12.2 Replication of DNA
Learning Objectives:
1. Summarize the role of enzymes involved in the replication of DNA
2. Explain how leading and lagging strands of DNA are synthesized differently
Scientists have discovered that DNA was in fact the genetic material AND the structure of DNA.
BUT… they still did not know how it replicated itself.
Watson and Crick speculated on how DNA might replicate, but actual evidence came from other’s experiments.
How is DNA copied?• Replication of DNA
– base pairing suggests that it will allow each side to serve as a template for a new strand
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.” — Watson & Crick
Models of DNA Replication• Alternative models
– become experimental predictions
conservative semiconservative
Can you designa nifty experiment
to verify?
dispersive
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2
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• Meselson & Stahl– label “parent” nucleotides in DNA strands with
heavy nitrogen = 15N– label new nucleotides with lighter isotope = 14N
“The Most Beautiful Experiment in Biology”
1958
parent replication
15N parent strands
15N/15N
Semiconservative replication
Predictions
1st round of replication
conservative
15N/15N
14N/14N
semi-conservative
15N/14N
dispersive
15N/14N
conservative
15N/15N
14N/14N
semi-conservative
15N/14N
dispersive
15N/14N
2nd round of replication
14N/14N
15N parent strands
15N/15N
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2
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Franklin Stahl
Matthew Meselson
Matthew Meselson Franklin Stahl
Meselson & Stahl
• The experiment showed that DNA replication was Semiconservative.
• This means, the DNA molecule splits and each half gets replicated, creating strands that are half old and half new.
Steps of DNA Replication(background information)
First, some background info…
#1. DNA is antiparallel
What does that mean?• The DNA strands
run opposite of each other.
Steps of DNA Replication(background information)
#2. DNA can only be replicated in the 5’ to 3’ direction.
– This means that when the DNA splits, it will be replicated in opposite directions.
Steps of DNA Replication
Step 1: The enzyme DNA Helicase unwinds and unzips the parent DNA molecule.
Steps of DNA ReplicationStep 2: RNA primase (an enzyme) adds a
short segment of RNA, called an RNA primer on each DNA strand. This gives the DNA bases something to attach to when they are laid down.
Steps of DNA ReplicationStep 3: DNA polymerase (an enzyme)
adds appropriate nucleotides to the new DNA strand in the 5’ to 3’ direction.
Notice that thestrands are beingreplicated in the
opposite direction!“antiparallel”
Steps of DNA ReplicationStep 3 (continued): The strands of DNA are being copied in
opposite directions.• Leading Strand: Gets replicated in one
continuous piece. • Lagging Strand: is replicated
discontinuously in small pieces called Okazaki Fragments.
Steps of DNA Replication• Step 4: Each Okazaki Fragment on the
Lagging strand of DNA must be joined together by an enzyme called Ligase.
End Product…
• Two complete DNA strands. – Each strand is half new and half old
Cut to the Video…
• http://www.youtube.com/watch?v=FBmO_rmXxIw
Time for Questions!!!Learning Objectives: Summarize the role of
enzymes involved in the replication of DNA.
Match the enzyme with it’s function:
a. Helicaseb. Ligasec. DNA Polymerased. RNA Polymerase
1. Joins Okazaki fragments together
2. Unwinds and unzips the parent DNA molecule
3. Adds short segments of RNA primers to each DNA strand.
4. Adds the appropriate nucleotides to the new DNA strand.
Learning Objective 2: Explain how leading and lagging strands of DNA are synthesized differently.
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