Download 2013 DNA/Replication Notes

DNA: The Genetic Material
Chapter 9 Section 1
TAKS/TEKS
• TAKS Objective 2
– The student will demonstrate an understanding of the
organization of living systems.
• TEKS
– Bio (6) The student knows the structures and functions
of nucleic acids in the mechanisms of genetics.
• (A) describe components of deoxyribonucleic acid (DNA), and
illustrate how information for specifying the traits of an
organism is carried in the DNA.
• (B) explain replication, transcription, translation using models
of DNA and ribonucleic acid (RNA).
Objectives
• Relate Griffith’s conclusions to the
observations he made during the
transformation experiments
• Summarize the steps involved in Avery’s
transformation experiments, and state the
results.
• Evaluate the results of the Hershey &
Chase experiment.
Key Terms
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Vaccine
Virulent
Transformation
Bacteriophage
Griffith’s Experiment
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1928
Fredrick Griffith
Bacteriologist
Trying to prepare a
vaccine against
pneumonia
Vaccine
• A substance that is prepared from killed or
weakened disease-causing agents, including
certain bacteria.
• The vaccine is introduced into the body to
protect the body against future infections by
the disease-causing agent.
Griffith’s Experiment
• Two types, or strains, of S. pneumoniae
• First strain is enclosed in a capsule composed of
polysaccharides.
– Capsule protects the bacterium from the body’s defense
system.
– Forms smooth-edges (S) when grown in a petri dish
• Helps make the microorganism virulent, or able to
cause disease.
Griffith’s Experiment
• Second strain lacks the polysaccharide
capsule and does not cause disease.
– Forms rough-edges (R) when grown in a petri
dish
Griffith’s Experiment
Griffith’s Discovery
• The harmless R bacteria had changed and
became virulent S bacteria.
• Transformation is a change in genotype
caused when cells take up foreign material.
Avery’s Experiment
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1944
Oswald Avery
Rockefeller Institute in New York
Repeated Griffith’s experiment to determine
which molecule in heat-killed bacteria was
most important for transformation.
Avery’s Experiment
• Made an extract, or juice, from the heatkilled bacteria.
• Treated the extract with enzymes that
destroyed proteins, lipids, carbohydrates,
and other molecules, including nucleic acid
RNA.
• Transformation still occurred
Avery’s Discovery
• Repeated the experiment using an enzymes
that would break down DNA.
• Transformation did not occur.
• DNA was the transforming factor!
What Scientists Knew
• Avery’s experiment clearly indicated
genetic material is composed of DNA
• Many scientist remain skeptical
• Proteins are important to many aspects of
the cell structure & metabolism, so most
suspected that proteins were the genetic
material
• Scientist knew very little about DNA
What Scientists Knew
• Viruses are composed of DNA or RNA
surrounded by a protective protein coat.
• Bacteriophage (phage) is a virus that infects
bacteria.
• When phages infect bacterial cells, the
pages are able to produce more viruses
– Released when the bacterial cells rupture.
What Scientists Didn’t Know
• How the bacteriophage reprograms the
bacterial cell to make viruses.
• Does the phage DNA, the protein, or both
issue instructions to the bacteria?
The Hershey-Chase Experiment
• 1952
• Alfred Hershey & Martha Chase
• Scientists at Cold Spring Harbor
Laboratory, in New York
• Used the bacteriophage T2 to answer this
question.
The Hershey-Chase Experiment
• Knew the only molecule in the phage that
contains phosphorus is its DNA.
• The only phage molecules that contain
sulfur are the proteins in its coat.
The Hershey-Chase Experiment
• Grew T2 with E. coli
bacteria in a nutrient
medium that contained
radioactive sulfur (35S)
– The protein coat would
incorporate the 35S
• Grew T2 with E. coli
bacteria in a nutrient
medium that contained
radioactive phosphorus
(32P)
– The radioactive phosphorus
would become part of the
cell’s DNA
The Hershey-Chase Experiment
•
35S-labeled
&
32P-labeled
phages were
used to infect
two separate
batches of E.
coli bacteria
The Hershey-Chase Experiment
• They waited a few
minutes for the viruses
to inject their genetic
material
• Next, they separated
the viruses from the
bacteria & tested the
bacteria for
radioactivity
Hershey-Chase Discovery
• Nearly all the radioactivity in the bacteria was
from phosphorus (32P), the marker found in DNA.
• Concluded that the DNA of viruses is injected into
the bacterial cell, while most of the viral proteins
remained outside.
• Causes bacterial cells to produce more viral DNA
and proteins.
• DNA is the hereditary material.
The Structure of DNA
Chapter 9 Section 2
Objectives
• Describe the three components of a nucleotide.
• Develop a model of the structure of a DNA
molecule.
• Evaluate the contributions of Chargaff, Franklin,
and Wilkins in helping Watson & Crick determine
the double-helical structure of DNA
• Relate the role of the base-pairing rules to the
structure of DNA.
