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DNA Structure and Function
Chapter 13
Impacts, Issues
Here Kitty, Kitty, Kitty, Kitty, Kitty
 Clones made from adult cells have problems;
the cell’s DNA must be reprogrammed to
function like the DNA of an egg
13.1 The Hunt for DNA
 Investigations that led to our understanding that
DNA is the molecule of inheritance reveal how
science advances
Early and Puzzling Clues
 1800s: Miescher found DNA (deoxyribonucleic
acid) in nuclei
 Early 1900s: Griffith transferred hereditary
material from dead cells to live cells
•
•
•
•
Mice injected with live R cells lived
Mice injected with live S cells died
Mike injected with killed S cells lived
Mice injected with killed S cells and live R cells
died; live S cells were found in their blood
Griffith’s Experiments
R
R
A Mice injected
with live cells of
harmless strain R
do not die. Live R
cells are in their
blood.
S
B Mice injected
with live cells of
killer strain S die.
Live S cells are
in their blood.
C Mice injected
with heat-killed S
cells do not die.
No live S cells are
in their blood.
D Mice injected
with live R cells
plus heat-killed S
cells die. Live S
cells are in their
blood.
Fig. 13-2, p. 204
R
R
A Mice injected
with live cells of
harmless strain R
do not die. Live R
cells are in their
blood.
S
B Mice injected
with live cells of
killer strain S die.
Live S cells are
in their blood.
C Mice injected
with heat-killed S
cells do not die.
No live S cells are
in their blood.
D Mice injected
with live R cells
plus heat-killed S
cells die. Live S
cells are in their
blood.
Stepped Art
Fig. 13-2, p. 204
Animation: Griffith’s experiment
Avery and McCarty Find
the Transforming Principle
 1940: Avery and McCarty separated deadly S
cells (from Griffith’s experiments) into lipid,
protein, and nucleic acid components
 When lipids, proteins, and RNA were destroyed,
the remaining substance, DNA, still transformed
R cells to S cells
 Conclusion: DNA is the “transforming principle”
Confirmation of DNA’s Function
 1950s: Hershey and Chase experimented with
bacteriophages (viruses that infect bacteria)
• Protein parts of viruses, labeled with 35S, stayed
outside the bacteria
• DNA of viruses, labeled with 32P, entered the
bacteria
 Conclusion: DNA, not protein, is the material that
stores hereditary information
The Hershey-Chase Experiments
Fig. 13-3a, p. 205
Fig. 13-3b, p. 205
35S
Virus particle
coat proteins
labeled with 35S
remains
outside cells
DNA being
injected into
bacterium
B In one experiment, bacteria were infected with virus particles labeled with a
radioisotope of sulfur (35S). The sulfur had labeled only viral proteins. The
viruses were dislodged from the bacteria by whirling the mixture in a kitchen
blender. Most of the radioactive sulfur was detected in the viruses, not in the
bacterial cells. The viruses had not injected protein into the bacteria.
Fig. 13-3b, p. 205
Fig. 13-3c, p. 205
Virus DNA
labeled with 32P
32P
remains
inside cells
Labeled
DNA being
injected into
bacterium
C In another experiment, bacteria were infected with virus particles labeled with
a radioisotope of phosphorus (32P). The phosphorus had labeled only viral DNA.
When the viruses were dislodged from the bacteria, the radioactive phosphorus
was detected mainly inside the bacterial cells. The viruses had injected DNA into
the cells—evidence that DNA is the genetic material of this virus.
