Believing in DNA

Originally published 7 May 1990

A [Feb­ru­ary 1990] issue of Sci­ence con­tained a pho­to­graph, made with an elec­tron micro­scope, of a por­tion of DNA extract­ed from a sin­gle-celled organ­ism called try­panosome.

If you have ever seen the mess a kit­ten can make of a ball of twine, this it it. The DNA is twist­ed into knots and loops as intri­cate as a cro­cheted table­cloth. The author of the arti­cle that accom­pa­nies the pic­ture, sci­ence reporter Bar­ry Cipra, uses the image of “an elab­o­rate fish­net.” Then he goes on to say, “Yet some­how the fish­net man­ages to repro­duce itself.”

Some­how, indeed!

It has always seemed to me a mir­a­cle that even a sim­ple lin­ear strand of DNA can repro­duce itself. In prin­ci­ple, the idea is sim­ple enough. The DNA mol­e­cule is shaped like a spi­ral stair­case, the famous dou­ble-helix. When the time comes to repro­duce, the stair­case unzips down the mid­dle of the treads. The unzipped half-stair­cas­es turn them­selves into two new stair­cas­es by tak­ing chem­i­cal com­po­nents out of the sur­round­ing medi­um — new bits of tread and banister.

Writhing and twisting

The idea is stun­ning, inge­nious, per­haps the most impor­tant sci­en­tif­ic dis­cov­ery of the 20th cen­tu­ry. But is not quite as sim­ple as I made it sound. In each of our bod­ies there is enough DNA, if stretched out, to reach to the sun and back 100 times, an arm’s length in every cell. It is writhing and twist­ing all the time, zip­ping and unzip­ping, becom­ing part of fern-like trac­eries with RNA as the cells build pro­teins, or doing its mirac­u­lous dance of repro­duc­tion. This unceas­ing activ­i­ty is assist­ed by enzymes that loosen, snip, tie, dis­en­tan­gle, unwind.

Look at that try­panosome tan­gle again. It’s not just lay­ing there. It’s a puls­ing, undu­lat­ing jum­ble of threads, feath­ers, knobs, and whiskers. A myr­i­ad of tiny lace-mak­ers fre­net­i­cal­ly mak­ing a lace called life.

That all of this should hap­pen, hour by hour, with­out result­ing in a hope­less tan­gle is unbe­liev­able. No mat­ter how much I read about how it hap­pens, no mat­ter how hard I try to stretch my imag­i­na­tion, it still seems sim­ply impossible.

And yet I believe it.

But I don’t believe in UFOs, abom­inable snow­men, Loch Ness mon­sters, or fairies, all of which, on the face of it, are more plausible.

I live part of each year in rur­al Ire­land on a road known local­ly as “The Fairies’ Road.” Once, my neigh­bor expressed a reluc­tance to walk the road at night, appre­hen­sive, even in this age of sci­ence, of lit­tle folk under the hill. I smiled con­de­scend­ing­ly. Lat­er I men­tioned to this same per­son that our hill had once been cov­ered with an Ice Age glac­i­er. Now it was her turn to smile. “It is eas­i­er to believe in fairies under the hill,” she said, “than ice on top.”

And she’s right.

So why do I believe in the fan­dan­go dance of the DNA, which I can’t imag­ine no mat­ter how hard I try, and not fairies, which every child can imag­ine? The answer I like best is based on ideas of physi­cist-philoso­pher Hen­ry Margenau.

Perceptions and constructs

Mar­ge­nau uses a sim­ple dia­gram. At the right he draws a ver­ti­cal line that he calls the per­cep­tion plane. It rep­re­sents our imme­di­ate per­cep­tions of the world: sights, tastes, touch­es, sounds. To the left of the line he draws cir­cles rep­re­sent­ing “con­structs,” ideas we invent to explain our per­cep­tions. The more abstract the con­struct, the fur­ther the cir­cle is from the line.

“Fairy” is a con­struct that is close to the line. After all, a fairy is just a lit­tle per­son with drag­on­fly wings, and there’s noth­ing unfa­mil­iar about any of that. The con­struct explains things we per­ceive — a tool gone miss­ing from the gar­den, or a strange singing noise heard on the hill at night. Between the con­struct “fairy” and the miss­ing tool or singing noise we can draw a direct line.

The dance of DNA is a con­struct far removed from the per­cep­tion plane. There is noth­ing about the con­struct that relates direct­ly to any per­cep­tion we have ever had, which is why it’s so dif­fi­cult to imagine.

The con­struct “danc­ing DNA” is ulti­mate­ly con­nect­ed with our per­cep­tions by way of oth­er con­structs, lines con­nect­ing cir­cles to cir­cles in a vast web, ideas linked to oth­er ideas until at last we reach the rel­e­vant per­cep­tions — black lines on a pho­to­graph­ic film, the posi­tion of a nee­dle on an elec­tri­cal meter, the bal­ance of a scale.

“Fairy” is con­nect­ed to things we per­ceive by one or two lines, but to noth­ing else in sci­ence. Snip a few threads and the con­struct fairy falls away.

The dance of DNA is fixed in a vast web of inter-con­nect­ed ideas that ulti­mate­ly embraces all of sci­ence. It is the firm­ness of those many con­nec­tions, based on tens of thou­sands of exact, quan­ti­ta­tive exper­i­ments that anchors the con­struct to real­i­ty. Snip a line here and there and the web still holds.

And that’s why I believe the “impos­si­ble” dance of DNA.

But it still isn’t easy.