1. DNA Replication Chapter 12 Section 2

2. History of DNA
Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA

3. Transformation
Fred Griffith worked with virulent S and nonvirulent R strain Pneumoccocus bacteria
He found that R strain could become virulent when it took in DNA from heat-killed S strain
Study suggested that DNA was probably the genetic material (not proteins as previously thought)

4. Griffith Experiment

5. History of DNA Chromosomes are made of both DNA and protein
Experiments on bacteriophage viruses by Hershey & Chase proved that DNA was the cell’s genetic material
Radioactive 32P was injected into bacteria!

6. Discovery of DNA Structure
Chargaff’s rules:
Adenine must pair with Thymine
Guanine must pair with Cytosine
The bases form weak hydrogen bonds G C T A

7. DNA Structure
Rosalind Franklin took diffraction x-ray photographs of DNA crystals.
This showed the shape of DNA

8. DNA Structure
In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-ray diffraction.

9. DNA Deoxyribonucleic acid
Made up of subunits (monomers) called nucleotides
Nucleotide made of:
1. Phosphate group
2. 5-carbon sugar
3. Nitrogenous base

10. DNA Double Helix “Rungs of ladder” Nitrogenous Base (A,T,G or C)
“Legs of ladder”
Phosphate &
Sugar Backbone

11. DNA Replication

12. Replication Facts DNA has to be copied before a cell divides
DNA is copied during the synthesis phase (S) of interphase
New cells need identical DNA strands

13. DNA Replication Step 1: DNA unwinds (helicase) Step 2: DNA unzips
Two strands open to form Replication Forks
New strands grow at the forks (DNA polymerase)
Replication
Fork
Parental DNA Molecule 3’ 5’

14. Origins of Replication
Replication begins at special sites called origins of replication, where the two DNA strands separate and open up a replication “bubble”
A eukaryotic chromosome may have many origins of replication, prokaryotes only have one.
Replication occurs in both directions from each origin
At the end of each replication bubble is a replication fork, a Y-shaped region

15. Prokaryote Replication

16. Eukaryote Replication

17. Semiconservative Replication
Parent strands separate and serve as a template
They produce new DNA molecules that have one strand of parent DNA and one strand of new DNA
DNA Template
New DNA
Parental DNA

18. Antiparallel Structure of DNA
Two strands oriented in opposite directions.
DNA polymerases add nucleotides only to the free 3end of a growing strand; a new DNA strand can elongate only in the 5 to 3direction

19. Leading & Lagging Strands
Leading strand is built continuously, towards the replication fork
Lagging strand built discontinuously (in fragments) away from the replication fork.
fragments called Okazaki fragments
DNA ligase joins the fragments together
Replication Video

20. Steps of DNA Replication
Unwinding the helix
DNA helicase
Base Pairing (add complimentary nucleotides)
DNA Polymerase
Joining of Okazaki Fragments
DNA Ligase

21. Proofreading New DNA Enzymes are involved in many
steps of DNA replication
Their final role in replication is to proofread and correct mistakes.

22. DNA Damage & Repair
Chemicals & ultraviolet radiation damage the DNA in our body cells.
Cells must continuously repair damaged DNA.
Excision repair occurs when a repair enzyme removes damaged parts of DNA
Enzymes replace and bond the new nucleotides together