1. DNA REPLICATION AND REPAIR
SBI 4UI – 4.3

2. Reproduction A crucial property of organisms
Ability to reproduce
Reproduction requires DNA replication
Mitosis
Daughter cell has exact copy of parent’s DNA

3. DNA Replication DNA has a double-helical structure
How might it replicate most easily?
Hydrogen bonds break between bases
Each strand acts as a template
Resulting in 2 identical DNA molecules

6. Semiconservative Replication
Each DNA molecule consists of a parent strand and a newly synthesized strand
Meselson and Stahl
Does E. Coli use semiconservative replication?
Used a centrifugation process to separate DNA of successive generations

8. Semiconservative Replication
Found that parent DNA remained intact through several generations
Other scientists soon proved the same thing for eukaryotic cells

9. Process of DNA Replication
Replication begins at a specific site known as the replication origin
Prokaryotic DNA has only one, eukaryotic has several
To replicate DNA must be first unwound and separated

10. Process of DNA Replication
DNA helicase
Unwinds double helix by breaking H bonds between complementary bases
Single-stranded binding proteins
Binds to exposed DNA to block hydrogen bonds from reforming

11. Process of DNA Replication
DNA gyrase
Relieves tension from unwinding DNA

12. Replication Fork
Junctions where DNA still joined is called the replication fork
In eukaryotes
Multiple sites of origin
Therefore, many replication forks
Thus, rapid replication of DNA

13. Replication Rules
DNA is always synthesized in a 5’ to 3’ direction

15. Building Complementary Strands
DNA Polymerase III
Adds nucleotides in a 5’ to 3’ direction
RNA primer
Sequence of RNA bases used to initiate DNA replication
Primase: the enzyme that build RNA primers

16. Leading Strand Uses 3’ to 5’ template as a guide
Built towards replication fork
Continuous

17. Lagging Strand Built away from replication fork Discontinuous
DNA polymerase III
Builds using short fragments called Okazaki fragments

18. Building Complementary Strands
DNA polymerase I
Removes RNA primers from both strands
Replaces them with appropriate nucleotides
DNA ligase
Joins Okazaki fragments together
New strand automatically twists into helix

19. Quality Control DNA polymerase III and I Exonuclease Proofread
Either enzyme can delete mistakes and replace them with appropriate nucleotides

20. Questions?