Meselson & Stahl (1958) Experiment, Fig. 4
The
two numbered experiments demonstrate that DNA
replication occurs semi-conservatively, as
anticipated from the Watson-Crick (1953)
model. This experiment is often shown in textbooks as though
it were done in a standard preparative centrifuge. Instead,
the method as stated in the first line of the legend is
"density-gradient [ultra-]centrifugation ... at various
times after the addition of an excess of 14N
substrate to a growing 15N-labelled
culture." Also, the bands are "ultraviolet absorption
photographs": what are these, and how were they
obtained?
2) At what points, and on what evidence and argument, are the alternative conservative and dispersive models disproved?
3) Based on the ultraviolet absorption photographs, construct a set of "fake evidence" to show the expected results if either the conservative or dispersive models were true. Answer the same questions in (1) & (2).
4) What is the purpose of the two 'mixed' experiments shown at the bottom? In each case, what was mixed?
HOMEWORK:
1) Trace successive
stages of the two experiments through time. At what point,
and on what evidence and argument, is the semi-conservative
model proved? 2) At what points, and on what evidence and argument, are the alternative conservative and dispersive models disproved?
3) Based on the ultraviolet absorption photographs, construct a set of "fake evidence" to show the expected results if either the conservative or dispersive models were true. Answer the same questions in (1) & (2).
4) What is the purpose of the two 'mixed' experiments shown at the bottom? In each case, what was mixed?