Most of us are taught in school that Watson and Crick were the guys who first described the structure of DNA. Some of us may have even been told that they actually shared the Nobel Prize for their work with a man named Wilkins. What is mostly left out of the story is how much work had been done solving the puzzle of DNA before Watson and Crick ever turned their attention to it.
Horace Freeland Judson, in his book, The Eighth Day Of Creation rectifies this situation by giving what is probably the fullest account ever printed of the pursuit of the secrets of DNA.
It is a thick book, and dense, and Judson does an excellent job explaining real science– neither dumbing it down nor lofting it all over our heads. And when an author neither over-simplifies nor skims over the material at hand, this means he actually gets it himself, which is always nice being he’s the one we’re trusting to explain it to us. So kudos to Judson.
That said, I actually stopped reading 8th Day about halfway through, not because it is a bad book, but simply because I needed a break from all the details of the people and the science behind the achievement. One day I hope to go back and read the final section, which appears to be more about Proteins.
Briefly as possible, using Judson’s book as my guide, I’d like to summarize the parade of discoveries, deductions, and sheer artistry that preceded Watson’s and Crick’s arrival on the DNA stage…
There’s no particular ideal starting point at which to begin when talking about science, as it is such a cumulative field. But let us skip over the work of Aristotle and Galen and the vast army of searchers who have been, for thousands of years, fighting the good fight for Truth and Understanding; let us start with a picture…
A man named Bernal figured out how to use X-ray diffraction to take pictures of enzymes which had crystallized. From these pictures (patterns of light and shadow that would appear meaningless to you and me), scientists could then infer to a remarkable extent the structure of the particular bombarded speck.
This is due to the fact that X-rays have wave-lengths roughly 4,000 times smaller than those of lightwaves. This allows them to pass through the spaces between atoms composing a solid. To oversimplify drastically: where the X-rays shoot through a molecule without being diffracted– that’s a gap, and when X-rays are diffracted (bent), that means they’ve bumped into something. When X-rays get bumped off-course by a group of atoms forming a molecule, the pattern produced by all the different diffractions produce an interference pattern which those trained in the field can read.
One of the things discovered about DNA fairly early on was that it was quite dense. To get that much density into the long molecule, it was accepted by many that DNA would have to be composed of multiple strands, maybe as many as four, though two and three strands were the top contenders.
There was also talk, before Watson and Crick, that the strands could not run beside each other in an identical pattern since this would cause too much repulsion between the strands, which seemed actually quite close together.
Wilkins and Rosalind Franklin took some darn good pictures of DNA X-ray diffraction-patterns for their day. Judson says that one of Franklin’s images turned out to be the most important, but sadly, she had already died before Wilkins, Crick, and Watson shared the Nobel for their DNA work.
Watson, sitting-in on a lecture of Wilkins’, learned that that Wilkins was using crystallized DNA for his X-ray analysis work. This gave Watson the notion, in his own words, that “if DNA could form crystals, it must have a repeating, regular, orderly structure, and so could be solved.”
Pauling, in his general chemistry work, had already surmised that if a certain unit-component of a structure repeats itself in a backward formation farther down the line, then we can imagine this as the unit moving up and around one half-turn. One more half-turn and the unit comes back to symmetry– then you have the beginnings of a Helix (a spiral curving up and around an axis or cylinder).
Since DNA is a linear molecule thought to form a repeating pattern, this meant the chances were good that it moved along its very long line in a Helix pattern.
Alexander Todd came up with how the “backbone” of DeOxyRibose Sugar Rings and Phosphate Groups would hook up in the DNA molecule in such a way that there would be no branching, as illustrated here.
Meanwhile, Chargaff had found that the ratio of Purines (Adenine and Guanine) to Pyrimidines (Thymine and Cytosine) in DNA is one-to-one. Even more specifically he discovered that there are one-to-one ratios of Adenine-to-Thymine and of Guanine-to-Cytosine.
Crick, thinking of DNA replication, asked John Griffith to find out exactly what type of attraction would exist between, say, two Adenines or two Thymines. Griffith dutifully went away to figure it out– only to come back with the news that, well actually, Adenine attracts Thymine and Guanine attracts Cytosine. Crick, in his wisdom, immediately saw that this meant the copying of genetic information was then most likely done, not via like-like pairings but with “complementary replication.” This fit in well with work already done by Pauling showing that complementary molecules can interact closer and with more stability than same-same molecules.
In his models, Watson originally tried to make DNA work with two identical strands, and with the Sugar-Phosphate backbones on the inside. However he ran into a very physical problem: for the molecule to spiral up and return to symmetry as quickly as the work of others showed that it should, he had to implement ridiculously tight turns at the bonds. However, he found that if he reversed one of the strands so the two strands ran in opposite directions, the turns would only need to be half as tight.
The example used in the book to demonstrate how this works is a pair of pencils. If you lay them side by side in the same direction, you have to spin them on the table a full 360 degrees before they return to symmetry (have the same configuration they started in). However, if you face them in opposite directions (sharpened end of one next to the eraser end of the other), they return to symmetry in just one half-turn.
Eventually, Watson had to give up his idea that the “backbone” ran along the inside of the DNA molecule. One can see why he would have reluctance do this, however; it seems counter-intuitive to enclose the side chains– the part of the structure sending out the instructions– on the inside. Nevertheless, this is how the crazy DNA molecule works– splitting itself apart down the middle to allow the side-chains to do their thing.
As you can see, a lot went into the discovery of the basics of DNA before Watson and Crick ever came into the picture. Their genius, of course, was in putting it altogether, which they did and published in Nature Vol 171 on the 25th of April 1953. Still, even after their achievement, there was much left to be discovered about DNA… and still is.