12.3 DNA Replication
How Does DNA Copy
Itself?
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
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
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
3 Models of Replication:
Conservative Replication

Leaves the original DNA
intact and produces another
molecule of DNA identical to
the first
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
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
DNA Replication: Process

@Replication begins when the enzyme DNA
helicase opens the DNA forming replication
bubbles@
DNA Replication: Process

Multiple replication bubbles are opened
simultaneously allows the molecule to be
replicated quickly
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
DNA Replication
 The ends of the replication bubbles known as the
replication fork is where replication begins
DNA Replication – DNA Polymerase

@The enzyme DNA polymerase
brings new nucleotides to the
replication fork@
- it pairs them according to base
pairing rules A pairs with T
C pairs with G
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
DNA Replication – Lagging Strand

lagging strand- replication
occurs in short segments
- called Okazaki fragments
- the new DNA strand grows
away from the replication
fork
DNA Replication- The Big Picture
Each Bubble has 2 Forks – each fork has a leading and
lagging strand
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 replication@

Known as semi-conservative replication
DNA Replication
DNA Replication
http://highered.mcgrawhill.com/sites/9834092339/student_view0/chapter
14/dna_replication.html
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 telomeres@
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.
Which enzyme is used to separate
the DNA?
A.
B.
C.
D.
Helicase
Ligase
Polymerase
Telomerase
Which enzyme adds nucleotides to
the strands?
A.
B.
C.
D.
Helicase
Ligase
Polymerase
Telomerase
Which enzyme prevents DNA from
being lost during replication?
A.
B.
C.
D.
Helicase
Ligase
Polymerase
Telomerase
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.