DNA replication is semi-conservative and involves several key enzymes. DNA helicase unwinds and unzips the parental DNA, then RNA primase adds short RNA primers to each strand. DNA polymerase adds complementary nucleotides to each new strand in the 5' to 3' direction, with the leading strand synthesized continuously and the lagging strand synthesized discontinuously in fragments called Okazaki fragments. DNA ligase joins the Okazaki fragments together. The end result is two double-stranded DNA molecules that each contain one old and one new strand.

1. 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

2. 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.

3. 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

4. Models of DNA Replication
• Alternative models
– become experimental predictions
conservative semiconservative
Can you design
a nifty experiment
to verify?
dispersive
1
2
P

5. • Meselson & Stahl
– label “parent” nucleotides in DNA strands with
heavy nitrogen = 15
N
– label new nucleotides with lighter isotope = 14
N
“The Most Beautiful Experiment in Biology”
1958
parent replication
15
N parent
strands
15
N/15
N
Semiconservative replication

6. Predictions
1st round of
replication
conservative
15
N/15
N
14
N/14
N
semi-
conservative
15
N/14
N
dispersive
15
N/14
N
conservative
15
N/15
N
14
N/14
N
semi-
conservative
15
N/14
N
dispersive
15
N/14
N
2nd round of
replication
14
N/14
N
15
N parent
strands
15
N/15
N1
2
P

7. Franklin Stahl
Matthew Meselson
Matthew Meselson Franklin Stahl
Meselson & Stahl

8. • 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.

9. 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.

10. 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.

11. Steps of DNA Replication
Step 1: The enzyme
DNA Helicase
unwinds and unzips
the parent DNA
molecule.

13. Steps of DNA Replication
Step 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.

14. Steps of DNA Replication
Step 3: DNA polymerase (an enzyme)
adds appropriate nucleotides to the new
DNA strand in the 5’ to 3’ direction.
Notice that the
strands are being
replicated in the
opposite direction!
“antiparallel”

15. Steps of DNA Replication
Step 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.

16. Steps of DNA Replication
• Step 4: Each Okazaki Fragment on the
Lagging strand of DNA must be joined
together by an enzyme called Ligase.

17. End Product…
• Two complete DNA strands.
– Each strand is half new and half old

18. Cut to the Video…
• http://www.youtube.com/watch?v=FBmO_rmXxIw

19. Time for Questions!!!
Learning Objectives: Summarize the role of
enzymes involved in the replication of DNA.
Match the enzyme
with it’s function:
a. Helicase
b. Ligase
c. DNA Polymerase
d. 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.

20. Learning Objective 2: Explain how leading
and lagging strands of DNA are
synthesized differently.