Replication of DNA
• Before a cell can divide by mitosis or meiosis, it
must first make a copy of its chromosomes.
• The DNA in the chromosomes is copied in a
process called DNA replication.
• Without DNA replication, new cells would have
only half the DNA of their parents.
• DNA is copied during interphase prior to mitosis
and meiosis.
• It is important that the new copies are exactly like
the original molecules.
Website upload 2015
Replication of DNA
• DNA replication depends on specific base pairing
• In DNA replication, the strands separate
– Enzymes use each strand as a template to
assemble the new strands
Nucleotides
Parental molecule
of DNA
Both parental strands serve
Website upload 2015
as templates
Two identical daughter
molecules of DNA
Replication of DNA
Semiconservative replication:
• Parental strands of DNA separate, serve as templates,
and produce 2 molecules of DNA that have one
strand of parental DNA and one strand of ‘new’ DNA
Website upload 2015
Copying DNA
• Matching bases allows DNA to
be easily copied
Website upload 2015
Separating of DNA
• DNA Helicase: enzyme responsible for uncoiling
the double helix and unzipping the weak
hydrogen bonds between the base pairs
Website upload 2015
DNA replication
DNA bases
in nucleus
• Enzyme
– DNA polymerase
– adds new bases
DNA
polymerase
Website upload 2015
Replicating DNA
 Build daughter DNA
strand
use original parent
strand as “template”
 add new matching bases
 synthesis enzyme =
DNA polymerase

DNA
Polymerase
Website upload 2015
Replicating DNA
• Adding bases
– only add
nucleotides to
3 end of a growing
DNA strand
• need a “starter”
nucleotide to
bond to
– strand only grows
53
5
3
energy
DNA
Polymerase III
energy
DNA
Polymerase III
energy
DNA
Polymerase III
energy
DNA
Polymerase III
3
Website upload 2015
5
Leading & Lagging strands
• Leading strand - elongates as DNA unwinds
• Lagging strand - elongates in opposite
direction.
– Synthesized discontinuously into small
segments called Okazaki fragments.
• DNA ligase (enzyme) links these sections.
Website upload 2015
Okazaki
Leading & Lagging strands
Limits of DNA polymerase III

can only build onto 3 end of an
existing DNA strand
5
3
5
5
5
3
3
5
5

Lagging strand
ligase
growing
3
replication fork
Leading strand

3
Lagging strand


3
Okazaki fragments
joined by ligase
 “spot welder” enzyme
5
3
DNA polymerase III
Leading strand
Website upload 2015

continuous synthesis
New copies of DNA
• Results in 2 exact copies of DNA to split between
reproduced cells
DNA
polymerase
DNA
polymerase
Website upload 2015