Francis Crick: When I looked at what Jim had done, low and behold, although he hadn't put it in,
there was this isosceles triangle. In other words, something had come out in the base
pairing, which of course fitted Chargaff's rules, that he hadn't put it in. I think
it was from that moment that we knew that we had it.
From there, the next step was that we had to build a model with coordinates to show
that it was acceptable. We had to incorporate our ideas and the phosphates and base
pairs, and that's what we did. The reason that we had to do model building was that,
although we had the basic idea of the structure - mainly that the phosphates were
on the outside, and the base pairing and so on. We still had to show that we could
build a model with acceptable coordinates.
When doing this, it is quite unnecessary to build a model like that! All that's necessary
to build is one residue, one bit, if you've got that right, and you've got it's relation
to the next bit right, you can just imagine the rest of the model, or mathematics
will generate it. We didn't even use a base pair. We just used this much here. I had
worked out a theorem that if this is the diode axis here, then as you move things
around, there's a constraint. If you built this half right, then this half would automatically
come right as well.
The amount we built was just one residue, but you had to have one atom repeating;
so that you had a phosphate, sugar, and one base. You made sure that the relationship
between this phosphate and this phosphate was what you wanted. That is to say, up
3.4 angstroms, and around 36 degrees. The aim we had was not to get the atoms in the
right place to show that at least one model of the atom could be built.