1. DNA Replication
Pre-AP Biology

2. Back to last class… Draw & label a nucleotide
Draw a deoxyribose & label the 5’ & 3’ ends
Draw a DNA strand that is at least 5 nucleotides long—label the 5’ & 3’ ends of each strand as well as each part of the nucleotides

3. Why do we need to replicate DNA?
In order to grow, reproduce, and repair, an organism’s cells must divide
Before a cell can divide, it must copy its DNA so that both new cells end up with the exact same information

4. Describe what you see happening in the diagram…

5. Describe what you see happening in the diagram…

6. Overview of DNA Replication

7. The model for DNA replication is known as the semiconservative model because half of the parental molecule is maintained (conserved) in each daughter molecule.

8. Did you know??? New nucleotides are added at the rate of …
50 per second in mammals
500 per second in bacteria

9. Hmmm…
How does complementary base pairing make DNA replication possible?
When an adenine is exposed,
a thymine is added.
When a cytosine is exposed,
a guanine is added.

10. DNA Replication
During replication, each strand serves as a template to make a new DNA molecule
Steps
1. DNA is “unzipped”
2. Free floating nucleotides attach by complementary base pairing
(Each strand is a complement of one of the original parent strands)
3. Results in the formation of two DNA molecules, identical to the original strand

11. Origins of Replication
Where DNA replication begins
Replication proceeds in both directions (bi-directional)
Creates “bubbles”
There are many origins of replication, and there can be thousands of bubbles along a chromosome at once

12. 3’ and 5’ ends Remember that… the sugar’s 3’ carbon is attached to
hydroxyl (-OH)
the sugar’s 5’ carbon is attached to
phosphate
The two strands run in opposite directions= antiparallel

13. Important Enzymes Helicase- unwinds the DNA strands
Topoisomerase- prevents tangling of DNA

14. Important Enzymes Primase- lays down a short piece of RNA primer
DNA polymerase- adds new DNA nucleotides and replaces RNA with DNA

16. Important Enzymes
DNA polymerase can only add new nucleotides to the 3’ end of the strand (NOT to the 5’ end)
A daughter strand of DNA can only grow 5’-3’.
One of the daughter strands is made continuously- leading strand

17. Important Enzymes
The other daughter strand (the lagging strand) is made in small pieces, called Okazaki fragments
These fragments must be glued together, using an enzyme called DNA ligase.

18. Summary of DNA Replication

19. DNA Replication in Action…
Animation
DNA Replication Song

20. Why do we need to copy our DNA?
Before our cells can divide, they need to make a copy of DNA. Every cell needs instructions!
Proper replication of DNA ensures that all body cells in an organism have the same genetic information.