1. DNA REPLICATION

2. (b) Separation of strands
Fig A T A T A T A T C G C G C G C G T A T A T A T A A T A T A T A T G C G C G C G C
(a) Parent molecule
(b) Separation of strands
(c) “Daughter” DNA molecules, each consisting of one parental strand and one new strand
Figure 16.9 A model for DNA replication: the basic concept

3. DNA Replication is semi-conservative:
each replicated DNA molecule consists of one “old” and on “new” strand.

4. Getting Started
Replication begins at special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble” A eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin, until the entire molecule is copied

5. Parental (template) strand
Fig a
Origin of replication
Parental (template) strand
Daughter (new) strand
Replication fork
Double-stranded DNA molecule
Replication bubble
0.5 µm
Two daughter DNA molecules
Figure Origins of replication in E. coli and eukaryotes
(a) Origins of replication in E. coli

6. Double-stranded DNA molecule
Fig b
Origin of replication
Double-stranded DNA molecule
Parental (template) strand
Daughter (new) strand
0.25 µm
Bubble
Replication fork
Figure Origins of replication in E. coli and eukaryotes
Two daughter DNA molecules
(b) Origins of replication in eukaryotes

7. 1. Helicase breaks hydrogen bonds between bases, unzips and unwinds the helix

8. Helicase: Is an enzyme (a protein that speeds up chemical reactions)
Is made during G1
Begins to unwind the DNA at the ORIGIN OF REPLICATION (a specific nucleotide sequence)

9. Helicase enzymes move in both directions from the
Point of Origin, forming a REPLICATION
BUBBLE.
At either end of the replication bubble is a
REPLICATION FORK, a Y-shaped region
where the new strands of DNA are elongating.

10. 2. Single stranded binding proteins hold the DNA strands apart
Keeps the separated strands apart and stabilize the unwound DNA

11. RNA nucleotides bind with complementary base sequences under the direction of RNA primase. These RNA nucleotides act as a primer for DNA nucleotides.
Primers are short segments of RNA, about 10 nucleotides long
Must have a primer because DNA polymerase can only add nucleotides to another nucleotide

12. Primase

13. 4. DNA polymerase III adds DNA nucleoside triphosphates to the RNA primer sequence in a 5’ to 3’ direction.
Two of the
phosphates
are stripped
off in the
bonding

14. New nucleotides can only be added to the 3’ end of a growing DNA chain
So we say DNA grows 5’ to 3’

15. Leading Strand:
DNA polymerase III can synthesize a complementary strand on one side of the template in the 5’ to 3’ direction with no problem.

16. What about the other strand??5’ 3’
DNA Polymerase III
Can only add to this side …
AWAY from the
replication fork

17. Lagging Strand
DNA polymerase III must work away from the replication fork.
Makes a short strand of DNA, called an Okazaki fragment.
As the bubble widens, it can make another short strand, and so on.

19. RNA primers are removed and replaced with DNA nucleotides by DNA Polymerase I.

20. Along the lagging strand the Okazaki fragments are joined by DNA Ligase to form a single DNA strand.

21. Proofreading by DNA Polymerase III and I occurs, and replication is complete.

22. The Animation

23. Animation: DNA Replication
Fig
Add onto the 3’ side (synthesized 5→3)
Primase
Single-strand binding proteins 3
Topoisomerase 5 3
RNA primer
Figure Some of the proteins involved in the initiation of DNA replication 5 5 3
Helicase
Animation: DNA Replication

24. Overall directions of replication
Fig
Overview
Origin of replication
Leading strand
Lagging strand
Primer
Lagging strand
Leading strand
Overall directions of replication
Origin of replication 3 5
RNA primer 5
“Sliding clamp” 3 5
DNA poll III
Parental DNA
Figure Synthesis of the leading strand during DNA replication 3 5 5 3 5

25. Overall directions of replication
Fig
Overview
Origin of replication
Leading strand
Lagging strand
Lagging strand 2 1
Leading strand
Overall directions of replication 3 5 5 3
Template strand 3
RNA primer 3 5 1 5
Okazaki fragment 3 5 3 1 5 5 3 3
Figure 16.6 Synthesis of the lagging strand 2 1 5 3 5 3 5 2 1 5 3 3 1 5 2
Overall direction of replication

26. Table 16-1

27. Single-strand binding protein Overall directions of replication
Fig
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
Figure A summary of bacterial DNA replication
DNA pol III
Lagging strand 5
DNA pol I
DNA ligase 4 3 5 3 2 1 3 5

28. DNA pol III synthesizes leading strand continuously
Fig. 16-UN3
DNA pol III synthesizes leading strand continuously 3 5
Parental DNA
DNA pol III starts DNA synthesis at 3 end of primer, continues in 5  3 direction 5 3 5
Lagging strand synthesized in short Okazaki fragments, later joined by DNA ligase
Primase synthesizes a short RNA primer 3 5

29. Fig. 16-UN5

30. Accurate Only 1:1,000,000,000 nucleotides are incorrectly paired
DNA replication is ...
Accurate
Only 1:1,000,000,000 nucleotides are incorrectly paired

31. DNA replication is ... Extremely rapid
In prokaryotes, up to 500 nucleotides are added per second
50 per second in eukaryotes

32. Other Good Animations http://www.ncc.gmu.edu/dna/repanim.htm