DNA replication is the process by which a cell makes an identical copy of its DNA when it divides. It occurs in S phase of the cell cycle and is essential for accurate transmission of genetic information from parent to daughter cells. There are three main steps: 1) unwinding and separating the DNA double helix, 2) adding complementary nucleotides to each strand to form two new double helices, and 3) completing replication to produce two identical DNA molecules, each with one original and one new strand. Replication differs between prokaryotes and eukaryotes in that prokaryotes replicate from a single origin of replication, while eukaryotes replicate from many origins simultaneously to complete replication more quickly.

1. OBJECTIVES
Essential Question: Why & how do our
cells reproduce DNA?
Restate what replications is and why it’s
important.
Describe the steps of DNA replication.
Compare the roles of DNA helicase, DNA
polymerase, and ligase.
Compare the process of DNA replication in
prokaryotes and in eukaryotes.
Illustrate the steps of replication in a
drawing.

2. DNA replication
DNA helicase
DNA polymerase
VOCABULARY

3. DNA SYNTHESIS
These subunits
are comprised of
a phosphate
group, a ribose
sugar, and one of
4 nitrogenous
bases.
You should also recall that in
DNA
A bonds with T and
In the first section of this
unit you learned what
DNA is.
The second section you
learned what happens in
G1: the process of gene
expression;
transcription and
translation.
You should also know that
DNA is a large molecule
built of billions of
subunits.

4. Every cell in an organism has
a particular life span… they
don’t last forever.
Cells are born, grow and do
their jobs, divide, and finally
create two new daughter
cells.
The life span is cyclical & with
every new generation the
cells that are formed carry
out the functions of the
parent cell because they are
given each an exact copy of
the parent cell’s DNA.
The cyclical life span of a cell
is called the CELL CYCLE.
CELLS DON’T LAST FOREVER

5. THE CELL’S LIFE. THE CELL CYCLE

6. Every time a cell goes
through a cycle it must
DUPLICATE ITS DNA SO
THAT WHEN IT MAKES
NEW BABY CELLS THEY
BOTH HAVE THE EXACT
SAME DNA.
Happens in ‘S’ phase
In this section we are going
explore the process of DNA
synthesis, called DNA
replication.
This process is extremely
precise and an incredibly
DNA SYNTHESIS = DNA DUPLICATION

7. THE CELL CYCLE: THE PART
WE’RE ISOLATING.
From single-copy of
each chromosome
•To double-copy

8. WHAT’S DIFFERENT ABOUT THESE CELLS?
REPLICATION
•These are identical strands of DNA
•There are two copies formed in replication (S phase) because each are destined for the
2 identical new cells that are formed, called daughter cells, after the cell divides.
•Identical DNA must be synthesized in order to achieve this.
•The process of DNA synthesis is REPLICATION.
1st Gap Phase DNA Synthesis Phase

9. HOW IS DNA REPLICATED?
There are three ways that
DNA can be
duplicated…
1. Conservative: original
is left completely
original.
2. Semi-conservative:
new DNA is half
original and half new.
3. Dispersive: DNA is
randomly duplicated.
The Meselson-Stahl experiment:
•Showed that DNA is replicated semi-conservatively.
•This means, the end product is one strand of original
DNA and one strand of newly-formed DNA.

10. SO. WHAT IS THE POINT OF REPLICATION?
The point of replication is to
produce exact copies of the
original cell’s DNA.

11. DNA REPLICATION: THE STEPS
The process of replicating DNA is broken down into three
major steps.
Your job is to know these 3 steps and be able to
summarize what happens in each.
The three steps are:
1. Unwinding and Separating DNA Strands
2. Adding complimentary bases
a. Leading Strand:
b. Lagging Strand: RNA Primers attach
3. Formation of Two Identical DNA molecules

12. DNA REPLICATION
• Step 1:
• DNA helicases unwinds &
separates the original DNA
double helix.
• These proteins wedge themselves
between the two strands of the
double helix and break the
hydrogen bonds between the base
pairs.
• Forms Replication Forks
• As the double helix unwinds, the
two complementary strands of DNA
separate from each other and form
a Y shape.

