Replication
of DNA



The process of making a copy of DNA
is called DNA REPLICATION.
Why does DNA need to be copied???
DNA contains all the information
required for the function of a cell and
the organisms.

Remember the base pair rule:
A
always pairs with T
 G always pairs with C
 This
pairing allows DNA to act
as a template (a guide or
pattern) for its own
replication.
SUMMARY OF DNA REPLICATION
Do not copy
DNA replication occurs in 4 stages:
1. “Unzipping” of DNA helix
2. Priming of DNA
3. Addition of nucleotides to the
exposed nitrogen bases.
4. Proofreading

ENZYMES play a major role in DNA
replication
1. “UNZIPPING” OF DNA STRANDS

DNA replication
occurs during the
synthesis (S) phase
of the cell cycle,
before a cell divides.
a)The double helix
unwinds with the
help of DNA
HELICASES. These
enzymes break the
hydrogen bonds
between the base
pairs.
b. Once the two
strands are
separated,
additional proteins
hold them apart
preventing them
from rewinding.
The areas where
the double helix
separates are
called replication
forks because of
their Y shape.
2. PRIMING OF DNA

The exposed bases have to be
“primed” (prepared) to accept new
nucleotides. This is done with the help
of PRIMASE:
 short
molecules called primers attach to
the exposed bases and serve as a starting
point for DNA synthesis.
3. ADDITION OF NUCLEOTIDES
Once the helix is unwound and primed
an enzyme called DNA POLYMERASE
moves along each strand of DNA
adding free-floating nucleotides to the
exposed bases, according to the basepairing rules.
 HOWEVER NOTHING IS AS SIMPLE AS
IT SEEMS!!

Remember the numbering of the sugar molecules in
DNA:
One strand of nucleotides will be pointing with the 5’
end at the top while the other strand will be going in
the opposite direction.
3’
2’
P
C
4’
5’
O
Nitrogen
Base
C
3’
C
C
C
2’
C
1’
C
1’ Nitrogen
Base
4’
O
5’
C
P
ADDITION OF NUCLEOTIDES CONT.

DNA polymerase can only synthesize a
new strand of DNA in a 5’ 3’ direction.

Since replication can
only proceed from a
3’ to the 5’ direction
of the original
molecule, on the
contiguous
(LEADING) strand,
nucleotides are
inserted in a smooth
continuous manner.

The strand that is oriented in the 3’ to
5’ direction is known as the lagging
strand because nucleotides are added
in small segments called “Okazaki”
fragments. These fragments are
eventually joined by an enzyme called
“ligase” into one long strand.
DO NOT COPY

The human DNA is up to 80 million base pairs
long. As a result DNA is”unzipped” at multiple
places along its length and DNA replication
steps are carried out simultaneously at many
places.
Fact File: The human DNA is copied at about
50 base pairs per second. The multiple
location of DNA replication process takes about
1 hour to complete. If this were not the case,
then it would take about a month to finish
replicating the entire DNA strand!
4. PROOFREADING

DNA replication is not
complete until several
enzymes including DNA
polymerase have
proofread the nucleotides
being added to the strand.
If they are not found to be
complementary they are
removed and the correct
ones are added.

DNA replication is considered “semiconservative” since ½ of the original
strand is preserved in each new
double helix.
Step Name
1. Unzipping
2. Priming
3. Adding
nucleotides
4. Proofreading
Enzyme(s) Used
Process
DNA helicase
Helicase unwinds DNA at
replication forks by breaking
hydrogen bonds between bases.
DNA primase
Primase adds short segments of
bases (5-10) to 5’ ends of new
strand.
DNA polymerase
DNA ligase
DNA polymerase
Other enzymes
Polymerase adds bases to 5’
ends of new strand. Outside
strand called leading strand.
Inside strand called lagging
strand. Lagging strand forces
polymerase to work “upsidedown” making it fall off at many
points. Forms Okazaki
fragments. Ligase adds bases to
join Okazaki fragments.
Polymerase and other enzymes
make sure correct bases are
added. If incorrect, removes
them and adds correct ones.