Meselson and Stahl (1958), The Replication of DNA in Escherichia coli. PNAS 44:671-682.
1. Why is Figure 3 included in the paper?
2. What does footnote 14 mean?
3. The mechanism depicted in Figure 6 has been reproduced in nearly all molecular biology textbooks. Did Meselson and Stahl rigorously prove this mechanism?
4. Meselson and Stahl state that their results disprove the DNA duplication scheme of Delbruck (PNAS 40:783, 1954) shown below. How was this model ruled out?
Figure 3.--Resolution of an interlock in a replicating duplex by breaking both old chains at each half-turn of the helix and rejoining the lower terminals of the breaks to the open ends of equal polarity of the new chains. Lateral view. a, Location of first pair of breaks. b, Rejoining of lower terminals of breaks. c, Location of second pair of breaks. d, rejoining of lower terminals.
Parental chains are represented by solid lines: new chains, by dashed lines. At the overlaps the lower chains are dotted.
5. It is now understood that when Meselson and Stahl prepared DNA from E. coli, they inadvertently fragmented it into pieces, each about 10,000 base pairs long. Would their results have been different if they had instead isolated intact E. coli chromosomes?
6. If you wanted to repeat the density-shift experiment today, what technological improvements would you make?
7. How does the equilibrium density gradient centrifugation described in this paper differ from velocity sedimentation (e.g., centrifugation through a sucrose gradient, a procedure which separates molecules according to size and share)?