Originally published 15 January 1996
Every high school biology student learns something like this: “Before a cell divides, the DNA replicates itself. A complete copy of the DNA moves to each side of the cell. Then the cell splits down the middle.”
Cell reproduction takes place in a matter of minutes or hours, depending on the cell. One makes two. Two make four. Four make eight. Et cetera, et cetera. Zip, split. Zip, split. Life goes on.
Sounds simple. The DNA double-helix unzips into two strands. Each strand forms its complement. The two copies of the genetic material move apart. The cell divides. Bingo!
A piece of cake.
Uh, yeah. Consider for a moment that each human cell contains a couple of meters of DNA. No kidding. If you stretched out the DNA in any human cell it would reach from fingertip to fingertip of your outstretched arms. How can two meters of thread be balled up into a cell that is invisibly small? The answer is that the DNA strand is a million times narrower than sewing thread. The tangled genetic material fits comfortably inside a cell.
(If you’re a high school student, try this exercise. It’s a good example of the power of mathematics to aid the imagination: DNA is about 3 nanometers in diameter. A typical animal cell is 15 microns in diameter. Show that the volume of the DNA is hundreds of times smaller than that of a cell.)
The DNA in a human cell is distributed over 46 chromosomes in a tangle like you wouldn’t believe. Replication starts at hundreds or thousands of sites, at precisely defined moments in the cell’s reproductive cycle. Billions of chemical units in the DNA thread must be copied exactly, exactly once. No more, no less. Any foul-up can be damaging or fatal. The imagination falters before the complexity of the process.
Where else can we find an aid to the imagination? I can think of only one thing in our common experience that begins to approach the complexity and finesse of cellular reproduction: computers. In a computer, tens or hundreds of millions of microscopically small electronic switches are opened and shut each second, in precise sequence, with essentially zero tolerance for error. If even one of millions of switches fails to open or shut at the correct moment, the system can crash.
Granted that even the largest computers are still pale imitations of the complexity of a living organism, but the fact that computers work at all, as reliably as they do, makes cellular reproduction easier to grasp.
A [December 1995] issue of the journal Science was partly devoted to progress in understanding DNA replication. The articles were technical, but it is clear to even the casual reader that substantial progress is being made toward unraveling the riddle of life. To paraphrase Churchill: We are not at the end of understanding DNA replication. We are not even at the beginning of the end. But we are at the end of the beginning.
It’s striking how often their authors use the metaphor of machines. “Cellular machinery.” “Molecular machines.” “Molecular motors.” “Replication mechanisms.” “Mechanisms for maintenance of DNA ends.” And so on. Life as a machine. It is a metaphor that has been deeply ingrained in scientific thought since the 17th century, when a scientific revolution coincided with a time of mechanical innovation.
The old metaphor apparently has some life in it yet. At least, no more fruitful metaphor has come along. But the mechanical metaphor has taken on a new slant. It is not the clockwork of gears and levers that best represents the machinery of life, but the silicon chip.
A completely functional digital computer could be made out of mechanical gears and levers, but such a machine would be mammoth, cumbersome and slow. What goes on inside an electronic computer is closer in scale — of size and speed — to what happens inside a living cell. Indeed, the computer has become an indispensable tool of molecular biologists. Only with computers can they begin to give visible representation to the exquisite chemical machinery of cellular reproduction.
Many people recoil from the mechanical metaphor for life. They cling to the notion that there is something magical, irreducible and transcendent about life, something that will forever escape the grasp of the molecular biologists with their computer models of chemical structures.
Two things to keep in mind:
1) “Life is a machine” is only a metaphor. All understanding is metaphorical — in science, in poetry, even in theology. No one mistakes the gray-bearded man on the ceiling of the Sistine Chapel for God, but Michelangelo’s powerful metaphor evokes awe and understanding of something essential to the believer’s idea of God. In science, too, we use the metaphors that most fruitfully advance our understanding of nature.
2) The mechanical metaphor for life does not so much reduce the miraculous to the mundane as it elevates the mundane to the miraculous. Remember, “mundane” comes from the Latin mundus, meaning “world.” The more we understand the staggeringly complex molecular machinery of life, the more truly miraculous the world seems.