Fig. 13-3c, p. 205
35S
remains
outside cells
Virus particle
coat proteins
labeled with 35S
DNA being
injected into
bacterium
Virus DNA
labeled with 32P
32P
remains
inside cells
Labeled
DNA being
injected into
bacterium
Stepped Art
Fig. 13-3, p. 205
Animation: Hershey-Chase experiments
13.1 Key Concepts
Discovery of DNA’s Function
 The work of many scientists over more than a
century led to the discovery that DNA is the
molecule that stores hereditary information
about traits
13.2 The Discovery of DNA’s Structure
 Watson and Crick’s discovery of DNA’s structure
was based on almost fifty years of research by
other scientists
DNA’s Building Blocks
 Nucleotide
• A nucleic acid monomer consisting of a fivecarbon sugar (deoxyribose), three phosphate
groups, and one of four nitrogen-containing bases
 DNA consists of four nucleotide building blocks
• Two pyrimidines: thymine and cytosine
• Two purines: adenine and guanine
Four Kinds of Nucleotides in DNA
adenine (A)
deoxyadenosine triphosphate, a purine
Fig. 13-4a, p. 206
guanine (G)
deoxyguanosine triphosphate, a purine
Fig. 13-4b, p. 206
thymine (T)
deoxythymidine triphosphate, a pyrimidine
Fig. 13-4c, p. 206
cytosine (C)
deoxycytidine triphosphate, a pyrimidine
Fig. 13-4d, p. 206
Chargaff’s Rules
 The amounts of thymine and adenine in DNA
are the same, and the amounts of cytosine and
guanine are the same: A = T and G = C
 The proportion of adenine and guanine differs
among species
Franklin, Watson and Crick
 Rosalind Franklin’s research in x-ray
crystallography revealed the dimensions and
shape of the DNA molecule: an alpha helix
 This was the final piece of information Watson
and Crick needed to build their model of DNA
Watson and Crick’s DNA Model
 A DNA molecule consists of two nucleotide
chains (strands), running in opposite directions
and coiled into a double helix
 Base pairs form on the inside of the helix, held
together by hydrogen bonds (A-T and G-C)
Patterns of Base Pairing
 Bases in DNA strands can pair in only one way
• A always pairs with T; G always pairs with C
 The sequence of bases is the genetic code
• Variation in base sequences gives life diversity
Structure of DNA
Fig. 13-5a, p. 207
2-nanometer
diameter
0.34 nanometer
between each base pair
3.4-nanometer length of each
full twist of the double helix
The numbers indicate the carbon of
the ribose sugars (compare Figure
13.4). The 3’ carbon of each sugar
is joined by the phosphate group
to the 5’ carbon of the next sugar.
These links form each strand’s sugar–
phosphate backbone.
The two sugar–phosphate
backbones run in parallel
but opposite directions
(green arrows). Think of
one strand as upside
down compared
with the other.
Fig. 13-5b, p. 207
Animation: DNA close up
13.2 Key Concepts
Discovery of DNA’s Structure
 A DNA molecule consists of two long chains of
nucleotides coiled into a double helix
 Four kinds of nucleotides make up the chains,
which are held together along their length by
hydrogen bonds
13.3 DNA Replication and Repair
 A cell copies its DNA before mitosis or meiosis I
 DNA repair mechanisms and proofreading
correct most replication errors
Semiconservative DNA Replication
 Each strand of a DNA double helix is a template
for synthesis of a complementary strand of DNA
 One template builds DNA continuously; the other
builds DNA discontinuously, in segments
 Each new DNA molecule consist of one old
strand and one new strand
Enzymes of DNA Replication
 DNA helicase
• Breaks hydrogen bonds between DNA strands
 DNA polymerase
• Joins free nucleotides into a new strand of DNA
 DNA ligase
• Joins DNA segments on discontinuous strand
DNA Replication
adenine (A)
deoxyadenosine triphosphate, a purine
Fig. 13-4a, p. 206
guanine (G)
deoxyguanosine triphosphate, a purine
Fig. 13-4b, p. 206
thymine (T)
deoxythymidine triphosphate, a pyrimidine
Fig. 13-4c, p. 206
cytosine (C)
deoxycytidine triphosphate, a pyrimidine
Fig. 13-4d, p. 206
A A DNA molecule is double-stranded.
The two strands of DNA stay zippered up
together because they are complementary:
their nucleotides match up according to
base-pairing rules (G to C, T to A).
B As replication starts, the two
strands of DNA are unwound. In
cells, the unwinding occurs simultaneously at many sites along the
length of each double helix.
C Each of the two parent strands serves
as a template for assembly of a new DNA
strand from free nucleotides, according to
base-pairing rules (G to C, T to A). Thus, the
two new DNA strands are complementary
in sequence to the parental strands.
D DNA ligase seals any gaps that remain
between bases of the ―new‖ DNA, so a
continuous strand forms. The base sequence
of each half-old, half-new DNA molecule is
identical to that of the parent DNA molecule.
Stepped Art
Fig. 13-6, p. 208
Animation: DNA replication details
Semiconservative Replication of DNA
Discontinuous Synthesis of DNA
A Each DNA strand has two
ends: one with a 5’ carbon,
and one with a 3’ carbon.
DNA polymerase can add
nucleotides only at the 3’
carbon. In other words, DNA
synthesis proceeds only in
the 5’ to 3’ direction.
Fig. 13-8a, p. 209
The parent DNA
double helix
unwinds in this
direction.
