10/12/2014
Review
DNA Replication
• What does replication mean?
• Why do we need to replicate DNA?
• When does this occur?
Date ___________
DNA Replication
• Purpose: make more DNA
before cell division so new
daughter cells will have
the same DNA as the
parent cell
• When?: S phase of
interphase
• How much?: ALL OF IT!
Step 1: Unwinding
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DNA helicase (enzyme) unwinds and unzips the
double helix. This creates a replication fork, or
bubble.
Once unzipped, hydrogen bonds between the
bases are broken, leaving single strands of DNA.
Single-stranded binding proteins associate with
the DNA to keep the strands separate during
replication.
As the helix unwinds, RNA primase (enzyme)
adds a short segment of RNA, called an RNA
primer, to each DNA strand.
Step 2: Base Pairing
Step 3: Joining
– DNA polymerase (enzyme) adds nucleotides to the 3’
end of the new DNA strand: A – T, C – G.
– The templates allow identical copies of the original
double-stranded DNA to be produced.
• Even though the leading strand is synthesized
continuously, in eukaryotic DNA replication there
often are many areas along the chromosome where
replication begins.
• When the DNA polymerase comes to an RNA primer
on the DNA, it removes the primer and fills in the
place with DNA nucleotides.
• When the RNA primer has been replaced, DNA ligase
(enzyme) links the two sections.
• The leading strand is made as DNA unwinds. This strand is
built continuously by the addition of nucleotides to the 3’
end.
• The lagging strand is made away from the replication fork.
It is synthesized discontinuously in the 3’  5’ direction
into small segments called Okazaki fragments. These are
later connected by DNA ligase (enzyme).
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10/12/2014
A few more notes:
• DNA is anti-parallel
– This means that each strand runs in opposite
directions. As a reference, one side is said to run
in a 5’  3’ direction while the other runs 3’  5’.
– DNA replication must always occur 5’  3’, so the
leading strand is 3’  5’ while the lagging strand
is 5’  3’.
• DNA replication is semi-conservative
– This means that each NEW DNA molecule is ½ old
(parental) and ½ new material.
Eukaryotes v. Prokaryotes
Eukaryotic DNA Replication
Prokaryotic DNA Replication
Linear DNA
Circular DNA
Many origins of replication
Only one origin of replication
Occurs in two directions at
once
Occurs in two directions at
once
DNA is longer
DNA is shorter
DNA is in the nucleus
DNA remains in the cytoplasm
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