Ever heard of four-stranded DNA?

[Byung-Dong Kim]

Space-filling model

On my way home to Kingston, where I was a faculty at University of Rhode Island, I visited Tom Steitz at Yale University in New Haven, Connecticut. I wanted to have a look at his space-filling models of DNA and protein. Space-filling models he showed to me were the CPK models, the same kind the canonical B-DNA had been built of and shown on the cover of a text book of Molecular Biology of the Gene. The model was built to the atomic scale. It was heavy and was supported by central axis and transparent ladders. To build a stem-and-loop DNA many sets of those models would be needed. To my dismay, price of it was way out of my reach.

As I was checking through the piles of the mail in my office I found one brochure that caught my eyes. It was about an introduction of a new space-filling model made of rather tiny plastic units that was distributed by the publisher Academy Press (AP). The plastic model had some unique features. This AP model had bases and ribose units separately and were connectable with oxygen and phosphate atoms. Caps for van der Waal's radius were removable to reveal the backbones of DNA molecule. Unlike CPK model the AP model allowed bond angles adjustable at 5 degree increments and stayed fixed through out global manipulation of the DNA strands. Being a plastic model it was small and light, and easy to handle. Most important, it was cheap. I asked for an opinion of a biophysics professor in the Department of Chemistry about the plastic model and my interest in four-stranded DNA. His response was down right simple and clear. "It is energetically impossible. Don't waste your time on it." On the next day I visited the associate dean for research. I briefly explained to him about what I am up to with the DNA space-filling model. I somehow persuaded him to make his historical contribution on research by buying me the AP model of DNA.

I never took any class about X-ray crystallography nor about atomic torsion angles. No wonder I did not dare talk to any knowledgeable scientist lest he would talk me out of this absurdity. The entire tool I had were the books "Structures of Nucleic Acids," "Double Helix," and the reprints about DNA structure that I made the copies of at the library of Cold Spring Harbor Laboratory. However feeble the possibility to be the truth, it was too good and too important not to take the chance. In my saltbox house on the 36 Heritage Drive, Kingston, Rhode Island my solo game of the DNA puzzle began. The DNA model I made was too big to take pictures of. I put the model on a white blanket on the bottom of the basement and I took the pictures on the top of the stair case.