1. Agenda Review homework DNA replication
Begin HW (DNA Replication worksheet)
Quiz will be Wednesday instead and will include DNA replication

2. #1
A, T, G or C

3. #2

4. #3
They match up perfectly to form hydrogen bonds (2 for AT, 3 for GC)

5. #4
The two bases can’t H-bond, so nothing holds them together. It will stick out

6. #5
H-bonds are weak! DNA must become separated to be copied or expressed

7. #6
GATTACA CTAATGT

8. #7
If 15% is Adenine, 15% is also Thymine That leaves 70% split between Guanine and Cytosine. So 35% each

9. #8
X-ray crystallography, helped determine shape

10. #9
Structure How it’s copied How it leads to proteins Sequence

11. #10
Why 99% of it is non-coding What that 99% does How the environment affects gene expression Where it came from/how it originated

12. DNA Replication

13. DNA Replication Must occur every time a new cell is to be made
E.g. prior to mitosis or meiosis

14. DNA Replication
Watson and Crick noticed the huge benefit of double strands
Each strand can serve as a template for making the other

15. Semiconservative Model
Each strand serves as a template for the creation of a new strand of DNA
2 DNA molecules are created, each containing 1 strand of the original DNA

16. Semiconservative Replication

17. DNA Replication is Remarkably Fast and Accurate!
Humans have 46 chromosomes, and thus 46 DNA molecules
About 6 billion base pairs
DNA replication takes just a few hours, even in humans
Only 1 error per 1 billion nucleotides

18. Step 1 – Separate the Strands
Helicase breaks the hydrogen bonds

19. Step 2-
Primase adds a short RNA primer to begin new strands

20. Step 3
DNA polymerase builds a new DNA strand

21. DNA is Antiparallel Strands run opposite directions
DNA polymerase only works in one direction
So the two strands are built in opposite directions

22. Leading vs. Lagging Strands
Built in one piece
Built in the overall direction of replication
Only one primer needed
Built in pieces called Okazaki fragments
Built in the opposite direction of replication
Many primers needed

24. Step 4
DNA ligase seals together Okazaki fragments

25. Step 5 - Proofreading
Nucleases cut out (incise) the incorrect nucleotide
DNA polymerase adds the correct nucleotide
Ligase connects the new nucleotide to the strand

26. Topoisomerase
Helicase
Single Stranded Binding Proteins

27. 5’ 3’ DNA Ligase Lagging Strand Okazaki Fragments Leading Strand
DNA Polymerase
Primase

28. Mutations are Rare… But Happen
Most errors are fixed in proofreading, so DNA replication is incredibly accurate
1 error every billion nucleotides. We have 6 billion. Every time DNA is copied there are ___ errors
We’ll talk about the significance of these mutations later