Key Terms
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Double Helix
Nucleotide
Deoxyribose
Base-pairing Rules
Complementary Base Pair
Structure of DNA
• Double helix- two strands twisted around each
other, like a winding staircase.
• Each strand is made of linked nucleotides.
• If all the DNA in your body was put end to end, it would
reach to the sun and back over 600 times (100 trillion times
six feet divided by 92 million miles).
• Our entire DNA sequence would fill 200 1,000-page New
York City telephone directories
• Over 99% of our DNA sequence is the same as other
humans’.
• If you unwrap all the DNA you have in all your cells, you
could reach the moon 6000 times!
• The average gene is 10,000 to 15,000 bases long.
• There are an estimated 20,000 to 25,000 genes in our
genome.
• It would take a person typing 60 words per minute, eight
hours a day, around 50 years to type the human genome
Nucleotides
• The subunits that
make up DNA.
• 3 parts
– Phosphate group
– A 5-Carbon sugar
molecule (deoxyribose)
– Nitrogen-containing
base
• Any one of 4 different
bases
Purines & Pyrimidines
• Purines are nitrogen bases made of 2 rings
of carbon & nitrogen atoms
– Adenine
– Guanine
• Pyrimidines are nitrogen bases made of a
single ring of carbon & nitrogen atoms
– Thymine
– Cytosine
Base Pairing Rule:
C G
A T
Nitrogen Bases
How was the actual structure of
DNA discovered?
Chargaff’s Observation
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1949
Erwin Chargaff
Biochemist at Columbia University
The amount of adenine (A) always equaled the amount of
thymine (T)
– A=T
• The amount of guanine (G) always equaled the amount of
cytosine (C)
– G=C
• The amount of adenine & thymine and of guanine &
cytosine varied between different organisms
– A/T ≠ G/C
Wilkins & Franklin’s Photographs
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1952
Maurice Wilkins & Rosalind Franklin
King’s College in London
Developed high-quality X-ray diffraction
photographs of strands of DNA
• Suggested DNA molecule resembled a
tightly coiled helix & was composed of 2 or
3 chains of nucleotides
• The photograph provided key information that was
essential for developing a model of B-form (hydrated)
DNA. In particular, it could be determined from the
diffraction pattern, and was openly discussed by Franklin in
lectures attended by Watson and in reports accessible to
Watson and Crick, that DNA (1) was helical, (2) was likely
a double helix with antiparallel strands, and, (3) had the
phosphate backbone on the outside (thus the bases of DNA,
which are the "code" for inheritance, were on the inside of
the helix). Calculations from the photograph also provided
crucial parameters for the size of the helix and its structure,
all of which were critical for the molecular modeling
undertaken by Watson and Crick.
James Watson & Francis Crick
• 1953
• Cambridge University
• Developed the first 3dimensional model of
DNA
• Had to take into account
both Chargaff’s findings
& Frankin and Wilkins’s
X-ray diffraction data
Base-pairings
• Watson & Crick determined that a purine on
one strand of DNA is always paired with a
pyrimidine on the opposite strand.
• An adenine on one strand always pairs with
a thymine on the opposite strand.
• A guanine on one strand always pairs with a
cytosine on the opposite strand.
• Supported by Chargaff’s findings.
Complementary Base Pairs
What is the complementary base
pair?
TCGAACT
AGCTTGA
The Replication of DNA
Chapter 9 Section 3
Objectives
• Summarize the process of DNA replication.
• Describe how errors are corrected during
DNA replication.
• Compare the number of replication forks in
prokaryotic and eukaryotic DNA.
Key Terms
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DNA Replication
DNA Helicase
Replication Fork
DNA Polymerase
DNA Replication
• DNA replication is the process of making a
copy of DNA.
• Watson & Crick proposed that one DNA
strand serves as a template, or pattern, on
which the other strand is built.
DNA Replication
1. The double helix unwinds,
caused by an enzyme
(DNA helicase). These
open the double helix by
breaking the hydrogen
bonds that link
complementary base pairs.
2. Once separated additional
proteins attach to the ends
to keep them apart.
Replication Forks
DNA Replication
3. At the replication fork,
enzymes known as DNA
polymerases move along each of
the DNA strands.
4. DNA polymerases add
nucleotides to the exposed
nitrogen bases, according to the
base-pairing rules. the process
continues until all of the DNA has
been copied & the polymerase is
signaled to detach.
DNA Replication
5. Two new double helixes
are formed
Checking for Errors
• DNA polymerase has a “proofreading” role.
• It can only add a new nucleotide if the
previous nucleotide was correct.
• If it is incorrect, the polymerase will go
back and remove the incorrect nucleotide &
replace it with the correct one.
• Reduces errors in DNA replication to 1
error per 1 billion nucleotides!
Rate of Replication
• The replication of a typical human chromosome
with one pair of replication forks spreading from a
single point, would take 33 days!
• Each human chromosome is replicated in about
100 sections that are 100,000 nucleotides long,
each section with its own starting point.
• As a result, an entire human chromosome can be
replicated in about 8 hours.
Replication Forks