James Watson: Astbury had the idea that a DNA molecule was a long linear molecule - where the bases stacked on top of each other like piles
The other thing we used was what Maurice Wilkins told us from the dimensions of the crystalline form, there was clearly more
than one chain in a nucleic acid molecule. It was difficult to decide exactly how many because you had to measure the density
accurately, and that wasn't that simple, but he thought there was between two and three. My recollection is that their preference
was for three.
The other thing that we thought about was that since it crystallizes, it must be very regular. But it was sort of our tenet
that the sequence of bases along this molecule were going to be irregular, and we thought they were going to be irregular
because we thought that's what would convey the genetic information. People now think that was a great idea, but there was
no other idea you could have. It was as simple minded as it could be - if nucleic acids conveyed information, it was with
the sequence of the four bases.
Now, if they crystallize, well the bases being irregular, we thought, four different types, four different shapes. That meant
that the regular pattern must come from the sugar-phosphate backbone. We thought that if there were several chains, and if
they pack together regularly to form crystal, then the sugar phosphate would have to be in the center of the molecule.