DNA Replication-----(Biochem)

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DNADNA
ReplicationReplication
copyright cmassengale

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DNA StructureDNA Structure
• Rosalind Franklin took
diffraction x-ray
photographs of DNA
crystals
• In the 1950’s, Watson &
Crick built the first model
of DNA using Franklin’s
x-rays

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Discovery of DNADiscovery of DNA
StructureStructure
• Erwin Chargaff showed the
amounts of the four bases on
DNA ( A,T,C,G)
• In a body or somatic cell:
A = 30.3%
T = 30.3%
G = 19.5%
C = 19.9%

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Chargaff’s RuleChargaff’s Rule
• AdenineAdenine must pair with
ThymineThymine
• GuanineGuanine must pair with
CytosineCytosine
• The bases form weak
hydrogen bonds
G CT A

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DNADNA
StructureStructure

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DNADNA
• Two strands coiled called
a double helix
• Sides made of a pentose
sugar Deoxyribose bonded
to phosphate (PO4) groups
by phosphodiester bonds
• Center made of nitrogen
bases bonded together by
weak hydrogen bonds

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DNA Double HelixDNA Double Helix
NitrogenousNitrogenous
Base (A,T,G or C)Base (A,T,G or C)
““Rungs of ladder”Rungs of ladder”
““Legs of ladder”Legs of ladder”
Phosphate &Phosphate &
Sugar BackboneSugar Backbone

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HelixHelix
• Most DNA has aMost DNA has a right-handright-hand
twist withtwist with 10 base pairs10 base pairs in ain a
complete turncomplete turn
• Left twisted DNA is calledLeft twisted DNA is called
Z-DNAZ-DNA oror southpawsouthpaw DNADNA
• Hot spotsHot spots occur where rightoccur where right
and left twisted DNA meetand left twisted DNA meet
producingproducing mutationsmutations

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DNADNA
• Stands for
Deoxyribonucleic acid
• Made up of subunits
called nucleotidesnucleotides
• NucleotideNucleotide made of:made of:
1. Phosphate groupPhosphate group
2. 5-carbon sugar5-carbon sugar
3. Nitrogenous baseNitrogenous base

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DNA NucleotideDNA Nucleotide
O=P-O
O
PhosphatePhosphate
GroupGroup
N
Nitrogenous baseNitrogenous base
(A, G, C, or T)(A, G, C, or T)
CH2
O
C1
C4
C3
C2
5
SugarSugar
(deoxyribose)(deoxyribose)
O

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Pentose SugarPentose Sugar
• Carbons are numbered clockwise
1’ to 5’
CH2
O
C1
C4
C3
C2
5
SugarSugar

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DNADNA
P
P
P
O
O
O
1
2
3
4
5
5
3
3
5
P
P
P
O
O
O
1
2 3
4
5
5
3
5
3
G C
T A

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Antiparallel StrandsAntiparallel Strands
• One strand of
DNA goes from
5’ to 3’ (sugars)
• The other
strand is
opposite in
direction going
3’ to 5’ (sugars)

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Nitrogenous BasesNitrogenous Bases
• Double ringDouble ring PURINESPURINES
Adenine (A)Adenine (A)
Guanine (G)Guanine (G)
• Single ringSingle ring PYRIMIDINESPYRIMIDINES
Thymine (T)Thymine (T)
Cytosine (C)Cytosine (C) T or C
A or G

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Base-PairingsBase-Pairings
• Purines only pair with
Pyrimidines
• Three hydrogen bonds
required to bond Guanine
& Cytosine
CG
3 H-bonds

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T A
•Two hydrogen bonds are
required to bond Adenine &
Thymine

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Question:Question:
• If there is 30%
AdenineAdenine, how much
CytosineCytosine is present?

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Answer:Answer:
• There would be 20%
CytosineCytosine
• Adenine (30%) = ThymineAdenine (30%) = Thymine
(30%)(30%)
• Guanine (20%) = CytosineGuanine (20%) = Cytosine
(20%)(20%)
• Therefore,Therefore, 60% A-T and60% A-T and
40% C-G40% C-G

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DNADNA

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Replication FactsReplication Facts
• DNA has to be copiedDNA has to be copied
before a cell dividesbefore a cell divides
• DNA is copied during theDNA is copied during the SS
or synthesis phaseor synthesis phase
• New cells will needNew cells will need identicalidentical
DNA strandsDNA strands

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DNA ReplicationDNA Replication
• As the 2 DNA strands open atAs the 2 DNA strands open at
the origin,the origin, Replication BubblesReplication Bubbles
formform
• Prokaryotes (bacteria) have a
single bubble
• Eukaryotic chromosomes have
MANY bubbles
Bubbles Bubbles

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• Begins atBegins at Origins of ReplicationOrigins of Replication
• Two strands open formingTwo strands open forming ReplicationReplication
Forks (Y-shaped region)Forks (Y-shaped region)
• New strands grow at the forksNew strands grow at the forks
ForkFork
Parental DNA MoleculeParental DNA Molecule
3’
5’
3’
5’

