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

1

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.