Download DNA - Structure & Function

Chapter 13
DNA Structure
Ch. 13 & Function
DNA Structure
&
Function
DNA Structure
& Function
DNA as Genetic Material
Johann Miescher (1869)
 Removed nuclei from pus cells
 Found they contained a chemical he called
nuclein
 This was rich in phosphorus and had no sulfur;
thus it could not be a protein
Later scientists realized there were two types of
nucleic acids: DNA (deoxyribonucleic acid) and
RNA (ribonucleic acid)
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DNA Structure
& Function
Frederick Griffith (1931)
Investigated virulence of Streptococcus pneumoniae in
mice in following manner:
1. S strain bacteria have a smooth capsule & are
capable of killing mice
2. R strain have no capsule & don’t kill mice
3. Injected heat-killed S strain bacteria into
mice; they did not die
4. Injected mice with mixture of heat-killed S strain &
live R strain. These mice had living S strain
bacteria & died
 Concluded that virulence passed from the dead strain
to the living strain; transformation had occurred
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Griffith’s Transformation Experiment
DNA Structure
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Avery, MacLeod & McCarty (1944)
(Refer to transparency here first)
Discovered that DNA is the transforming substance.
1. Took DNA only from the S bacteria and mixed it with R
bacteria.
2. S strain DNA was then incorporated into
genome of living R strain bacteria and they were then
transformed into S strain bacteria.
3. Enzymes that degrade proteins or RNA did not
prevent transformation while those that digest DNA
did.
DNA Structure
& Function
Reproduction of Viruses
Viruses consist of a protein coat (capsid)
surrounding a nucleic acid core
Bacteriophages are viruses that infect bacteria
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DNA Structure
& Function
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Hershey and Chase (1952)
Did an experiment to determine whether the
bacteriophages inject the protein or DNA into the
bacteria.
Radioactively labeled the DNA core and protein capsid of
a bacteriophage
1. Radioactive P (found in DNA & not in protein) was
found inside cells
2. Radioactive S (found in protein & not in DNA) was
found mainly outside of cells
Results indicated that DNA, not the protein, enters the
host
The DNA of the phage contains genetic information for
producing new phages
Hershey and Chase Experiments
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DNA Structure
& Function
Structure of DNA
DNA contains:
Two nucleotides with purine bases. These are
double ring nitrogenous bases.
- Adenine (A)
- Guanine (G)
Two nucleotides with pyrimidine bases. These
are single ring nitrogenous bases.
- Thymine (T)
- Cytosine (C)
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Nucleotide Composition of DNA
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DNA Structure
& Function
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Chargaff’s Rules
The amounts of A, T, G, and C in DNA:
Identical in identical twins
Varies between individuals of a species
Varies more from species to species
In each species, there are equal amounts of:
A & T
G & C
All this suggests DNA uses complementary base
pairing to store genetic information.
Human chromosome estimated to contain, on
average, 140 million base pairs.
Number of possible nucleotide sequences
4^140,000,000.
DNA Structure
& Function
Diffraction Data
Rosalind Franklin:
Studied structure of DNA using X-rays.
Found that if a concentrated solution of DNA is
made it forms into a crystal like structure.
When X-rayed, an X-ray diffraction pattern
results.
The pattern of DNA shows that it is a helix.
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X-Ray Diffraction of DNA
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DNA Structure
& Function
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Watson and Crick Model (1953)
Using data provided by Franklin’s X-ray diffraction and
other knowledge about DNA, they eventually
determined that DNA is a double-helix
- Sugar-phosphate backbones make up the sides
- Hydrogen-bonded bases make up the rungs.
Complementary bases (A-T; C-G) pair up.
 Model matched data of both Franklin & Chargaff
Received a Nobel Prize in 1962
Watson/Crick Model of DNA
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DNA Replication:
DNA Structure
& Function
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Replication = process of copying a DNA molecule
1. During DNA replication, each old DNA strand of
the parental molecule (original double helix) serves
as a template for a new strand in a daughter
molecule.
