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DNA DNA ReplicatiReplicati
onon
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Replication FactsReplication Facts
• DNA has to be copied DNA has to be copied before a cell dividesbefore a cell divides
• DNA is copied during DNA is copied during the the SS or synthesis phase or synthesis phase of of interphaseinterphase
• New cells will need New cells will need identical identical DNA strandsDNA strands
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DNA ReplicationDNA Replication
• Begins atBegins at Origins of ReplicationOrigins of Replication• Two strands open forming Two strands open forming
Replication Forks (Y-shaped Replication Forks (Y-shaped region)region)
• New strands grow at the forksNew strands grow at the forks
ReplicationReplicationForkFork
Parental DNA MoleculeParental DNA Molecule
3’
5’
3’
5’copyright cmassengale
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DNA ReplicationDNA Replication
• Enzyme Enzyme HelicaseHelicase unwinds and separates unwinds and separates the 2 DNA strands by the 2 DNA strands by breaking the breaking the weak weak hydrogen bondshydrogen bonds
• Single-Strand Binding Single-Strand Binding ProteinsProteins attach and keep the 2 DNA strands separated and untwisted
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DNA ReplicationDNA Replication• BeforeBefore new DNA strands can
form, there must be RNA RNA 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 ReplicationDNA Replication• DNA polymeraseDNA polymerase can only add can only add
nucleotides to the nucleotides to the 3’ end3’ end of of the DNA the DNA
• This causes the This causes the NEWNEW strand to strand to be built in a be built in a 5’ to 3’ direction5’ to 3’ direction
RNARNAPrimerPrimerDNA PolymeraseDNA Polymerase
NucleotideNucleotide
5’
5’ 3’
Direction of ReplicationDirection of Replicationcopyright cmassengale
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Remember the Strands are Remember the Strands are AntiparallelAntiparallel
P
P
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O
O
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5
5
3
3
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P
P
PO
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2 3
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G C
T A
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Synthesis of the New Synthesis of the New DNA StrandsDNA Strands
• The The Leading StrandLeading Strand is synthesized as a single single strand strand from the point of origin toward the opening replication fork
RNARNAPrimerPrimerDNA PolymeraseDNA PolymeraseNucleotidesNucleotides
3’5’
5’
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Synthesis of the New DNA Synthesis of the New DNA StrandsStrands
• The The Lagging StrandLagging Strand is is 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’
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Lagging Strand Lagging Strand SegmentsSegments
• Okazaki FragmentsOkazaki Fragments - - series of short segments on the lagging strandlagging strand
• Must be joined together by Must be joined together by an an enzymeenzyme
Lagging Strand
RNARNAPrimerPrimer
DNADNAPolymerasePolymerase
3’
3’
5’
5’
Okazaki FragmentOkazaki Fragment
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Joining of Okazaki Joining of Okazaki FragmentsFragments
• The enzyme The enzyme LigaseLigase joins the joins the Okazaki fragments together Okazaki fragments together to make one strandto make 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 Replication of StrandsStrands
Replication Fork
Point of Origin
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Proofreading New Proofreading New DNADNA
• DNA polymerase initially DNA polymerase initially makes about makes about 1 in 10,0001 in 10,000 base base pairing errorspairing errors
• EnzymesEnzymes proofread and proofread and correct these mistakescorrect these mistakes
• The new error rate for DNA The new error rate for DNA that has been proofread is that has been proofread is 1 1 in 1 billionin 1 billion base pairing errors base pairing errors
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Semiconservative Model Semiconservative Model of Replicationof Replication
• Idea presented by Idea 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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Question:Question:
•What would be the complementary DNA strand for the following DNA sequence?
DNA 5’-CGTATG-3’DNA 5’-CGTATG-3’copyright cmassengale
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Answer:Answer:
DNA 5’-CGTATG-3’DNA 5’-CGTATG-3’
DNA 3’-GCATAC-5’DNA 3’-GCATAC-5’
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DNA Replication
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