In his letter Klug gave electron density calculations derived from experiments with synthetic DNA made of one chain consisting
of a sequence of only adenine bases, called poly-A, plus the complementary second chain. Despite Crick's earlier skepticism,
research with simple, repeating synthetic sequences of DNA and RNA consisting of just one base yielded many important experimental
results in deciphering the genetic code.
Item is a photocopy.
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1961-01-16 (January 16, 1961)
Original Repository: Wellcome Library for the History and Understanding of Medicine. Francis Harry Compton Crick Papers
Just a note to let you know that we have now obtained a fibre of poly-A which is predominantly in the B form which, if you
remember, was the least protonated form. This was achieved by standing a fibre over ammonia for a few days, so this is cleared
We have now also measured the density in a gradient and find it to be 1.58. If I remember correctly this was Alex's original
value before he managed to meet the theoretical value half way with a revised value of 1.53. On calculating the density I
get a value of 1.50 for the pure acid, 1.54 if there is a half a potassium atom per nucleotide and 1.58 for two potassium
atoms per nucleotide. In view of the fact that Beers dialysed his preparation of poly-A against distilled water, I do not
think he could have removed all the potassium. I, therefore, think there is now no anomaly about the density, and that the
two strand structure is certain. Do you agree? I have used a value of 0.54 for the partial specific volume, having forgotten
the value you gave me.
I now feel we have carried the problem far enough to warrant accurate model building and I should like to come up to do this
with you some time. Do you forsee any time in the next few weeks when you could tear yourself away from protein synthesis
to do this.
P.S. I discussed with David Davies the problem of whether Alex and Jim's photographs are the same as ours. I am sure
now that they are. The two spots at 4.4 and 4.7 A into which their third layer line can sometimes be resolved, corresponds
essentially to reflections found in the two phase system we have. The equatorial reflections can also be squared with a super
position of our two phases. I therefore have little doubt that there is no separate 8-fold screw structure.