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Testing Models for DNA replication
Matthew Meselson and Franklin Stahl (1958)
Matthew Meselson and Franklin Stahl more recently
Faculty member at Harvard
Mechanisms of Molecular Evolution
Faculty Chair for CBW Studies
Faculty member at U. of Oregon
Meiotic Recombination
Testing Models for DNA replication
Meselson and Stahl (1958)
Density labeling experiment on E. coli (bacterial) DNA
Bacterial culture
Grow for several generations
15NH Cl
4
(Sole N source)
Bacterial culture
with dense DNA
This is the starting material for the experiment
Meselson and Stahl (continued)
Harvest cells and resuspend in media with
14NH Cl
4
as the sole N source
Bacterial culture
with dense DNA
Grow for another
generation
etc
14NH Cl
4
Bacterial culture
“0 generation”
Grow for another
generation
Harvest some cells
“2nd generation”
Grow for 1
generation
Harvest some cells
“1st generation”
For each generation isolate the DNA and spin through a density (CsCl) gradient).
Detect DNA in the gradient (eg by UV absorption)
Monitor how many DNA bands there are after each generation
Anticipated Results for each Possible Model
Gray = Heavy
orange = light
3 light, 1 heavy
i.e. two bands
2 light, 2 intermediate
i.e. two bands
4 intermediate
i.e. one band
Meselson and Stahl Original Data
DNA Replication
…Is semiconservative
• Matthew Meselson and Franklin Stahl showed
that DNA replication results in new DNA duplex
molecules in which one strand is from the parent
duplex and the other is completely new
• Since DNA replication is semiconservative,
therefore the helix must be unwound.
• Unwinding generates torsional stress (supercoils)
which must be removed by topoisomerases
(Chapter 12)