate group
– A nitrogen-containing base
• Four bases
– Adenine, Guanine, Thymine, Cytosine
Nucleotide Bases
sugar
(deoxyribose)
adenine
A
base with a
double-ring
structure
guanine
(G)
base with a
double-ring
structure
thymine
(T)
base with a
single-ring
structure
cytosine
(C)
base with a
single-ring
structure
Composition of DNA
• Chargaff showed:
– Amount of adenine relative to guanine
differs among species
– Amount of adenine always equals amount
of thymine and amount of guanine always
equals amount of cytosine
A=T and G=C
Watson-Crick Model
• DNA consists of two nucleotide strands
• Strands run in opposite directions
• Strands are held together by hydrogen bonds
between bases
• A binds with T and C with G
• Molecule is a double helix
Watson-Crick
Model
Patterns of Base Pairing
Rosalind Franklin’s Work
• Was an expert in x-ray crystallography
• Used this technique to examine DNA
fibers
• Concluded that DNA was some sort of
helix
DNA Structure Helps
Explain How It Duplicates
• DNA is two nucleotide strands held
together by hydrogen bonds
• Hydrogen bonds between two strands
are easily broken
• Each single strand then serves as
template for new strand
DNA
Replication
• Each parent
strand remains
intact
• Every DNA
molecule is half
“old” and half
“new”
new
old
old
new
Base Pairing
during
Replication
Each old strand
serves as the template
for complementary
new strand
Semiconservative Replication
of DNA
One DNA
double helix
DNA replication
Two identical DNA
double helices, each
with one parental
strand (blue) and
one new strand (red)
Fig. 11-7
A Closer Look at
Strand Assembly
Energy for strand
assembly is
provided by
removal of two
phosphate groups
from free
nucleotides
newly
forming
DNA
strand
one parent
DNA strand
Continuous and Discontinuous
Assembly
Strands can
only be
assembled in
the 5’ to 3’
direction
Enzymes in Replication
• Enzymes unwind the two strands
• DNA polymerase attaches
complementary nucleotides
• DNA ligase fills in gaps
• Enzymes wind two strands together
DNA Repair
• Mistakes can occur during replication
• DNA polymerase can read correct
sequence from complementary strand
and, together with DNA ligase, can
repair mistakes in incorrect strand
What about Cloning?
Moral Dilemma or Technological
Revolution
• Making a genetically identical copy of
an individual
• Researchers have been creating clones
for decades
• These clones were created by embryo
splitting
Dolly:
Cloned from an Adult Cell
• Showed that differentiated cells could
be used to create clones
• Sheep udder cell was combined with
enucleated egg cell
• Dolly is genetically identical to the
sheep that donated the udder cell
More Clones
•
•
•
•
•
Mice
Cows
Pigs
Goats
Guar (endangered species)
How Do Mutations Occur?
• Mistakes do happen
– DNA is altered or damaged in a number of
ways
• Mistakes are made during normal DNA
replication
• Certain chemicals (some components of
cigarette smoke, for example) increase DNA
errors during and after replication
• Ultraviolet radiation or X-rays also contribute to
incorrect base pairing
Mutations
Fig. 11-8a
Mutations
Fig. 11-8b
Mutations
Fig. 11-8c
Mutations
Fig. 11-8d
11.5 How Do Mutations
Occur?
• Mutations may have varying effects on
function
– Mutations are often harmful, and an
organism inheriting them may quickly die
– Some mutations may have no functional
effect
– Some mutations may be beneficial and
provide an advantage to the organism in
certain environments