DNA REPLICATION
What does it mean
to replicate?
The production of
exact copies of
complex molecules,
such as DNA
molecules, that
occurs during growth
of living organisms
When does
replication occur?
• DNA replication takes
place during the
synthesis (S) phase
of the cell cycle.
Early Models of DNA Replication
Conservative Replication
Dispersive Replication
The original helix of DNA is
completely copied to make a
new helix.
The original helix of DNA is
broken down and
interspersed with new pieces
of DNA.
The Accepted Model of Replication
Semi-Conservative Replication
In this model, the
parental strands of DNA
separate. Each strand
serves as a template in
order to produce DNA
molecules that have one
strand of parental DNA
and one strand of new
DNA.
General Information
Since DNA is antiparallel the strands are replicated
differently. Replication begins with the original
strands unwinding at a location called the
REPLICATION FORK.
• One strand follows continuous, smooth replication
and is called the LEADING STRAND. The leading
strand is synthesized in the 5’ to 3’ direction.
• One strand is synthesized in several small pieces and
is not continuous. This strand is called the LAGGING
STRAND. The lagging strand is also synthesized in
the 5’ to 3’ direction.
DNA Replication: The Leading Strand
1. DNA HELICASE unwinds and unzips the DNA double helix by
2.
3.
4.
5.
breaking the hydrogen bonds between the bases.
Proteins called SINGLE-STRANDED BINDING PROTEINS
bind with the DNA to keep the strands separate during
replication.
RNA PRIMASE adds a short segment of RNA, called an RNA
primer, to the DNA strand.
DNA POLYMERASE attaches to the RNA primer and begins
adding DNA bases to the 3’ end of the strand, therefore
synthesizing new DNA. Polymerase adds by matching the
correct base pairs to the new strand.
RNAse H removes the RNA primers initially put on the strand.
Practice Building DNA
• If your original strand had the following base sequence, what
bases would RNA primase add to create the RNA primer?
A C A G T
T
A G
• If your original strand had the following base sequence, what
bases would DNA polymerase add to create a complementary
strand OF DNA?
A C A G T T A G
DNA Replication: The Lagging Strand
1. DNA HELICASE unwinds and unzips the DNA double helix
2.
3.
4.
5.
by breaking the hydrogen bonds between the bases.
Proteins called SINGLE-STRANDED BINDING PROTEINS
bind with the DNA to keep the strands separate during
replication.
RNA PRIMASE adds a short segment of RNA, called an
RNA primer, to the DNA strand. Since the strand is
synthesized discontinuously, many primers are added.
DNA polymerase adds bases to the RNA primers to form
many small fragments, known as OKAZAKI FRAGMENTS.
RNAse H removes the RNA primers initially put on the
strand and the Okazaki fragments are joined by DNA
LIGASE.
Lets watch replication take place!
Click on the link below!
http://www.wiley.com/college/pratt/0471393878/student/ani
mations/dna_replication/index.html
Comparing Replication
Eukaryotes
Prokaryotes
• DNA is able to unwind in
• DNA is unwound at one
multiple areas along the
strand during replication
• Occurs in the nucleus
origin of replication and
takes place in both
directions.
• Occurs in the cytoplasm