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• EnzymeEnzyme HelicaseHelicase unwindsunwinds
and separates the 2 DNAand separates the 2 DNA
strands by breaking thestrands by breaking the
weak hydrogen bondsweak hydrogen bonds
• Single-Strand BindingSingle-Strand Binding
ProteinsProteins attach and keep
the 2 DNA strands
separated and untwisted

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• EnzymeEnzyme TopoisomeraseTopoisomerase attaches
to the 2 forks of the bubble to
relieve stressrelieve stress on the DNADNA
moleculemolecule as it separates
Enzyme
DNA
Enzyme

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• BeforeBefore new DNA strands can
form, there must be RNARNA
primersprimers present to start the
addition of new nucleotides
• PrimasePrimase is the enzyme that
synthesizes the RNA Primer
• DNA polymerase can then add
the new nucleotides

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• DNA polymeraseDNA polymerase can only addcan only add
nucleotides to thenucleotides to the 3’ end3’ end of theof the
DNADNA
• This causes theThis causes the NEWNEW strand to bestrand to be
built in abuilt in a 5’ to 3’ direction5’ to 3’ direction
RNARNA
PrimerPrimerDNA PolymeraseDNA Polymerase
NucleotideNucleotide
5’
5’ 3’
Direction of ReplicationDirection of Replicationcopyright cmassengale

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Remember HOW theRemember HOW the
Carbons Are Numbered!Carbons Are Numbered!
O
O=P-O
O
PhosphatePhosphate
GroupGroup
N
Nitrogenous baseNitrogenous base
(A, G, C, or T)(A, G, C, or T)
CH2
O
C1
C4
C3
C2
5
SugarSugar

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Remember the Strands areRemember the Strands are
AntiparallelAntiparallel
P
P
P
O
O
O
1
2
3
4
5
5
3
3
5
P
P
P
O
O
O
1
2 3
4
5
5
3
5
3
G C
T A

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Synthesis of the New DNASynthesis of the New DNA
StrandsStrands
• TheThe Leading StrandLeading Strand is
synthesized as a single strandsingle strand
from the point of origin toward
the opening replication fork
RNARNA
PrimerPrimerDNA PolymeraseDNA PolymeraseNucleotidesNucleotides
3’5’
5’

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Synthesis of the New DNASynthesis of the New DNA
StrandsStrands
• TheThe Lagging StrandLagging Strand isis synthesized
discontinuouslydiscontinuously against overall direction of
replication
• This strand is made in MANY short segments
It is replicated from the replication fork
toward the origin
RNA PrimerRNA Primer
Leading StrandLeading Strand
DNA PolymeraseDNA Polymerase
5
’
5’
3’
3’
Lagging StrandLagging Strand
5’
5’
3’
3’ copyright cmassengale

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Lagging Strand SegmentsLagging Strand Segments
• Okazaki FragmentsOkazaki Fragments -- series of
short segments on the lagginglagging
strandstrand
• Must be joined together by anMust be joined together by an
enzymeenzyme
Lagging Strand
RNARNA
PrimerPrimer
DNADNA
PolymerasePolymerase
3’
3’
5’
5’
Okazaki FragmentOkazaki Fragment

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Joining of Okazaki FragmentsJoining of Okazaki Fragments
• The enzymeThe enzyme LigaseLigase joins thejoins the
Okazaki fragments together toOkazaki fragments together to
make one strandmake one strand
Lagging Strand
Okazaki Fragment 2Okazaki Fragment 2
DNA ligaseDNA ligase
Okazaki Fragment 1Okazaki Fragment 1
5’
5’
3’
3’

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Replication of StrandsReplication of Strands
Replication
Fork
Point of Origin

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Proofreading New DNAProofreading New DNA
• DNA polymerase initially makesDNA polymerase initially makes
aboutabout 1 in 10,0001 in 10,000 base pairingbase pairing
errorserrors
• EnzymesEnzymes proofread and correctproofread and correct
these mistakesthese mistakes
• The new error rate for DNA thatThe new error rate for DNA that
has been proofread ishas been proofread is 1 in 1 billion1 in 1 billion
base pairing errorsbase pairing errors

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Semiconservative Model ofSemiconservative Model of
• Idea presented byIdea presented by Watson & CrickWatson & Crick
• TheThe two strands of the parental
molecule separate, and each acts as a
template for a new complementary
strand
• New DNA consists of 1
PARENTAL (original) and 1 NEW
strand of DNA
Parental DNA
DNA Template
New DNA

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DNA Damage & RepairDNA Damage & Repair
• Chemicals & ultraviolet radiation
damage the DNA in our body cells
• Cells must continuously repair
DAMAGED DNA
• Excision repair occurs when any of
over 50 repair enzymes remove
damaged parts of DNA
• DNA polymerase and DNA ligase
replace and bond the new nucleotides
together

DNA Replication-----(Biochem)

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