SEMI-CONSERVATIVE DNA
REPLICATION
Pages 333-335
Essential Questions
What is replication and how is it done?
What’s the role of the enzymes
helicase and DNA polymerase?
Replication
When a complete copy of the
DNA is made during the S-Phase
of the cell cycle.
mitosis
& cell
division
WHY MUST DNA SYNTHESIZE
COPIES OF ITSELF ?
TO ENSURE THAT EVERY
NEW CELL CONTAINS
SAME NUMBER
EXACTLY THE ______________
SAME KIND
AND EXACTLY THE ____________
OF GENES THE
PARENT CELL HAD.
The DNA code is in the middle of the helix, so how
does it get copied if it’s obscured by the side chains
and twist of the helix shape?
DNA Synthesis
Step One:
Hydrogen bonds
are broken
by the enzyme,
Helicase.
Hydrogen =
Bonds are weak
bonds
REVIEW:
Hydrogen bonds
are located betwee
nitrogenous bases
Step Two: DNA Polymerase
adds free nucleotides to both
strands.
Step 3: New DNA strands are Proofread and mistakes are repaired:
Replication occurs in 3
Steps:
1. Helicase enzyme “unzips” the double helix
by weakening H-bonds creating a
replication fork where the two DNA chains
separate.
2. DNA polymerase enzyme assembles new
DNA nucleotides using each original
strand as a template.
3. The replicated DNA is proofread and any
mistakes are edited.
Replication Fork
Leading and Lagging Strands
• The leading strand is the strand of DNA that is
made continuously.
• The lagging strand is the strand of DNA that is
made discontinuously.
Boring person explaining 
• http://highered.mcgrawhill.com/olcweb/cgi/pluginpop.cgi?it=swf::535
::535::/sites/dl/free/0072437316/120076/mic
ro04.swf::DNA%20Replication%20Fork
Replication is Discontinuous on the Lagging
Strand
 Short fragments of DNA called Okazaki
fragments are created near the
replication fork.
 Gaps are filled in when DNA
polymerase adds nucleotides.
 Bases are added following the base
pairing rules (A-T, C-G)
* The lengths of Okazaki fragments are between 1,000 to 2,000
nucleotides long in bacteria and are generally between 100 to 200
nucleotides long in eukaryotes.
DNA Foldables
1/2
1/4
Copy the following sequence onto your foldable on the left side.
Write the new base pairs on the right side.
T-- --A
A-- --T
C-- --G
Remember that
A-- --T
H-bonds hold
A-- --T
complementary
A-- --T
bases together
C-- --G
T-- --A
T-- --A
A-- --T
C-- --G
T-- --A
Unzip sequence on your foldable.
T
A
C
A
A
A
C
T
T
A
C
T
A
T
G
T
T
T
G
A
A
T
G
A
Step 1:
Helicase
enzyme
“unzips”
double helix
by weakening
H-bonds
Using the original DNA sequence on the foldable make a copy.
T
A
C
A
A
A
C
T
T
A
C
T
A
T
G
T
T
T
G
A
A
T
G
A
Step 2: DNA
polymerase
enzyme
adds DNA
bases to the
exposed
nucleotides
on the
leading
strand
A
T
G
T
T
T
G
A
A
T
G
A
Using the original DNA sequence on the foldable make a copy.
T
A
C
A
A
A
C
T
T
A
C
T
A
T
G
T
T
T
G
A
A
T
G
A
While Okazaki
fragments are
added on the
lagging strand
A
C
A
A
C
A
C
T
A
T
G
T
T
T
G
A
A
T
G
A
Using the original DNA sequence on the foldable make a copy.
T
A
C
A
A
A
C
T
T
A
C
T
A
T
G
T
T
T
G
A
A
T
G
A
Step 3:
Polymerase
also
proofreads
and edits any
gaps
T
A
C
A
A
A
C
T
T
A
C
T
A
T
G
T
T
T
G
A
A
T
G
A
RESULTS
• TWO strands of identical DNA
• DNA replication is semiconservative because each
new DNA molecule contains one original strand
and one newly made strand.
original
double
helix
original
double
helix
Replication fork
replication
fork
two identical
new strands
are formed
free nucleotides
are attracted to
their opposites
forming two
new strands
two identical
new strands
are formed
1.Why must DNA synthesize copies of
itself?
2. If the code below is for a parent
DNA molecule, write the code for the
offspring:
AATTCCGGTATACCCGCCAAG
3. List and describe the three steps in
the process of DNA replication.
4. During which phase of the cell cycle
does DNA replicate?
5. Explain the process of DNA replication
Have Your DNA & eat it too!
1. Now replicate the DNA, using 2
more pieces of licorice but use
black sticks
3-2-1
3 steps cells undergo in replication
2 words meaning the structure of DNA
1 word for duplicating cell DNA