1. How Does DNA Copy Itself?
12.3 DNA Replication
How Does DNA Copy Itself?

2. DNA Replication – What and Why
Replication = DNA making copies of itself
DNA must be copied before a cell can divide
Each new cell will have
a complete set of DNA

3. History of Replication
Watson & Crick – realized that
complementary base pairing
provided a way for DNA to copy itself
- base pairing could allow a new
strand to be built on an old strand
- 3 possible models of DNA
replication

4. 3 Models of Replication: Semi-conservative Replication
Each strand in a DNA molecule
is used as a template to build
a new strand using
complementary base pairing
Results in new molecule
with one original DNA strand
and one new strand

5. 3 Models of Replication: Conservative Replication
Leaves the original DNA
intact and produces another
molecule of DNA identical to
the first

6. 3 Models of Replication: Dispersive Replication
Creates two molecules that
are a mix of new and old DNA
interspersed along each
strand of the molecule

7. The Question is Answered
1957 Matthew Meselson & Franklin Stahl
-conducted an
experiment proving
replication is
semiconservative
- each DNA molecule
has one new strand
and one old strand

8. DNA Replication: Process
@Replication begins when the enzyme DNA helicase opens the DNA forming replication

9. DNA Replication: Process
Multiple replication bubbles are opened simultaneously allows the molecule to be replicated quickly

10. DNA Replication: Process
The nitrogen bases
on the original DNA
strands are exposed
in the replication
bubbles.
They serve
as a template to
build new DNA
strands

11. DNA Replication The ends of the replication bubbles known as the
replication fork is where replication begins

12. DNA Replication – DNA Polymerase
@The enzyme DNA polymerase brings new nucleotides to the
replication
- it pairs them according to base pairing rules A pairs with T
C pairs with G

13. DNA Replication – Leading Strand
The process of replication
proceeds from 5’ to 3’
leading strand- replication
is continuous
– it is built toward the replication fork
Up to 1000 base pairs/ second

14. DNA Replication – Lagging Strand
lagging strand- replication
occurs in short segments
- called Okazaki fragments
- the new DNA strand grows
away from the replication
fork

15. DNA Replication- The Big Picture
Each Bubble has 2 Forks – each fork has a leading and
lagging strand

16. DNA Replication
The process continues until 2 complete copies of the DNA are produced
@Each copy of the DNA has 1 strand of DNA from the original DNA and one new strand that was produced by
Known as semi-conservative replication

17. DNA Replication

18. DNA Replication

19. Telomeres The tips of chromosomes are known as telomeres.
Over time, DNA may actually be lost from telomeres each time a chromosome is replicated.
@An enzyme, telomerase, solves this problem by adding short, repeated DNA sequences to

20. Prokaryotes vs. Eukaryotes
Prokaryotes- starts from a single point and goes in two directions until the entire chromosome is copied.
Eukaryotes- replication begins at dozens or even hundreds of spots on the DNA, going in both directions until each chromosome is completely copied.

21. Which enzyme is used to separate the DNA?
Helicase
Ligase
Polymerase
Telomerase

22. Which enzyme adds nucleotides to the strands?
Helicase
Ligase
Polymerase
Telomerase

23. Which enzyme prevents DNA from being lost during replication?
Helicase
Ligase
Polymerase
Telomerase

24. How is Prokaryotic DNA different from Eukaryotic?
Prokaryotic DNA is circular and replication begins at 1 point.
Eukaryotic DNA is linear and replication begins at many points.