NUCLEIC ACID STRUCTURE AND
DNA REPLICATION
Genetic material must be able to:
Contain the information necessary to
construct an entire organism
Pass from parent to offspring and from cell
to cell during cell division
Be accurately copied
Account for the known variation within and
between species
History of Nucleic Acids
Late 1800s scientists postulated a
biochemical basis
Researchers became convinced
chromosomes carry genetic information
1920s to 1940s expected the protein
portion of chromosomes to be the genetic
material
Griffiths bacterial transformations
Late 1920s Frederick Griffith was working
with Streptococcus pneumoniae
S. pneumoniae
Strains
that secrete capsules look smooth and
can cause fatal infections in mice
Strains that do not secrete capsules look
rough and infections are not fatal in mice
Rough strains (R) without
capsule are not fatal
Smooth strains (S) with
capsule are fatal
No living bacteria found in
blood
Capsule prevents immune
system from killing bacteria
Living bacteria found in
blood
If mice are injected with
heat-killed type S, they
survive
Mixing live R with heatkilled S kills the mouse
Blood contains living
S bacteria
Transformation
Results of Griffiths Experiments
Genetic material from the heat-killed type
S bacteria had been transferred to the
living type R bacteria
This trait gave them the capsule and was
passed on to their offspring
Griffith did not know the biochemical basis
of his transforming principle
Avery, MacLeod, and McCarty used purification
methods to reveal that DNA is the genetic material
1940s interested in bacterial transformation
Only purified DNA from type S could transform
type R
Purified DNA might still contain traces of
contamination that may be the transforming
principle
Added DNase, RNase and proteases
RNase and protease had no effect
With DNase no transformation
DNA is the genetic material
HYPOTHESIS: A purified macromolecule from type S bacteria, which functions
as the genetic material, will be able to convert type R bacteria into type S.
Starting Materials: Type R and Type S strains of S. pneumoniae.
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Hershey and Chase
1952, studying T2 virus infecting Escherichia coli
Bacteriophage or phage
Phage coat made entirely of protein
DNA found inside capsid
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Hershey and Chase
Shearing force from a blender will separate the
phage coat from the bacteria
35S will label proteins only
32P will label DNA only
Experiment to find what is injected into bacteriaDNA or protein?
Results support DNA as the genetic material
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Levels of DNA structure
1.
2.
3.
4.
5.
Nucleotides are the
building blocks of DNA
(and RNA).
Form a strand of DNA
(or RNA)
Two strands form a
double helix.
In living cells, DNA is
associated with an
array of different
proteins to form
chromosomes.
A genome is the
complete complement
of an organisms
genetic material.
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DNA
3 components
Phosphate
group
Pentose sugar
Deoxyribose
Nitrogenous
Purines
base
Adenine (A),
guanine (G)
Pyrimidines
Cytosine (C),
thymine (T),
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RNA
3 components
Phosphate
group
Pentose sugar
Ribose
Nitrogenous
Purines
base
Adenine (A),
guanine (G)
Pyrimidines
Cytosine (C),
uracil (U)
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Conventional numbering system
Sugar carbons 1 to 5
Base attached to 1
Phosphate attached to 5
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Strands
Nucleotides covalently
bonded
Phosphodiester bond
phosphate group links 2
sugars
Phosphates and sugars
form the backbone
Bases project from
backbone
Directionality- 5 to 3
5 TACG 3
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Solving DNA structure
1953, James Watson and Francis Crick, with
Maurice Wilkins, proposed the structure of the
DNA double helix
Watson and Crick used Linus Paulings method
of working out protein structures using simple
ball-and-stick models
Rosalind Franklins X-ray diffraction results
provided crucial information
Erwin Chargoff analyzed base composition of
DNA that also provided important information
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Watson and Crick put
together these pieces
of information
Found ball-and-stick
model consistent with
data
Watson, Crick &
Wilkins awarded
Nobel Prize in 1962
Rosalind Franklin had
died and the Nobel is
not awarded
posthumously
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DNA is
Double
stranded
Helical
Sugar-phosphate
backbone
Bases on the inside
Stabilized by
hydrogen bonding
Base pairs with
specific pairing
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AT/GC or Chargoffs rule
10 base pairs per turn
2 DNA strands are
complementary
A pairs with T
G pairs with C
5 GCGGATTT 3
3 CGCCTAAA 5
2 strands are antiparallel
One strand 5 to 3
Other stand 3 to 5
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Space-filling model shows grooves
Major
groove
Where proteins bind
Minor
groove
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Replication
3 different models for DNA replication
proposed in late 1950s
Semiconservative
Conservative
Dispersive
Newly made strands are daughter strands
