1. The wonderful world of DNA
We looked at mitosis last week, now we are going to see what happens inside of interphase.
So then the question is, what happened in interphase?

2. Replication
This should be an easy topic – after all, it is the specialty of many high school students – but not our Biology students of course…

3. Crazy, but right
Watson and Crick, in their paper describing the structure of DNA, proposed the mechanism of replication.
They suggested that since each strand is complementary to its partner, it could serve as the template for a newly synthesized strand.

4. Meselson and Stahl (important people who paved the way for W&C)
The exact way this happens was demonstrated by Meselson and Stahl.
Their experiments supported the idea of semiconservative replication.
(Not in your book, however important) Lets break down the previous word.

5. The possibilities

6. The experiment

7. DNA polymerase and its posse
Replication requires a key enzyme (DNA polymerase III) and a whole posse of enzymes.
You DO need to know these enzymes and their functions…in case you were wondering…

9. DNA Polymerase III (you can leave off the III)
DNA Polymerase III synthesizes a new DNA strand from free nucleotides. It does this by reading the template strand and attaching the correct nucleotide according to the AT and CG base pairing rules.
It does this at a rate of 50 nucelotides/second.
Q: How does it accomplish the replication of the entire human genome in only a few hours?
A: There are MANY DNA polymerases working simultanesouly, each having a different origin of replication.

11. The Origin
An origin is a point at which replication begins. Along a linear chromosomes, there are many origins.
An origin will expand into a replication bubble.
Each replication bubble has two replication forks.

13. The posse
DNA replication requires several highly specialized enzymes and other molecules. We’ll attempt to go through them in a somewhat chronological order.

14. DNA Helicase
Helicase unzips the helix. It opens the DNA so that DNA polymerase can have access to the template.

15. Single Stranded Binding Proteins
SSBPs bind to the DNA at the replication fork and keep it unwound.
DNA doesn’t like to be single stranded and would otherwise close when it had the chance.

16. Primase Primase lays down an RNA primer.
DNA polymerase requires a primer to begin because it needs a free 3’ OH to add nucleotides onto.

17. DNA polymerase III
The all-star!

18. DNA polymerase I
This enzyme removes the RNA primer and fills in the space with DNA nucleotides.
This enzyme is also involved in proofreading and repair.

19. Ligase
Ligase is an enzyme that seals the gaps between adjacent nucleotides that have not yet been linked by a phosphodiester bond.

20. A special problem
Because DNA polymerase can only add onto a 3’ OH, it synthesizes one strand directly, but the other strand runs in the ‘wrong direction.’
Another way of stating this problem is to say that DNA READS the template in the 3’ to 5’ direction and SYNTHESIZES in the 5’ to 3’ direction.

21. Problem Solved
To deal with this problem, DNA polymerase synthesizes the ‘leading strand’ continuously and synthesizes the ‘lagging strand’ discontinuously.

23. Okazaki fragments
This ‘discontinuous’ synthesis is done in the form of Okazaki fragments. The DNA of the lagging strand is looped back and around so that it ‘appears’ to be in the correct orientation for the DNA Polymerase III. This is done repeatedly at short intervals. Each interval is called an Okazaki fragment.
These fragments each need a primer.

25. Coming soon…
We’ll next discuss transcription – the process of DNA serving as a template for a molecule of RNA.

26. For your interest
Some very odd genetic mutations recently discovered living inside a dormant volcano on an island in the pacific.