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DNA REPLICATION
What would you do?
• Imagine being presented with a book that is
composed of about 1 billion pages, and you
are asked to find a way to make 100 copies, at
once, within a 20 minute time period
Your body, every day
• If you can wrap your head around the fact that an
average cell cycle requires a full 20 minutes to
complete – and the fact that each new cell that
comes along requires a brand-spanking new copy
of your DNA, you can imagine the legwork
required to maintain the health of your body
• Every time a cell is replaced in order to repair
damage, or every time growth has to occur, your
DNA has to be copied so a new cell has the
complete instruction manual
DNA is annoying to copy
• The truth is, never mind having to worry about copying
3 billion base pairs accurately (mistakes are bad –
remember this is your body we are talking about) but
you also have to deal with an enormous molecule that
is twisted and complex in structure
• So imagine that your task just got more annoying
because the instruction manual you have been asked
to copy is housed in complicated binders that you have
to open, remove papers from, put in the copier, and
copy – all at the same time keeping track of everything
and not mixing up pages
On top of that…
• Think very carefully about DNA structure – the
coded information for your cell is found within
the base pair sequence of DNA
• But in truth, where are these base pair
sequences? They are in fact closed off from
the outside world; remember that the
structure of DNA is a double helix – it is very
difficult to get to the base pairs because DNA
is all wound up to begin with
The challenges of DNA replication
• Therefore, DNA replication requires some finesse the
major steps involved include:
– Unwinding DNA to expose the base pairs
– Copying the base pairs
– Putting the base pairs together in a new DNA molecule
• Not too much different from photocopying a big
book:
– Take the pages from the original
– Photocopy the pages from the original
– Put the new pages together in the new copy
Use a template
• There are a few ways to get around this that
DNA replication uses to minimize mistakes
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Meselson%20and%20Stahl%20Experiment
http://www.tvdsb.on.ca/westmin/scienc
e/sbi3a1/genetics/replicat.htm
Matthew Meselson and Franklin Stahl
• Provided experimental evidence that DNA replicates semiconservatively
• They grew E.coli bacteria in two different cultures: one
containing heavy nitrogen (AMU = 15, therefore 15N) and one
containing light nitrogen, 14N
• The first generation of bacteria was grown in the heavy
nitrogen, and then switched to the lighter nitrogen
• The idea behind their experiment was that as each generation
of bacteria grew, the density of the DNA would change as
they incorporated more and more of the lighter DNA
• Therefore, if they were to separate the DNA from the group
of bacteria afterwards via centrifuge, they would find that
each successive generation would yield a different density in
their DNA
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22.swf::Meselson%20and%20Stahl%20Experi
ment
Yay! Proteins!
• These hosts of proteins go to the DNA strand
in order to complete DNA replication
Prep to copy
• Before DNA replication, the chromosomes
start to unwind in order to reveal the
chromatin that makes up DNA
• The DNA is unwound until individual strands
can be accessed by the proteins responsible
for DNA replication
Unwinding
• The next step is to unwind DNA – DNA is somewhat like a rope
composed of two strands – what happens when you try to
pull apart the two strands?
• Without
topoisomera
ses, the
unwinding
DNA strand
will bunch
up
eventually
and get
knotted up –
which would
bring DNA
replication to
an end
Getting started
• To begin, a small piece of DNA is laid down by an enzyme known as
RNA PRIMASE
• This enzymes “primes” or gets DNA replication started by laying down
a short piece of RNA on the original strands of DNA
DNA Polymerase III
• As its name indicates, DNA polymerase III “polymerizes”
nucleotides – it joins them together to form a new DNA strand
Fork it
• As DNA is being replicated, it is being split open to form a Yshaped structure termed a DNA REPLICATION FORK
Leading and lagging
• Therefore, there is a difference in the manner in which the two new daughter
strands are synthesized
• The LEADING STRAND
• On the LAGGING STRAND
Okazaki fragments
• On the lagging strand, RNA primase makes short RNA primers all
along the length of the strand
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RNA
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o23.swf::How%20Nucleotides%20are%20Added%20in%20DNA%20Replication
Telomeres
• In prokaryotes, circular DNA means that DNA
polymerase, at the end, always has another
nucleotide to attach to
• However, in many eukaryotes, DNA is linear
• So what happens to those primers that are left
at the ends of the daughter strands?
• http://web.centre.edu/bmb/movies/Telomere
s.html
Save the end bits
• Some theories suggest that telomeres may be responsible for aging
– a theory proposed after the early death of the cloned sheep Dolly
Name
Topoisomerase
Helicase
ssBP
RNA Primase
Leading strand
Lagging strand
DNA Polymerase III
DNA Polymerase I
Ligase
Okazaki fragments
Function