Wednesday, October 03, 2012

It has not escaped our notice…

It was short and sweet, taking up no more than one page in the journal Nature. It was surely the most important scientific communications of the second half of the 20th century, the most important communication in biology since Darwin. The paper had the unprepossessing title "A Structure for Deoxyribose Nucleic Acid." The authors were J. D. Watson and F. H. C. Crick. The date of publication: April 25, 1953.

I took high school biology in 1951-52. DNA was not mentioned. Within a decade, every respectable biology text or course had DNA front and center. The molecule of life.

You can read the Watson and Crick paper in an annotated version here. You can see that it is fairly technical. It proposes a double helix structure for the DNA molecule: sugar-phosphate railings and paired organic bases stair-treads. Only four bases: adenine, which uniquely pairs with thymine, and guanine, which uniquely pairs with cytosine. A-T. G-C.

Who, glancing at the paper, could have guessed?

Watson and Crick guessed. The colossal importance of the paper is hidden in the last sentence: "It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material."

Well, it's all history now. But mysteries remain. There's an arm's length of DNA in almost every cell of our bodies. Every time a cell reproduces, the DNA is copied. Kenneth Johnson in a recent Nature says: "However, the structural and energetic differences between the canonical Watson-Crick base pairs and mismatched base pairs are so subtle that one is left wondering how cellular machinery can catalyse DNA replication with sufficient speed and accuracy to sustain life." Add to this the infernal tangle of DNA within a cell and one wonders how life is possible at all.

But that is sure to change. In the July 12 issue of Nature, a group of researchers from the U. S., Japan, and China managed to observe a DNA repair enzyme in the act of replicating DNA. They stopped the reaction at various stages by cooling, then observed the interrupted process with X-ray crystallography.

Here is an illustration from the paper, showing the molecular machinery at work at various stages (click to enlarge). This, remember, on a scale a hundred thousand times smaller than the period at the end of this sentence.

We don't have to understand the details of what's going on -- the replication, the quality control, the repair. The miracle is that it's going on at all, all the time, in every cell of every living creature on Earth. And that humans have devised the exacting skills to lay it out for our delectation.