Download PowerLecture: Chapter 13

PowerLecture:
Chapter 13
DNA Structure and Function
13.1 The Hunt
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Originally believed to be an unknown class
of proteins
Thinking was
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Heritable traits are diverse
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Molecules encoding traits must be diverse
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Proteins are made of 20 amino acids and are
structurally diverse
Miescher Discovered DNA
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1868
Johann Miescher investigated the chemical
composition of the nucleus
Isolated an organic acid that was high in
phosphorus
He called it nuclein
Griffith Discovers
Transformation
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1928
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Attempting to develop a vaccine
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Isolated two strains of Streptococcus
pneumoniae
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Rough strain was harmless
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Smooth strain was pathogenic
Transformation
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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
Avery, McCarty, and MacLeod
Repeated Griffith’s Experiment
Oswald Avery
Maclyn McCarty
Colin MacLeod
Avery, McCarty, and MacLeod
Added the non-deadly Rough Type of
Bacteria to the Heat-Killed Smooth Type
To the Heat-Killed Smooth Type,
added enzymes that destroyed…
Carbohydrates
Lipids
Proteins
RNA
DNA
S-Type Carbohydrates S-Type Lipids
Destroyed
Destroyed
S-Type
Proteins
Destroyed
S-Type RNA
Destroyed
S-Type DNA
Destroyed
Conclusion:
DNA was the
transforming factor!
Oswald & Avery
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What is the transforming material?
Cell extracts treated with protein-digesting
enzymes could still transform bacteria
Cell extracts treated with DNA-digesting
enzymes lost their transforming ability
Concluded that DNA, not protein,
transforms bacteria
The Hershey-Chase Experiment
Alfred Hershey &
Martha Chase
worked with a
bacteriophage:
A virus that
invades bacteria.
It consists of a
DNA core and a
protein coat
Protein coat
DNA
Protein coats of bacteriophages labeled with Sulfur-35
Phage
Bacterium
1.
Phage
Bacterium
DNA of bacteriophages labeled with Phosphorus-32
Hershey and Chase mixed the
radioactively-labeled viruses
with the bacteria
The viruses infect the
bacterial cells.
Protein coats of bacteriophages labeled with Sulfur-35
1.
DNA of bacteriophages labeled with Phosphorus-32
Separated the viruses from
the bacteria by agitating the
virus-bacteria mixture in a
blender
Protein coats of bacteriophages labeled with Sulfur-35
1.
Centrifuged the mixture so that the bacteria
would form a pellet at the bottom of the test
tube
1.
Measured the radioactivity in the pellet and in
the liquid
DNA of bacteriophages labeled with Phosphorus-32
Hershey & Chase’s Experiments
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Created labeled bacteriophages
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Radioactive sulfur
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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
35P
remains
inside cells
DNA (blue)
being injected
into bacterium
Fig. 13-4ab, p.209
Structure of the
Hereditary Material
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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
Figure 13.6
Page 211
13.2 Structure of Nucleotides
in DNA
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Each nucleotide consists of
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Deoxyribose (5-carbon sugar)
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Phosphate group
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A nitrogen-containing base
Four bases
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Adenine, Guanine, Thymine, Cytosine
Nucleotide Bases
ADENINE
(A)
phosphate
group
GUANINE
(G)
deoxyribose
THYMINE
(T)
CYTOSINE
(C)
Composition of DNA
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Chargaff showed:
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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
Rosalind Franklin’s Work
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Was an expert in X-ray
crystallography
Used this technique to examine
DNA fibers
Concluded that DNA was some sort
of helix
Watson-Crick
Model
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DNA consists of two
nucleotide strands
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Strands run in opposite
directions
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Strands are held together by
hydrogen bonds between
bases
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A binds with T and C with G
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Molecule is a double helix
13.3 DNA Structure Helps
Explain How It Duplicates
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DNA is two nucleotide strands held
together by hydrogen bonds
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Hydrogen bonds between two strands
are easily broken
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Each single strand then serves as
template for new strand
How does DNA replicate?
Hypotheses:
Conservative
Semi-Conservative
Dispersive
Meselson-Stahl Experiment
• Bacteria cultured in medium containing
a heavy isotope of Nitrogen (15N)
Meselson-Stahl Experiment
• Bacteria transferred to a medium
containing elemental Nitrogen (14N)
Meselson-Stahl Experiment
1. DNA sample centrifuged after First
replication
Meselson-Stahl Experiment
1. DNA sample centrifuged after Second
replication
DNA
Replication
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Each parent
strand remains
intact
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Every DNA
molecule is half
“old” and half
“new”
Fig. 13-7, p.212
Strand Assembly
Why the discontinuous
additions? Nucleotides can
only be joined to an exposed —
OH group that is attached to
the 3’ carbon of a growing
strand.
Energy for strand assembly is
provided by removal of two
phosphate groups from free
nucleotides
Fig. 13-8c, p.213
Continuous and Discontinuous
Assembly
• Strands can only be assembled in
the 5’ to 3’ direction
•continuous on just one parent
strand. This is because DNA
synthesis occurs only in the 5´ to 3´
direction.
• discontinuous: short, separate
stretches of nucleotides are added
to the template, and then ligase fill
in the gaps between them.
Base Pairing
during
Replication
Each old strand
serves as the
template for
complementary
new strand
Fig. 13-8, p. 213
Enzymes in Replication
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Enzymes unwind the two strands - helicase
DNA polymerase attaches complementary
nucleotides
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DNA ligase fills in gaps (Okazaki fragments)
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Enzymes wind two strands together
DNA Repair
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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
13.4 Cloning
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Making a genetically identical copy of an
individual
Researchers have been creating clones for
decades
These clones were created by embryo
splitting
Cloning
1 A microneedle
2 The microneedle has emptied
the sheep egg of its own nucleus.
3 DNA from a donor
cell is about to be
deposited in the
enucleated egg.
4 An electric spark will
stimulate the egg to enter
mitotic cell division.
the first cloned sheep
Fig. 13-9, p.214
Dolly:
Cloned from an Adult Cell
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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
Dolly: Cloned from an Adult Cell
Fig. 13-9, p.214
Impacts, Issues: Goodbye Dolly
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Ian Wilmut was the first to produce a
cloned sheep, which he named Dolly
Dolly experienced health problems similar
to other mammals cloned from adult DNA
Goodbye Dolly
Fig. 13-1a, p.206
Impacts, Issues: Goodbye Dolly
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The risk of defects in clones is huge
Possible benefit – patients in desperate need of organ
transplants
Genetically modified cloned animals may produce organs
that human donors are less likely to reject
Cloning humans – ethical?
Therapeutic Cloning
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SCNT – Somatic Cell Nuclear Transfer
Transplant DNA of a somatic cell from the
heart, liver, muscles, or nerves into a stem
cell (undifferentiated cell)
http://www.cyagra.com/gallery/jewel.htm
Cows
More Clones
http://www.popsci.com/scitech/article/2003-05/face-should-we-clone-fading-species
Fig. 13-10, p.215