2. DNA replication is termed semiconservative
replication because one of the old strands is
conserved, or present, in each daughter DNA
molecule.
Steps of Replication
DNA Structure
& Function
1. Unwinding
 DNA replication begins at numerous points
along linear chromosome called replication
forks.
 DNA unwinds and unzips into two strands.
Weak hydrogen bonds between paired
bases are broken.
 A special enzyme, DNA helicase, unwinds
the DNA.
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DNA Structure
& Function
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Replication (cont’d)
2. Complementary base pairing
 Each old strand of DNA serves as a template
for a new strand
 New complementary nucleotides are
positioned by process of complementary
base pairing
 A special enzyme, called DNA polymerase,
helps to position the complementary base
pairs
DNA Structure
& Function
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Replication (cont’d)
3. Joining
 The complementary nucleotides join to form
new strands.
 This is also helped by DNA polymerase
Semiconservative Replication
of DNA
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DNA Structure
& Function
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Meselson & Stahl’s experiment (1958)
Confirmed semiconservative replication theory
 They grew bacteria in a medium containing
heavy N-15 so only heavy DNAs were found.
- Switched bacteria to N-14 medium.
- After 1 division, only hybrid DNA was found
- After 2 divisions, half the DNA is light & half is
hybrid
- These are the results expected if DNA replication is
semiconservative.
Meselson and Stahl’s
DNA replication experiment
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DNA Structure
& Function
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Details of DNA Replication
Carbon atoms are numbered in the deoxyribose
molecule.
DNA strands are antiparallel. One of the strands runs
from 3’ to 5’ in one direction, and the other strand runs
from 3’ to 5’ in the opposite direction.
During replication, DNA polymerase has to synthesize
the daughter strand in the 5’ to 3’ direction.
Why? DNA polymerase can only join a nucleotide to a
free 3’ end of a previous nucleotide.
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Details of DNA Replication (cont’d)
This also means that DNA polymerase cannot start the
synthesis of a DNA chain.
An RNA polymerase lays down a short amount of RNA,
called an RNA primer, that is complementary to DNA.
Then DNA polymerase can join DNA nucleotides to the
3’ end of the growing daughter strand.
DNA Structure
& Function
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Details of DNA Replication (cont’d)
As helicase unwinds DNA, one parental strand runs in
the 3’ to 5’ direction toward the fork. Thus, the new
complementary daughter strand will be synthesized
from the 5’ to 3’ direction. This strand is called the
leading strand.
The other parental strand, however, is running in the
opposite direction (3’ to 5’ AWAY from the fork). The
daughter strand must begin at the fork and run in the
opposite direction to the leading strand. This is called
the lagging strand.
Antiparallel Replication of DNA
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DNA Structure
& Function
Details of DNA Replication (cont’d)
Replication of the lagging strand is discontinuous.
It results in segments called Okazaki fragments.
While proofreading, DNA polymerase will remove the
RNA primers and replace them with complementary
DNA nucleotides.
DNA ligase will then join the fragments together.
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Antiparallel Replication of DNA
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DNA Replication:
Prokaryotic
DNA Structure
& Function
Prokaryotic Replication
Bacteria have a single circular loop
Replication moves around the circular DNA
molecule in both directions. Takes about 40
minutes.
Produces two identical circles
Cell divides between circles, as fast as every 20
minutes
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Replication:
Prokaryotic vs. Eukaryotic
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DNA Structure
& Function
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Replication Errors
Genetic variations are the raw material for
evolutionary change
Mutation:
A permanent (but unplanned) change in base-pair
sequence
- Some due to errors in DNA replication.
Proofreading occurs which eliminates most errors.
Mistake rate is only 1 per 1 billion base pairs.
- Others are due to DNA damage like UV radiation
DNA repair enzymes are usually available to
reverse most errors
DNA Structure
& Function
Videos for Chapter 13
DNA Replication Animation I
http://www.courses.fas.harvard.edu/~biotext/
animations/replication1.html
DNA Replication Animation II
http://highered.mcgraw-hill.com/olc/dl/120076/bio23.swf
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