Only one new
DNA strand
is assembled
continuously.
3’ 5’
3’
5’
3’
The other new
DNA strand is
assembled in
many pieces.
Gaps are
sealed by
DNA ligase.
3’
5’
B Because DNA synthesis proceeds only in the 5’ to 3’ direction, only
one of the two new DNA strands can be assembled in a single piece.
The other new DNA strand forms in short segments, which are called
Okazaki fragments after the two scientists who discovered them. DNA
ligase joins the fragments into a continuous strand of DNA.
Fig. 13-8b, p. 209
Checking for Mistakes
 DNA repair mechanisms
• DNA polymerases proofread DNA sequences
during DNA replication and repair damaged DNA
 When proofreading and repair mechanisms fail,
an error becomes a mutation – a permanent
change in the DNA sequence
13.3 Key Concepts
How Cells Duplicate Their DNA
 Before a cell begins mitosis or meiosis, enzymes
and other proteins replicate its chromosome(s)
 Newly forming DNA strands are monitored for
errors
 Uncorrected errors may become mutations
13.4 Using DNA to
Duplicate Existing Mammals
 Reproductive cloning is a reproductive
intervention that results in an exact genetic copy
of an adult individual
Cloning
 Clones
• Exact copies of a molecule, cell, or individual
• Occur in nature by asexual reproduction or
embryo splitting (identical twins)
 Reproductive cloning technologies produce an
exact copy (clone) of an individual
Reproductive Cloning Technologies
 Somatic cell nuclear transfer (SCNT)
• Nuclear DNA of an adult is transferred to an
enucleated egg
• Egg cytoplasm reprograms differentiated (adult)
DNA to act like undifferentiated (egg) DNA
• The hybrid cell develops into an embryo that is
genetically identical to the donor individual
Somatic Cell Nuclear
Transfer (SCNT)
Fig. 13-9a, p. 210
A A cow egg is held in place by
suction through a hollow glass
tube called a micropipette. The
polar body (Section 10.5) and
chromosomes are identified by
a purple stain.
Fig. 13-9a, p. 210
Fig. 13-9b, p. 210
B A micropipette punctures the
egg and sucks out the polar body
and all of the chromosomes. All
that remains inside the egg’s
plasma membrane is cytoplasm.
Fig. 13-9b, p. 210
Fig. 13-9c, p. 210
C A new micropipette prepares to
enter the egg at the puncture site.
The pipette contains a cell grown
from the skin of a donor animal.
skin cell
Fig. 13-9c, p. 210
Fig. 13-9d, p. 210
D The micropipette enters the
egg and delivers the skin cell to
a region between the cytoplasm
and the plasma membrane.
Fig. 13-9d, p. 210
Fig. 13-9e, p. 210
E After the pipette is withdrawn,
the donor’s skin cell is visible next
to the cytoplasm of the egg. The
transfer is complete.
Fig. 13-9e, p. 210
Fig. 13-9f, p. 210
F The egg is exposed to an electric
current. This treatment causes the
foreign cell to fuse with and empty
its nucleus into the cytoplasm of the
egg. The egg begins to divide, and an
embryo forms. After a few days, the
embryo may be transplanted into a
surrogate mother.
Fig. 13-9f, p. 210
A Clone Produced by SCNT
Fig. 13-10, p. 211
Animation: How Dolly was created
Therapeutic Cloning
 Therapeutic cloning uses SCNT to produce
human embryos for research purposes
 Researchers harvest undifferentiated (stem)
cells from the cloned human embryos
13.4 Key Concepts
Cloning Animals
 Knowledge about the structure and function of
DNA is the basis of several methods of making
clones, which are identical copies of organisms
13.5 Fame and Glory
 In science, as in other professions, public
recognition does not always include everyone
who contributed to a discovery
 Rosalind Franklin was first to discover the
molecular structure of DNA, but did not share in
the Nobel prize which was given to Watson,
Crick, and Wilkins
Rosalind Franklin’s
X-Ray Diffraction Image
 Franklin died of cancer at age 37, possibly
related to extensive exposure to x-rays
13.5 Key Concepts
The Franklin Footnote
 Science proceeds as a joint effort; many
scientists contributed to the discovery of DNA’s
structure
Animation: DNA replication
Animation: Semidiscontinuous DNA
replication
Animation: Structure of DNA
Animation: Subunits of DNA
ABC video: DNA ark promise hope for
the future
Video: Goodbye, dolly