1. How is DNA replicated, ensuring consistency across generations?
Essential Question 3
How is DNA replicated, ensuring consistency across generations? ?

2. The Proposal: How DNA might replicate 1
(a) Parent molecule
(b) Separation of strands
(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand
Called “semi-conservative replication”

3. 3 alternative models of DNA replication
The Proposal: How DNA might replicate 2
Second replication
Parent cell
First replication
(a) Conservative model
3 alternative models of DNA replication
(b) Semiconserva- tive model
(c) Dispersive model

4. EXPERIMENT: Studying DNA Replication 1
Bacteria cultured in medium containing 15N 2
Bacteria transferred to medium containing 14N
RESULTS 3
DNA sample centrifuged after 20 min (after first application) 4
DNA sample centrifuged after 20 min (after second replication)
Less dense
More dense

5. EXPERIMENT: Studying DNA Replication 2 CONCLUSION
First replication
Second replication
Conservative model
Semiconservative model
Dispersive model

6. DNA Replication in Prokaryotes
Origin of replication
Daughter (new) strand
Replication fork
Double-stranded DNA molecule
Replication bubble
0.5 µm
Two daughter DNA molecules
(a) Replication in E. coli

7. DNA Replication 1 Origin of replication Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 µm
Bubble
Replication fork
Two daughter DNA molecules

8. Some proteins involved in initiating DNA replication
Primase
Single-strand binding proteins 3
Topoisomerase 5 3
RNA primer 5 5 3
Helicase

9. Nucleotides are added to the 3’ end of the growing strand
DNA Replication 3
Nucleotides are added to the 3’ end of the growing strand
New strand 5 end
Template strand 3 end
5 end
3 end A T A T
Base
Sugar
Phosphate C G C G G C G C
DNA polymerase A T A
3 end T C C
3 end
Nucleoside triphosphate
5 end
5 end

10. Overall directions of replication
DNA Replication 4
Overview
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Lagging strand
Leading strand
Overall directions of replication
Why so complicated? DNA can only be synthesized
in the 5’  3’ direction

11. Synthesis of the leading strand
Origin of replication
DNA Replication 5
Synthesis of the leading strand 3 5
RNA primer 5
“Sliding clamp” 3 5
DNA pol III
Parental DNA 3 5
The two strands are anti-parallel
Run in opposite directions 5 3 5

12. Synthesis of the Lagging Strand3 5
DNA Replication 6 5 3
Template strand 3 5
Synthesis of the Lagging Strand
RNA primer 3 1 5 3
Okazaki fragment 5 3 5 1 3 5 3 2 1 5

13. DNA Replication 7 DNA pol III synthesizes leading strand continuously3 5
Parental DNA
DNA pol III starts DNA synthesis at 3 end of primer, continues in 5  3 direction 5
Lagging strand synthesized in short Okazaki fragments, later joined by DNA ligase 3 5
Primase synthesizes a short RNA primer 3 5

14. Single-strand binding protein Overall directions of replication
DNA Replication 8
Bwa-ha-ha!!!
Overview
Origin of replication
Leading strand
Lagging strand
Leading strand
Lagging strand
Single-strand binding protein
Overall directions of replication
Helicase
Leading strand 5
DNA pol III 3 3
Primer
Primase 5
Parental DNA 3
DNA pol III
Lagging strand 5
DNA pol I
DNA ligase 4 3 5 3 2 1 3 5

15. How is DNA replicated, ensuring consistency across generations?
Essential Question 3
How is DNA replicated, ensuring consistency across generations?
By Semi-conservative replication
One strand old; one strand new

16. How are mistakes in replication corrected?
Essential Question 4
How are mistakes in replication corrected? ?

17. Nucleotide excision repair of DNA damage
Correcting Mistakes 1
Nuclease
Nucleotide excision repair of DNA damage
DNA polymerase
DNA ligase

18. The Telomere Problem Correcting Mistakes 25
Ends of parental DNA strands
Leading strand
Lagging strand 3
Correcting Mistakes 2
Last fragment
Previous fragment
RNA primer
Lagging strand 5 3
Parental strand
The Telomere Problem
Removal of primers and replacement with DNA where a 3 end is available 5
Telomeres: repetitive, noncoding sequences at the ends of the chromosomes 3
Second round of replication 5
New leading strand 3
New lagging strand 5 3
Further rounds of replication
Shorter and shorter daughter molecules

19. Levels of DNA coiling and folding
Nucleosome
(10 nm in diameter)
DNA double helix (2 nm in diameter) H1
Histone tail
Histones
DNA, the double helix
Histones
Nucleosomes, or “beads on a string” (10-nm fiber)

20. Looped domains (300-nm fiber) Metaphase chromosome
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber
Looped domains (300-nm fiber)
Metaphase chromosome

21. DNA Replication Animations, etc.
YouTube Mrs. Hamilton’s favorite for “lagging strand”
DNA Tutorial simple. Does not show replication bubble or a good diagram of the entire replication fork.
Nobel Prize Site. super accurate; thorough background reading
Bioteach Site: U British Columbia. Another simplified animation.
N. Harris College Biology Dept. Houston. Super links to other sites:
Science Videos DNA Replication Link list
McGraw-Hill Publisher’s animations