1. 7.2 DNA Replication Assessment Statements: I know that DNA replication occurs in a 5’ 3’ direction. I can explain the process of DNA replication in prokaryotes. I know that DNA replication is initiated at many points in eukaryotes.

2. DNA Replication
*We will focus on the process in prokaryotes.
DNA Replication is made possible by enzymes!
Helicase – untwists and separates double helix
Single-strand binding proteins – keep strands apart

3. DNA polymerase III – catalyzes elongation of new DNA at the replication fork
Can only add to the 3’ end of a growing strand, so new strands are built in a 5’ 3’ direction
Can add at a rate of 50/second in human cells

4. Nucleoside triphosphates are raw material
Two phosphates are broken off to provide energy for reaction
What molecule does this remind you of?
ATP!

5. Strands of the double-helix are anti-parallel – sugar-phosphate backbone runs in opposite directions
Strands have a 3’ OH end and a 5’ PO4 end
Label 5’ and 3’ ends of strands, draw an arrow to show 5’3’ direction

6. Strands of the double-helix are anti-parallel – sugar-phosphate backbone runs in opposite directions
Strands have a 3’ OH end and a 5’ PO4 end
Label 5’ and 3’ ends of strands, draw an arrow to show 5’3’ direction

7. Primase - adds RNA primer to parent/template strand
Since DNA polymerase III can only add on to an existing nucleotide, starting replication requires a primer
A primer is an RNA segment about 10 nucleotides

8. After a primer is in place, DNA pol III can add to the 3’ end of primer
DNA polymerase I later replaces the RNA primer with DNA nucleotides
Label primase, primer, DNA pol III, DNA pol I

9. After a primer is in place, polymerase can add to the 3’ end of primer
Another polymerase later replaces the RNA primer with DNA nucleotides
Label primase, primer, DNA pol III, DNA pol I

10. Ligase – joins fragments of copied DNA (called Okazaki fragments) together
Since DNA polymerase III can only add to a 3’ end, and the strands are antiparallel, the new strands are built in opposite directions

11. On the leading strand, DNA pol III can add continuously toward the replication fork
On the lagging strand, DNA pol III adds in chunks moving away from the fork (Okazaki fragments)
Label leading strand, lagging strand, Okazaki fragment, DNA pol III, ligase

12. On the leading strand, DNA polymerase III can add continuously toward the replication fork
On the lagging strand, DNA polymerase III adds in chunks moving away from the fork (Okazaki fragments)
Label leading strand, lagging strand, Okazaki fragment, DNA pol III, ligase

13. Additional polymerase enzymes – replace RNA primers with DNA (DNA pol I), proofread and correct mismatches
Final error rate is ~1/1 billion base pairs (bp)

14. There is a replication fork at each end of an origin, or “replication bubble.”
Bacterial DNA (circular) has a single origin
Eukaryotic DNA (linear) has thousands of origins.
Why might multiple origins of replication have been selected for by evolution in eukaryotes?
Accelerated replication time could benefit cell survival and reproduction.

15. The bubbles stretch as DNA is copied in each direction until two adjoining bubbles fuse.
Label origin of replication, replication fork, replication bubble.

16. Steps of DNA Replication: (Fill in the appropriate enzyme names.)
A _______enzyme unwinds the parental double helix
_______ stabilize the unwound parental DNA
A _______ enzyme synthesizes an RNA primer.

17. The leading strand is synthesized continuously in the 5’3’ direction by a _________ enzyme.
The lagging strand is synthesized discontinuously in Okazaki fragments.
A ________ enzyme replaces the RNA primer with DNA.

18. A ________ enzyme joins the Okazaki fragments into a continuous strand.
Other _______ enzymes proofread DNA and repair mismatches.
Two identical daughter DNA molecules result.

19. Replication Modeling Activity