Thursday, February 24, 2005

Making a worm

If you have not already done so, take a look at the little video clip referenced in the Student Activities section of this week's Musing, the one called "Normal Development." Watch a nematode make itself, starting with one cell, ending with 959.

In every one of those 959 cells there are identical copies of the genome, carried on six chromosomes, six little wound-up bundles of DNA, something like 17,800 genes, coded as 100 million paired chemical units of just four kinds along the DNA double helix.

As the DNA spins and weaves, fabricating proteins, it is controlled by its chemical environment; that is, by what has already been fabricated. The earliest cells -- the stem cells -- have the potential to become any other kind of cell. What they become depends upon which genes are expressed at each stage of development. It's a bootstrap process. The worm pulls itself into being.

Here's how geneticist Enrico Coen puts it in his book, The Art of Genes: "The software, the program, is responsible for organizing hardware, the organism. Yet throughout the process, it is the organism in its various stages of development that has to run the program." In other words, the hardware runs the software, while at the same time the software is generating the hardware.

Of course, the metaphor is not quite satisfactory, but then there is nothing in our familiar experienece quite as astonishing as embryogenesis.