1. DNA as the molecule of heredity: part A
What experiments identified DNA as the hereditary material?
What is the structure of DNA, and what does the structure suggest about DNA function?
How do cells replicate DNA?

2. Frederick Griffith 1928: bacterial transformation
Streptococcus pneumoniae

3. Avery, McLeod & McCarty - 1944
What is the material that transforms bacteria?
DNA transformed bacteria, but not proteins.
Treatment with DNase, but not RNase or proteases, eliminated transformation activity.

4. bacteriophage T2

5. Hershey-Chase experiment - 1952

6. Chargaff's rules Organism %A %G %C %T A/T G/C %GC %AT φX174 24.0 23.3
21.5
31.2
0.77
1.08
44.8
55.2
Maize
26.8
22.8
23.2
27.2
0.99
0.98
46.1
54.0
Octopus
33.2
17.6
31.6
1.05
1.00
35.2
64.8
Chicken
28.0
22.0
21.6
28.4
1.02
43.7
56.4
Rat
28.6
21.4
20.5
1.01
42.9
57.0
Human
29.3
20.7
20.0
30.0
1.04
40.7
59.3
Grasshopper
41.2
58.6
Sea Urchin
32.8
17.7
17.3
32.1
35.0
64.9
Wheat
27.3
22.7
27.1
45.5
54.4
Yeast
31.3
18.7
17.1
32.9
0.95
1.09
35.8
64.4
E. coli
24.7
26.0
25.7
23.6
51.7
48.3

7. Rosalind Franklin's X-ray crystallography
From Campbell & Reece, Biology 7th ed., Pearson

8. Watson-Crick model of DNA - 1953

9. DNA as the molecule of heredity: part B
How do cells replicate DNA?
Meselson-Stahl experiment
Model of DNA replication
“It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”
- Watson and Crick, 1953, Nature 171:

10. Possible modes of DNA replication

11. Meselson-Stahl: density-gradient ultracentrifugation
Bacteria are grown in medium with 15N (heavier isotope) or 14N to label DNA. Ultracentrifugation on CsCl gradients (20 h at 140,000 x g) separates DNA molecules by density.

12. Meselson-Stahl experiment

13. Meselson and Stahl’s results
From Meselson & Stahl 1958 PNAS

14. DNA polymerase adds new nucleotides to 3’-OH; DNA strands grow 5'3'

15. DNA replication proceeds bidirectionally from an origin of replication

16. Because DNA synthesis always occurs 5'3', the two strands replicate differently

17. DNA replication complex

18. More complex views of DNA replication
The trombone model
mboneFINALd.swf
Molecular visualization of DNA replication