DNA Replication
The Structure
of DNA
What we know…
 Double helix

Each nucleotide is
made up of:



Deoxyribose (sugar)
phosphate group
base
The Structure
of DNA

Base-Pairing Rules
(Chargaff)

DNA strands are
antiparallel

*Know
experiments:
Griffith, HersheyChase
Models of replication
1.Conservative

Two parental strands came back together
after replication
2.Semi-conservative

One new and one old strand in a newly
synthesized double stranded DNA
3.Dispersive

All 4 strands post-replication have a mixture
of old and new DNA

Meselsohn and Stahl labeled the old strand
with N15 (heavy). New strands will have N14.
 Centrifugation after replication supports the
semi-conservative model.
Semi-Conservative Replication

Each new DNA
molecule has one old
strand and one new
strand
DNA
Replication…
how it works

Occurs in the nucleus

A new and identical
molecule of DNA is
made, using the old
one as a template
DNA Replication

origin
of replication
begins at the

a special
sequence of DNA

2 strands are separated
by helicase, forming a
replication bubble

Replication fork is
formed at each end of
the replication bubble
DNA Replication
 At
replication fork, nucleotides “line up”
with their complementary mates
 DNA polymerase
III attaches the
nucleotides to the exposed bases of the
DNA strand
DNA Replication: A Summary
 http://www.youtube.com/watch?v=OnuspQ
G0Jd0 (bioflix)
 https://www.youtube.com/watch?v=27TxK
oFU2Nw (3d medical)
Leading
Strand

DNA polymerase can
only add nucleotides
to the 3’ end of a DNA
strand

In the leading strand,
DNA polymerase III
follows the replication
fork
Lagging
Strand

DNA polymerase
works in the opposite
direction of the
replication fork

Short segments of
DNA are made


Okazaki fragments
Okazaki fragments
are joined by DNA
ligase
DNA Proofreading
 DNA polymerase
I proofreads each
nucleotide as it is added to the DNA strand
 If



there’s a mistake…
it backs up
removes the wrong nucleotide
adds the right nucleotide
Enzymes & Their Job in Replication





Helicases- unwind the DNA strand
Primase- inserts RNA primer to begin replication
process.
DNA Polymerase III- adds complementary bases to
3’ end of primer or new DNA strand.
DNA Polymerase I- removes RNA primer & inserts
DNA nucleotides. (also proofreads)
DNA Ligase- “sews” Okasaki fragments of lagging
strand together with covalent bonds.