2. DNA Replication 12-2 Memory Check: Mitosis and Meiosis When Does DNA Replicate? Why Does DNA Replicate?

3. 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.

4. 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)

5. 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)

6. 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

7. 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

8. Let’s Visualize This!  http://www.lpscience.fatcow.com/jwanamaker/animations/DNA%20Replicatio n%20-%20long%20.html http://www.lpscience.fatcow.com/jwanamaker/animations/DNA%20Replicatio n%20-%20long%20.html  http://www.johnkyrk.com/DNAreplication.html http://www.johnkyrk.com/DNAreplication.html  http://www.ncc.gmu.edu/dna/repanim.htm http://www.ncc.gmu.edu/dna/repanim.htm  http://highered.mcgraw- hill.com/sites/0072437316/student_view0/chapter14/animations.html# http://highered.mcgraw- hill.com/sites/0072437316/student_view0/chapter14/animations.html#  http://207.207.4.198/pub/flash/24/menu.swf http://207.207.4.198/pub/flash/24/menu.swf