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DNA Replication
What
Why
Where
How
Steps of Replication
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DNA Replication WHAT
What:
Making a copy of the DNA
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DNA Replication - WHAT
Model: Semiconservative Model of Replication
Idea presented by Watson & Crick
The 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 Replication WHY
2. Why:
Because we need to make more DNA for new cells
DNA has to be copied before a cell divides
New cells will need identical DNA strands
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DNA Replication WHERE
3. Where:
Nucleus of eukaryotes
Because that is where the DNA is
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DNA Replication HOW
4. How:
Begins at Origins of Replication
Two strands open forming Replication Forks (Y-shaped region)
New strands grow at the forks
Replication
Fork
Parental DNA Molecule 3’ 5’
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DNA Replication HOW
As the 2 DNA strands open at the origin, Replication Bubbles form
Prokaryotes (bacteria) have a single bubble
Eukaryotic chromosomes have MANY bubbles
Bubbles
Bubbles
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8. DNA Replication - Process
Large team of enzymes coordinates replication
Enzymes
more than a dozen enzymes & other proteins participate in DNA replication

9. How: Replication: 1st step
Unwind DNA
helicase enzyme
unwinds part of DNA helix
by breaking the weak hydrogen bonds between the nitrogenous bases
single-stranded binding proteins
replication fork
helicase

10. HOW: Replication: 2nd step
Build daughter DNA strand
DNA polymerase III: adds new complementary bases
DNA
Polymerase III

11. 3rd step: Proofreading New DNA
DNA polymerase initially makes about 1 in 10,000 base pairing errors
Enzymes proofread and correct these mistakes
The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors
What do you notice about enzyme names?
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