Original strands are parental strands
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In 1958, Matthew Meselson and Franklin Stahl
devised an experiment to differentiate among 3
proposed mechanisms
Nitrogen comes in a common light form (14N)
and a rare heavy form (15N)
Grew E.coli in medium with only 15N
Then switched to medium with only 14N
Collected sample after each generation
Original parental strands would be 15N while
strands from later generations would be 14N
Results consistent with semiconservative
mechanism
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During replication 2
parental strands
separate and serve
as template strands
New nucleotides must
obey the AT/GC rule
End result 2 new
double helices with
same base sequence
as original
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Origin of replication
Site
of start point for
replication
Bidirectional
replication
Replication
proceeds
outward in opposite
directions
Bacteria have a single
origin
Eukaryotes require
multiple origins
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Origin of replication provides an opening
called a replication bubble that forms two
replication forks
DNA replication occurs near the fork
Synthesis begins with a primer
Proceeds 5 to 3
Leading strand made in direction fork is
moving
Synthesized
as one long continuous molecule
Lagging strand made as Okazaki
fragments that have to be connected later
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DNA helicase
Binds
to DNA and
uses ATP to separate
strand and move fork
forward
DNA topoisomerase
Relives
additional
coiling ahead of
replication fork
Single-strand binding
proteins
Keep
parental strands
open to act as
templates
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DNA polymerase
Covalently
links nucleotides
Deoxynuceloside triphosphates
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Deoxynuceloside triphosphates
Free
nucleotides with 3 phosphate groups
Breaking
covalent bond to release
pyrophosphate (2 phosphate groups) provides
energy to connect adjacent nucleotides
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DNA polymerase has 2 enzymatic
features to explain leading and lagging
strands
1.
DNA polymerase is unable to begin DNA
synthesis on a bare template strand
DNA primase must make a short RNA primer
2.
RNA primer will be removed and replaced with DNA
later
DNA polymerase can only work 5 to 3
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In the leading strand
DNA
primase makes one RNA primer
DNA polymerase attaches nucleotides in a 5
to 3 direction as it slides forward
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In the lagging strand
DNA synthesized 5 to 3 but
in a direction away from the
fork
Okazaki fragments are
relatively short fragments of
DNA with an RNA primer at
the 5' terminus
100-200 bp in Eukaryotes
1-2 kb in bacteria
RNA primers will be removed
by DNA polymerase and filled
in with DNA
DNA ligase will join adjacent
DNA fragments
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DNA replication is very accurate
3 reasons
1.
2.
3.
Hydrogen bonding between A and T or G
and C more stable than mismatches
Active site of DNA polymerase unlikely to
form bonds if pairs mismatched
DNA polymerase removes mismatched pairs
Proofreading results in DNA polymerase backing
up and digesting improper base pairing (e.g A-C)
Other DNA repair enzymes
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Telomeres
A telomere is a region of repetitive DNA at
the end of a chromosome protecting it
from deterioration.
Specialized form of DNA replication only in
eukaryotic telomeres
Telomere at 3 does not have a
complementary strand and is called a 3
overhang
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Telomeres
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Telomeres
DNA polymerase cannot copy the tip of
the DNA strand with a 3 end
No
place for upstream primer to be made
If this replication problem were not solved,
linear chromosomes would become
progressively shorter
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Telomeres
Telomerase prevents chromosome
shortening
Attaches many copies of repeated DNA
sequences to the ends of the
chromosomes
Provides upstream site for RNA primer
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Telomeres and aging
Body cells have a predetermined life span
Skin sample grown in a dish will double a
finite number of times
Infants,
about 80 times
Older person, 10 to 20 times
Senescent cells have lost the capacity to
divide
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Telomeres
Progressive shortening of telomeres
correlated with cellular senescence
Telomerase present in germ-line cells and
in rapidly dividing somatic cells
Telomerase function reduces with age
Inserting a highly active telomerase gene
into cells in the lab causes them to
continue to divide
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Telomeres and cancer
When cells become cancerous they divide
uncontrollably
In 90% of all types of human cancers,
telomerase is found at high levels
Prevents telomere shortening and may
play a role in continued growth of cancer
cells
Mechanism unknown
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