Sunday, July 12, 2009

A few more thoughts on twiners

Among the twining plants, some climb clockwise (bindweed), some climb anti-clockwise (honeysuckle). A single mutation in the genes of certain plants can cause a normally straight stem to spiral, by changing the shape of a crucial protein. The direction of spiraling is all there in the first cell that becomes the plant.

So ponder this. Every living thing begins life as a single cell, one-hundred times smaller than the period at the end of this sentence. And within that invisibly small space is the information that will determine whether the cell becomes a gnat or an elephant, a morning-glory or a sequoia, a right-twiner or a left-twiner. The more you think on this, the more miraculous it seems. I have written about it dozens of times and still I am astounded. It seems, quite frankly, impossible.

There are four kinds of nucleotides along the DNA double helix: adenine, guanine, cytosine and thymine. Adenine always pairs with thymine, and guanine always pairs with cytosine: A-T, T-A, G-C and C-G. Four possibilities. The instructions for making a living creature -- any living creature -- is written in a chemical code of just four letters. Print out the human genome in 12-point type and you'd need a warehouse to store all the books. Somewhere in that mass of text is the difference that makes the brown-eyed you different from your blue-eyed daughter.

A warehouse of information in a space a hundred times smaller than the dot at the end of this sentence!

I've done the calculation. You'll find it in Skeptics and True Believers, and no doubt somewhere in the archives of this blog. It all works out. There is no magic. No miracle. Every day we edge a little bit closer to figuring out the details of how the genes express themselves in the development of an organism. Maybe there is some big new thing we have yet to learn, maybe not.

Here's an analogy I worked out some years ago: Imagine the human DNA as strands of ordinary sewing thread. On this scale, the DNA in the 23 pairs of chromosomes in a typical human cell would be about 150 miles long, with about 600 nucleotide pairs per inch. That is, the DNA in a single cell is equivalent to 1000 spools of sewing thread! This represents two copies of the genetic code.

Take all that thread -- the 1000 spools worth -- and crumple it into 46 wads (the chromosomes). Stuff the wads into a shoe box (the cell nucleus) along with -- oh, say enough chicken-noodle soup to fill the box. Toss the shoe box into a steamer trunk (the cell), and fill the trunk with more soup.

Take the steamer trunk with its contents and shrink it down to an invisibly small object, one hundred times smaller than the period at the end of this sentence. Multiply that tiny object by a trillion and you have the trillion soma cells of the human body, each with its full complement of DNA.

Right-twiners, left twiners. Brown eyes, blue eyes. Gnat or elephant. Morning-glory or sequoia. Take a deep breath. As the writer/cartographer Tim Robinson suggested, miracles are explainable, it's the explanations that are miraculous.