13. DNA REPLICATION
Step 2: New DNA is
formed from DNA
template.
RNA Primers attach to
specific regions. New
nucleotides are added to
the primer by the enzyme
DNA Polymerase
according to the base-
pairing rules.
DNA polymerases create the
formation of the new
complimentary DNA
molecule by moving along
each strand in a 5’ to 3’
direction.
They add nucleotides to a
new daughter compliment

14. DNA REPLICATION
You know that DNA is anti-
parallel because of the bi-
directionality of DNA.
One side goes 3’ to 5’.
The other 5’ to 3’.
New nucleotides can only
be added to the 3’ end of
the existing chain.
One side (the top pictured
here) is the LEADING
STRAND. It has its new
strand continuously
synthesized as helicase
unwinds more DNA.
The other side, the
LAGGING STRAND (on
bottom) is discontinuous
replication because it the
bases are oriented in the
wrong direction.
5’
5’
DNA polymerase can only
add nucleotides in a 5’ to
3’ direction.

15. DNA REPLICATION
• Step 2: Leading strand
• Primers attach.
• Primers are pre-made
sequences of RNA. These
bind to complementary
regions of the original DNA
once it’s separated.
• Primers serve as tethers
from which replication can
proceed from the 3’ end.
• DNA Polymerase
attaches free
nucleotides to the 3’
end of the primer.
• On the leading strand
there is one primer and the
newly forming DNA follows
the replication fork as the
new DNA is synthesized.

17. DNA REPLICATION
• Step 2: Lagging strand
• The lagging strand creates a particular problem for
replication.
• Since DNA can only be synthesized from the 3’ end of
the primers, the lagging strand would be left
incomplete.
• Multiple primers attach to the lagging strand as the
replication fork moves forward, creating what are
known as “Okasaki Fragments”
• These are usually 1-2000 nucleotides long.

18. DNA REPLICATION
• Step 2: Lagging strand
• If you notice, the Okazaki Fragments are not
joined together.
• A special enzyme, call ligase, joins Okasaki
fragments together to form one continuous
molecule.
Ligas
e
Ligas
e

19. DNA REPLICATION, CONTINUED
Step 3:
The process completes
when all the original
bases have been paired
with a new
complementary
nucleotide.
Each double-stranded DNA
helix is made of one new
strand of DNA and one
original strand of DNA.

20. DNA REPLICATION
Click to animate the image.

21. VISUAL CONCEPT: DNA REPLICATION

22. SUMMARY
What is DNA replication?
Why does it happen?
Know the steps…
On your handout, label:
Where is…
a. Leading Strand
b. Lagging Strand
c. Okazaki fragments
d. DNA Ligase
e. Replication Fork
f. DNA Polymerase
g. All 3’ & 5’ ends.
h. Where helicase should be

23. REPLICATION PROTEINS…QUALITY CONTROL
• Replication involves many proteins that form a
machine-like complex of moving parts.
• These proteins play a key role in making sure that the
process is flawless.
• DNA Helicase unwinds DNA carefully so it doesn’t get
torn.
• Ligase is a protein that ensures proper bonding of
growing Okasaki fragments.
• DNA polymerase adds complimentary nucleotides.
• DNA polymerase also has a “proofreading” function.
• During DNA replication, errors sometime occur and the wrong
nucleotide is added to the new strand.
• This could lead to cancer if not detected.
• If a mismatch occurs, the DNA polymerase has the amazing
ability of being able to backtrack, remove the incorrect
nucleotide, and replace it with the correct one.
• This decreases the chances of the wrong DNA being made,
lessening our chances of CANCER!

24. REPLICATION V TRANSCRIPTION
Based upon what you’ve learned about transcription and
replication, shoulder partner share and come up with
a similarity between transcription and replication.
WHAT IS THE SIMILARITY BETWEEN TRANSCRIPTION &
REPLICATION?
Similarities: transcription and replication both use DNA
as the template for copying.
SHOULDER PARTNER AND ANSWER: WHAT IS A
CRITICAL DIFFERENCE BETWEEN REPLICATION AND
TRANSCRIPTION?
Differences:
In transcription, a new molecule of mRNA is made from
the template DNA by using the enzyme RNA
polymerase.
In DNA replication, a new molecule of DNA is made from
the template DNA by using the enzyme DNA
polymerase.

