DNA Replication
12-2
Memory Check: Mitosis and Meiosis
When Does DNA Replicate?
Why Does DNA Replicate?
When new cells are made…
 DNA must be replicated exactly
 All somatic cells that undergo mitosis get a
new strand of DNA and an “old” one
 This occurs in the S phase of interphase
 Cannot be accomplished without the aid of
enzymes.
Step 1
 Unwind and Unzip!
 Enzyme: DNA helicase
 Action: breaks H-bonds that link bases
together
 Result: Replication Fork made by using
additional proteins that keep the DNA from
re-twisting (keep site open)
Step 2
 Create New DNA Strands
 Enzyme: DNA polymerase
 Action: Adds new nucleotides to the
exposed bases using the base-pair rule; also
proofreads every connection at this time
(avg. 1 error per 2 billion nucleotides)
 Result: 2 new double DNA strands are
created (but still attached)
Step 3
 Detach Enzyme and Re-Coil
 Enzyme: Ligase
 Action: DNA polymerase and proteins
from helicase are removed, gaps filled in
 Result: 2 new DNA molecules twist into
their finalized shape
Food For Thought
 Does this “replication fork” only occur once
and go base by base?
 What could be the potential problem if it
occurred this way?
 Too Slow! In fact, from start to finish it
would take 33 days!
 There are multiple replication forks going at
once all over DNA to speed up the process
Let’s Visualize This!
 http://www.lpscience.fatcow.com/jwanamaker/animations/DNA%20Replicatio
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n%20-%20long%20.html
http://www.johnkyrk.com/DNAreplication.html
http://www.ncc.gmu.edu/dna/repanim.htm
http://highered.mcgrawhill.com/sites/0072437316/student_view0/chapter14/animations.html#
http://207.207.4.198/pub/flash/24/menu.swf