Meselson's experiments with Franklin Stahl on the replication of DNA in 1958 helped cement the concept of the double helix
by validating a model that many scientists still saw as speculation and demonstrated how two strands of a helix could physically
code for the material of inheritance. At Luria's request, Meselson provided more detailed instructions for performing
density gradient experiments.
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1958-10-28 (October 28, 1958)
California Institute of Technology
Luria, Salvador E.
Original Repository: American Philosophical Society. Library. Salvador Luria Papers
Reproduced with permission of Matthew Meselson.
Genetics Lessons from Bacteriophage, 1938-1944
Letter from Matthew Meselson to Salvador E. Luria (January 18, 1958)
[HANDWRITTEN NOTE: Lu, you are the victim of my first encounter with a dictaphone. Forbearance!]
Jean is in Mexico resting after a rather hectic month in Pasadena with the many persons who are visiting currently. Therefore
I will try to tell you some of the technical details of our experiment with lambda in cesium chloride density gradients.
(1) First of all, you should obtain your cesium chloride from the American Potash Company, 3030 W. Sixth Street, Los Angeles
54. You should direct your correspondence to Mr. Frank Radovitch. The cesium chloride supplied by American Potash is not
completely pure but contains about 3% rubidium. This contamination with rubidium, however, should not trouble your work with
viruses. Its only effect, so far as we know, is to very slightly change the actual density gradient from what one would expect
if the salt were pure cesium chloride.* [HANDWRITTEN NOTE: * For CsCl solutions centrifuged to equilibrium of 25 degrees C,
the value of 2P/2Y is 8.08x10^-10 w(^2)Y gmcm^-4. W is in radians sec^-1 and Y in cm. This value holds at P=1.70. For 1.7>P>1.4
it will be good to within 10% even at 20 degrees C.] (2) This change is only a percent or so and can be neglected. The cesium
chloride from American Potash costs about 25 dollars a pound. You should treat it very carefully overnight with a small amount
of acid-washed decolorizing charcoal. This should take the optical density of the saturated cesium chloride solution down
below 0.1. (3) [HANDWRITTEN NOTE: .01 M tris] You will find data for the composition and density of cesium chloride solutions
at various temperatures in the International Critical Tables. The density of lambda is 1.51 grams per cubic centimeter.
We usually work by diluting a stronger cesium chloride solution with a standard amount of lambda sample or water to give a
final density of 1.51. Also present in the cesium chloride solution which we use is .01 molar tris buffer and the solution
is brought to pH 7 as determined by a glass electrode pH meter before use. We have found in this solution the titre of lambda
remains constant at 20 degrees even for many days of exposure. If you want to find the density of a new virus in cesium chloride,
I would recommend that you place a great deal of it, let us say enough to give it optical density of 1, in the analytical
cell with cesium chloride solution of density, perhaps 1.5, and watch to see which way the bulk of the material moves. Then
you should adjust the density of the cesium chloride in the second experiment in the direction indicated by the first. Usually
in a few tries this will enable you to determine the density quite accurately. In the case of lambda we centrifuge at 27,690
revolutions a minute. At this speed the time required for essential equilibrium is approximately 12 hours. I would recommend
that in the (4) analytical run you use plastic centerpieces as supplied by Spinco. (5) In the case of the swinging bucket
machine which we use for experiments from which we want to recover the lambda you must spin for a somewhat longer time at
the same speed of 27, or 28,000 rpm. You should spin at least 16 hours to obtain equilibrium with respect to the cesium chloride
and then you should continue spinning if P-1 takes longer than the length of time I mentioned for lambda when run in the analytical
machine. Specifically, if P-1 takes longer than say, 12-14 hours to come to apparent equilibrium in the analytical machine
then in the swinging bucket use a length of time increased by a factor of 2 over the time you find in the analytical cell.
After a swinging bucket run the tubes which should be filled to just 3 milliliters can be removed to a lab bench where a small
hole is punctured in their bottoms and the falling drops are collected in separate test tubes. I would strongly advise you
to practice this exercise several times before actually performing an experiment with virus. It is very easy to let the drops
fall down the outside of the test tube. I can't think of any more details which would be of use to you off hand but if
there is anything please let me know and I will write to you immediately.
Give my best regards to all the people at M. I. T.