1. Copying the genetic blueprint
DNA replication
Copying the genetic blueprint

2. DNA replication The copying of a cell’s DNA Semiconservative
About 3 billion base pairs in the human genome
Semiconservative
Each strand in the DNA double helix acts as a template for making a new, complementary strand
1 strand2 daughter molecules
Each with 1 new & 1 old strand

3. A:T C:G 5’ 3’ 5’ 3’ DNA Replication = adenine = cytosine = guanine
= thymine
A:T
C:G 3’ 5’

4. 5’ 3’
Step 1…an enzyme breaks the hydrogen bonds between the nitrogen bases and the strand splits 3’ 5’

5. 5’ 3’ 5’
Step 2…each strand of DNA is a template for building a new, complementary strand 3’ 3’ 5’ 3’ 5’

6. 5’ 5’ 3’ 3’
Step 3…completion of the process results in two identical DNA double helices, each with one new and one old strand 3’ 5’ 3’ 5’

7. DNA replication Basically, that’s it!!
HOWEVER…cells need to do this very quickly and accurately
What do they need?
Enzymes & proteins

8. DNA polymerase Responsible for synthesizing (putting together) the DNA
There are several different DNA polymerases (5 in humans)
Add complementary nucleotides one at a time to the growing DNA chain
Order based on the template (old DNA strand)

9. DNA polymerase They always need a template
Can’t start out a strand from scratch
Require a pre-existing, short length of nucleotides called a primer
Can only add nucleotides to the 3’ end of the chain (5’  3’)
Proofread their work

10. DNA polymerase

11. DNA polymerase Addition of nucleotides requires ENERGY
Comes from the nucleotides
3 attached phosphates (like ATP)
When the bond is broken, energy is released to form the bond between the nucleotide and the DNA chain

12. Nucleoside triphosphate (=nucleotide)

13. How to start??? Origin of replication Replication always starts here
Humans can have up to 100,000

14. Replicaton forks

15. Helicase & SSB’s 1st replication enzyme at origin of replication
Move the replication forks forward by unwinding the DNA (breaking the H-bonds)
Single-stranded binding proteins coat the DNA strand to prevent re-annealing

16. Primase Enzyme Makes an RNA primer
Provides a 3’ end for DNA polymerase to add to
Typically 5-10 nucleotides long
Primes DNA synthesis (gets it started)

17. DNA polymerase Once the primer is in place: DNA polymerase extends it
Adds nucleotides one at a time
Can only extend in the 5’ to 3’ direction
Leading and lagging strands

18. Leading & Lagging strands

19. Leading strand Leading strand is the easy one! 5’ to 3’
Made continuously
DNA polymerase moving in the same direction
Can be extended from one primer

20. Leading & Lagging strands

21. Lagging strand The tricky strand Moves 3’ to 5’
Made in short fragments
Okazaki fragments
Requires a new primer for each of the short Okazaki fragments

22. Leading & Lagging strands

23. Maintenance & Cleanup Sliding clamp Ring-shaped enzyme
Keeps DNA polymerase in place
Keeps DNA pol of lagging strand from floating off when it re-starts at a new Okazaki fragment

24. Maintenance & Cleanup Topoisomerase
Prevents the double helix ahead of the replication fork from getting too tightly wound and the DNA opens up
Makes temporary nicks in the helix to release tension then seals the nicks

25. Quiz!!
Helicase…
Opens up the DNA at the replication fork

26. Quiz!! Single-stranded binding proteins…
Coat the DNA strand to prevent re-annealing

27. Quiz!!
Topoisomerase…
Works ahead of the replication fork to prevent supercoiling

28. Quiz!!
Primase…
Synthesizes RNA primers complementary to the DNA strand; allows DNA polymerase to add on to the strand

29. Quiz!!
DNA polymerase…
Extends the primers, adding to the 3’ end

30. Quiz!!
DNA ligase…
Seals the gaps between the Okazaki fragments