1. Testing Models for DNA replication Matthew Meselson and Franklin Stahl (1958)

2. 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

3. Testing Models for DNA replication Meselson and Stahl (1958) Density labeling experiment on E. coli (bacterial) DNA Bacterial culture 15 NH 4 Cl (Sole N source) Grow for several generations Bacterial culture with dense DNA This is the starting material for the experiment

4. Meselson and Stahl (continued) Harvest cells and resuspend in media with 14 NH 4 Cl as the sole N source Bacterial culture with dense DNA Grow for 1 generation Harvest some cells “1st generation” Grow for another generation Harvest some cells “2nd generation” Grow for another generation etc 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 Bacterial culture “0 generation” 14 NH 4 Cl

5. 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

7. Meselson and Stahl Original Data

8. 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)