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?
Frederick Griffith 1928: bacterial transformation
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.
bacteriophage T2
http://www.youtube.com/watch?v=41aqxcxsX2w
Hershey-Chase experiment - 1952
Chargaff's rules
Organis
m
%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
17.6
31.6
1.05
1.00
35.2
64.8
Chicken
28.0
22.0
21.6
28.4
0.99
1.02
43.7
56.4
Rat
28.6
21.4
20.5
28.4
1.01
1.00
42.9
57.0
Human
29.3
20.7
20.0
30.0
0.98
1.04
40.7
59.3
Grassho
pper
29.3
20.5
20.7
29.3
1.00
0.99
41.2
58.6
Sea
Urchin
32.8
17.7
17.3
32.1
1.02
1.02
35.0
64.9
Wheat
27.3
22.7
22.8
27.1
1.01
1.00
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
1.05
1.01
51.7
48.3
Rosalind Franklin's X-ray crystallography
From Campbell & Reece,
Biology 7th ed., Pearson
Watson-Crick model of DNA - 1953
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 171:737-738
Possible modes of DNA replication
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.
Meselson-Stahl experiment
Meselson and Stahl’s results
From Meselson & Stahl 1958 PNAS
DNA polymerase adds new
nucleotides to 3’-OH;
DNA strands grow 5'3'
DNA replication proceeds bidirectionally from an
origin of replication
http://www.web-books.com/MoBio/Free/Ch7A.htm
Because DNA synthesis always occurs 5'3', the
two strands replicate differently
DNA replication complex
http://www.nature.com/nature/journal/v421/n6921/box/nature01407_bx1.html
More complex views of DNA replication
• The trombone model
– http://www.mcb.harvard.edu/Losick/images/Tro
mboneFINALd.swf
• Molecular visualization of DNA replication