25. OBJECTIVES
Primary
Compare the process of DNA replication in
prokaryotes and in eukaryotes.
Identify the features of prokaryotic and Eukaryotic
replication on an illustration.
Secondary
Compare the number of nucleotides replicated in
eukaryotes and prokaryotes by calculating the
rates of replication.

26. PROKARYOTIC AND EUKARYOTIC REPLICATION
All cells have chromosomes, but eukaryotes
and prokaryotes replicate their
chromosomes differently.
The main difference between prokaryote and
eukaryote replication is how many start sites
each have.
Eukaryotes have hundreds of start sites.
The start sites then regulate how replication
proceeds in each organism type.

27. PROKARYOTIC DNA REPLICATION
Recall the structure of prokaryotic DNA.
Does anyone remember what the
structure is?
Prokaryotic cells usually have a single
chromosome which is a closed loop
attached to the inner cell membrane.
Replication in prokaryotes begins at a
single site along the loop. This site is
called the origin of replication.

28. PROKARYOTIC REPLICATION
Two replication forks begin at the
origin of replication.
Replication occurs in opposite
directions until the forks meet on
the opposite side of the loop.
The result is two identical loops of
DNA.

29. EUKARYOTIC REPLICATION
Eukaryotic cells often have several chromosomes
which are linear and contain both DNA and protein.
 How many chromosomes do humans have?
 What are the proteins called that hold DNA in nucleosomes?
Eukaryotic replication starts at many sites along the
chromosome.
This process allows eukaryotic cells to replicate their
DNA faster than prokaryotes.

30. EUKARYOTIC REPLICATION
Two distinct replication forks form at each start site and
replication occurs in opposite directions.
This process forms replication “bubbles” along the DNA
molecule.
Replication bubbles continue to get larger as more of the
DNA is copied.
The replication bubbles keep growing until they join
together with other bubbles and complete replication.
Replication is complete when two identical
complementary strands of DNA is formed.

31. EUKARYOTIC REPLICATION
Original DNA
Replication bubbles
Replication bubbles joining
Original + new DNA
New + original DNA

32. PROKARYOTIC AND EUKARYOTIC REPLICATION
Even the smallest eukaryotic chromosomes are often
10 times the size of a prokaryotic chromosome.
Eukaryotic chromosomes are so long that it would
take 33 days to replicate a typical human
chromosome if there were only one origin of
replication.
As such, evolution has allowed Human chromosomes
to replicate in about 100 sections that are 100,000
nucleotides long, each section with its own starting
point.
 Remember, the 46 human chromosomes laid end to end would
measure ≈ 2m. Bacterial chromosomes measure ≈ 0.25cm!
Because eukaryotic cells have multiple replication
forks working at the same time, an entire human
chromosome can be replicated much faster, in only
about 8 hours.
Bacteria replicate their small genomes in minutes…

33. IN-CLASS ACTIVITY: REPLICATION COMPARISON
A
B
C
D
E
F
G
•Label the letters in your notes.
•Save this paper for a follow-up activity that will be turned in at the end of
class.
•Word-bank: Original DNA (x2), New DNA (x2), Replication Forks (x2),
Replication bubble
•What is the difference between prokaryotic replication & eukaryotic
replication?
•Why are they different?

34. SUMMARY
In DNA replication, the DNA molecule unwinds, and the
two sides split. Then, new bases are added to each
side until two identical sequences result.
The replication of DNA involves many proteins that
form a machinelike complex of moving parts.
In prokaryotic cells, replication starts at a single site. In
eukaryotic cells, replication starts at many sites
along the chromosome.

35. Replication concept Check
1. What is the purpose, outcome of DNA replication AND what stage does it happen in the
cell cycle?
To create an identical, duplicate copy of original
DNA, “S”
2. What enzyme is responsible for “unzipping” the DNA double helix?
DNA Helicase
3. What enzyme is responsible for adding nucleotides to the “unzipped” DNA?
DNA polymerase
4. What is the enzyme responsible for bonding the fragments on the lagging strand?
Ligase
5. (Review) What is the enzyme responsible for creating mRNA copies of genes in DNA?
RNA polymerase
6. What is the enzyme responsible for “proofreading” the newly made DNA, checking for
mismatched base-pairs?
DNA polymerase

36. SUMMARY… THE MORE COMPLEX VERSION