STANFORD UNIVERSITY
   STANFORD, CALIFORNIA

DEPARTMENT OF GENETICS
   Jcbool of Medicine

March 28, 1959

Dear Arthur:

  I hope you won't have to spend much time answering this letter-- a few words
will do.

How close are you to getting DNA synthesis from individual molecules of
primer? It should be possible, ultimately-, to `plate out' single DNA
molecules so as to get clones from them, assuming we can superimpose some
mechanism for dissociating the immediate products of condensation so these
can more effectively act as new primers. Your priaw poly-AT, as you point out,
may se1 1 have xgaz~%rsl etarted out this way, being subject to very strong selection
for species lacking G and C,

  It strikes me then that a general method for getting other homogeneous
species of DNA, of greater complexity than poly-AT, would be equivalent to
clonal isolation by dilution: i.e., setting up a series of reaction f)raxkxxtubes
with all four triphosphates, and progressive dilutions of a DNA primer. At
some critical di lution, if you are lucky, you should run into tubes that show
substantial (and self-accelerating) condensation after appreciable delay, i.e.,
comparable to the first appearance of poly-AT, but perhaps containing other
more complex +UXE products with characteristic base ratios. Of course the con-
tami nat i ng nucl eases, as usual,cause all the trouble, and may make this experi-
menta quite unrealistic.  But I wonder if you can report any experience under
conditions where the amount of DNA primer was critical.

Is there any point trying to set up systems where the nucleases are momentarily
blocked by excess neutral DNA or RNA, or does this not work?
m----m----

What these questions boil down to, Art, ts that I feel quite inhibited about
plannling to set up any other new programs of genetic analysis when the treatsent
of molecules is so tantalizingly near by.  The alternative to think about is
artificial systems of molecular rep1 ication,  like the mixed cation-anion @4X poly-
mers I talked wi th you about:  I've had a little encouragement on doing something
about this wi th Frank Mayo, who used to be axz at GE and is now at Stanford Research
lnsti tute, and one of the few traditional polymer chemists in the area. The catch
is, there are strong hints that some of sak the commercial labs. may already
be on to the same game, viz. for condens&tions oriented by %a& complementary-ion
ablsorptionl of the monomers.

oshua Lederberg