Replication of DNA
Objectives:
1. Summarize the roles of the different
enzymes involved in replication of DNA.
2. Explain how the leading and lagging
strands are replicated differently
Semiconservative Replication
• When Watson and Crick proposed the model
of the structure of DNA they also proposed a
method of its replication, Semiconservative.
Aspects of Semiconservative
Replication
1. Original strands
separate, and serve as
template strands.
2. These template strands
copy themselves forming
two molecules of DNA
3. Each DNA molecule has
one strand of parental
DNA and one strand of
new DNA
3 stages of the process of
Semiconservative replication
Unwinding
Base pairing
Joining of the strands
Unwinding
The Double Helix of DNA must be unwound in
order to allow base pairing (copying) of the
DNA molecule.
An enzyme called Helicase unwinds the DNA
molecule.
Helicase does this by breaking the weak
hydrogen bonds between the two strand of
DNA.
The Enzyme Helicase
http://www.youtube.com/watch?v=UWNhwce
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Base Pairing
The enzyme DNA polymerase attaches
appropriate nucleotides to the new DNA
strands
The nucleotides are added to the 3’ so we say
that the DNA synthesis moves in the 5’ to 3’
direction.
Okazaki Fragments
and the leading and lagging strand
The two strands are made in a different manner.
One strand is called the leading strand. This strand
is built continuously from the 5’ to 3’ ends.
But, since the strands are antiparallel, the lagging
strand can not be built continuously.
This strand is built into small segments, called
Okazaki fragments.
The fragments are later connected by the enzyme
DNA ligase.
http://www.youtube.com/watch?v=teV62zrm2
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Joining the Strands
• When the DNA molecule is finished replicating
the enzymes release the strands and they
reform two double helix’s, Each with a
parental template strand, and with a new
strand from replication.
Comparing DNA Replication between
Prokaryotes and Eukaryotes
• In Prokaryotes the circular DNA strand is
opened at one origin of replication. In
Eukaryotes, with up to one million bases pairs
to copy there are multiple origins of
replication.
• Both replicate in two directions, that is to say
they are both semiconservative (one parent
strand and one new strand).