1. DNA Replication
Biology 12

2. Two Jokes today….because I forgot last time!

3. What is DNA replication?
Process by which DNA copies itself
Before a cell starts mitosis, it replicates its DNA

4. Semi-Conservative Replication:
Each strand acts as a template for a new double helix.
Each new double-stranded molecule is composed of one parental strand and one new strand. D ds

5. Important: The two strands in DNA are anti-parallel
One strand is in a 5' to a 3' while the other is in the opposite direction( 3' to 5' )

6. Short video clip (1st time):0

7. The Steps:

8. 1. Separation of Strands
Weak hydrogen bonds that hold bases together are “unzipped” by an enzyme called DNA helicase

9. 2. Keeping the strand separated
Single stranded binding proteins (SSBs) prevent the strands for sticking back together (reannealing)

10. Note:
Replication proceeds only in the 5’ to 3’ direction (on the daughter strand)!
This makes one side much simpler than the other.

11. 3. Building Complimentary Strands
Primase creates a RNA primer which allows DNA polymerase to start attaching new nucleotides
DNA polymerase continues until it reaches another RNA primer

12. 3. Building Complimentary Strands
Leading strand
Built continuously
From parent 3’ end toward replication fork

13. 3. Building Complimentary Strands
Lagging strand
Built in short Okazaki fragments between RNA primers
Discontinuous
From parent 3’ end
Away from
replication
fork

14. Another picture….
Okazaki fragments

15. 3. Building Complimentary Strands
DNA ligase
Joins Okazaki fragments together
Links all the new pieces of DNA together
Lagging strand only!

16. 4. Proofreading DNA polymerase checks for proper base pairing
If mistake found:
Removes
Replace with proper
nucleotide

17. Short video clip (2nd time):0

18. To do : Re-read page 217-222 Page 229: Page 230
Questions 1, 3-6, 9-15
Page 230
Questions 3, 6, 10,13,14
Biology 12 is so much fun!