Download DNA Replication

DNA Replication
Structure of DNA
• Understanding how
DNA is built is
important
• The structure allows us
to understand how the
different processes fit
can happen
• There are several
different processes that
can happen with DNA
Structure of DNA
• DNA structure and DNA
function are related
• The way that DNA is
built is very telling to
how it operates
• Let’s review and go
deeper into how DNA is
built
Draw In Your Notes
Time to Prime
• Remember DNA is
made from nucleotides
• The nucleotides are
made from a 5 carbon
sugar
• All five carbons of the
sugar are numbered
• Since there are 5
carbons they are
numbered one through
five
Time to Prime
• Each molecule of DNA
has two distinct and
different ends
• Each end has a 3’ (three
prime) and a 5’ (five
prime) end
• The 3’ and 5’ ends refer
to the carbon on the
sugar that is near the
end
Time to Prime
• Notice how the 5’ end
has a phosphate group
that is left free
• Also notice that the
three prime end has a
hydroxyl group left on
the end
• These are important for
the next step of DNA
replication
Video
• http://www.youtube.co
m/watch?v=IN8pKyQz2
RQ
DNA Replication Overview
• What happens during
the S Phase of the cell
cycle?
• Why does this need to
happen for mitosis to
happen?
• How long does the S
phase take?
DNA Replication Overview
• Watson and Crick
correctly projected that
making copies of DNA
would be based on its
structure
• They said if you peeled
apart DNA down the
middle it would be easy
to copy
• This is because all of the
unpaired bases must be
paired with a specific
base
DNA Replication Overview
• DNA replication is not
much more complicated
than Watson and Crick
predicted
• The basic idea is that DNA
is split down the middle
and each remaining
strand is the template for
a new strand
• This is called the
semiconservative model
because the parental
model is conserved in
DNA replication
DNA Replication Overview
• Human DNA has over 6
billion base pairs
• However it is copied in
under three hours
• During that three hours
DNA is copied
overwhelmingly
accurately
• Less than 1 mistake is
made in every 10,000,000
base pairs copied
Demo
• To understand how
accurate DNA
replication is… lets do
an experiment
• Try this…
• Have one partner sit
facing the blackboard,
one sit facing the
opposite direction
• The partner that faces
the direction of the
board will describe and
the other partner will
draw
Lets see how many mistakes you make!
Now Switch
• If DNA Replication did
that process 100 times
a day it would take
10,000 days before it
made a mistake
• That about 27.4 years
without a mistake
• That is about as old as
these celebrities
Meghan Fox
Dwight Howard
Mr. Owdij
DNA Replication Enzymes
• There are many
different enzymes that
contribute to DNA
replication
• However there are
three major enzymes
that are involved
• These three enzymes
allow DNA replication to
proceed
DNA Replication Enzymes
• DNA Helicase is the
enzyme unzips the DNA
strand
• Its job is to separate the
DNA strand and make
sure it does not
reattach
• It takes away the
hydrogen bonds that
hold together the bases
DNA Replication Enzymes
• DNA Polymerase is the
principal enzyme
involved with DNA
Replication
• It has 2 main functions.
• It “polymerizes” or
creates a complex
compound by linking
together nucleotides
• It also proofreads and
corrects any mistakes
that happen in the DNA
strands
DNA Replication Enzymes
• DNA ligase pieces
together the segments
of DNA to make one
complete molecule
• It fits together
fragments that allow
DNA to come together
completely
DNA Replication
• DNA Replication starts on
sections of DNA called
origins of replication
• These are specific
sections of DNA that code
in a specific way where
enzymes can attach and
separate the strands
• There can be hundreds of
origins of replication on
any given strand of DNA
DNA Replication
• This means that DNA
replication happens
many different places
on any given strand of
DNA
• This creates many
different replication
bubbles that will
eventually meet and
merge
DNA Replication
• When completed the
replication bubbles will
create four strands of
DNA
• One original strand
(daughter strand) will be
attached to an older
strand (parental strand)
• This means that DNA
conserved through the
process of DNA
replication
One Way Street
• DNA Replication actually
happens two different
ways on any given strand
• This is because DNA
replication can only
happen in the 5’ – 3’
direction
• Since both strands of DNA
are not going in the 5’ – 3’
directions there are
actually two different
methods for completing
DNA replication
One Way Street
• Lets start with the
strand that is already in
the 5’ – 3’ orientation
• This strand is the
simplest to replicate
• This strand uses a very
linear strategy to
replicate
• This strand is called the
leading strand
One Way Street
• First DNA Helicase will
break hydrogen bonds
at an origin of
replication
• The Helicase separates
the strands and creates
a replication fork
• This is a site where both
DNA strands are
separated and have
unattached nucleotides
One Way Street
• After the DNA Helicase
breaks the hydrogen
bonds DNA polymerase
follows and attaches
the appropriate
nucleotides to create a
stable new strand
• DNA polymerase also
“proofreads” the
nucleotides to ensure
the stability of the DNA
3’ – 5’
• The daughter strand that
is oriented in the 3’ – 5’
direction has to be done
differently
• This direction is
synthesized in segments
that are fitted together
• It is a slightly more
involved process but it
has the same general idea
• This strand is called the
lagging strand
3’ – 5’
• The process starts the
same way
• DNA helicase unzips the
DNA at a replication fork
• However DNA
polymerase has to fit
together the bases in
small segments called
Okazaki fragments
• These are small sections
of nucleotides that are
attached to the 3’ – 5’
strand
3’ – 5’
• After the Okazaki
fragments are fitted
DNA ligase is used to
attach all of the
fragments together
• Once this is completed
you have a daughter
strand to the 3’ – 5’
parent strand
Video
Animation
Animation 2
Routine Maintenance
• During the process of
DNA replication several
other steps are
happening to ensure
the health and well
being of DNA
• During replication DNA
ligase and polymerase
are fixing any routine
damage that could have
happened to DNA
Routine Maintenance
• DNA can have routine
damage from any
variety of sources
• X Rays, ultraviolet
radiation, toxic
chemicals and tobacco
are common chemicals
that damage DNA