Al Stahler: Evolution — Crazy, but it works
Back in the 1800s, a young girl named Alice visited some interesting places. One was Wonderland. Another was on the far side of a mirror in which she’d been looking at her reflection. After falling through the looking glass, Alice meets Humpty Dumpty. It bugs Alice that Humpty Dumpty makes up his own definitions for words … to which HD responds that you’ve got to decide who’s to be boss – you … or the word?
The dictionary offers two, very different definitions for the word “spiral.” According to the second definition, a spiral staircase is spiral-shaped. I maintain that it is not.
The first dictionary definition for “spiral” suits me fine. To see a true spiral, fill the sink with water, and pull the plug. The water swirls, ‘round and ‘round, down toward the plug-hole, swirling closer and closer to the center as it spirals downward. Even as it goes around, a true spiral swirls inward, closing down to a point.
A spiral staircase does not spiral down to a point … we’d be trouble if it did. No matter how many floors in your house, that so-called spiral staircase circles ‘round and ‘round, never spiraling down to point.
Such a staircase, I submit, is not a spiral, but a helix. A helix goes ‘round and ‘round, without ever coming closer to the center.
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Our genes – the instructions for assembling and running our bodies – are stored on microscopic charm bracelets – a couple-dozen teeny, tiny charm bracelets called chromosomes.
Each of the charms on a charm bracelet is one word in a coded message … the message adds up to all the instructions for running our bodies.
Lose that instruction manual and your life is over: Do not pass GO, do not collect two hundred dollars. So we do not want these bracelets to just flop around, like bracelets loose in a drawer. Instead, the instruction bracelets are bound up … into a helix. The instructions go ‘round and ‘round, not unlike a (so-called) spiral staircase.
So important is our instruction manual, we keep two copies. Each copy is wound into a helix, the two helices (HEL-ih-seez) winding around each other. Our DNA instruction manual is stored as a double helix.
With two copies of each charm bracelet, should one copy break, we can make repairs. Tiny bio-robots look at the good copy, and proceed to repair the broken one. Such repairs are essential – an uncorrected mistake is a mutation. Mutations cause problems: Parkinson’s disease … color-blindness … cancer.
Since this repair mechanism depends on having a good copy of one helix – one strand of the coded message – woe betide a body-cell whose DNA suffers a double-strand break – both helices busted. Ionizing radiation causes double-strand breaks, which is why ionizing radiation is so dangerous. With both helices – both bracelets – broken, there’s no good strand to look at. Worse, such breaks are seldom clean: When both strands break, we often lose charms at the break points – we lose parts of the message.
And yet, despite the potential havoc caused by a double-strand break, our bodies sometimes make double-strand breaks in our DNA on-purpose. What sort of extreme situation could tempt us to make such double-strand breaks?
Another person Alice met behind the looking glass was the Red Queen. The Red Queen was running … constantly. Alice also had to run, just to chat. But fast as they ran, they didn’t get anywhere. “Why can’t we just stop running?” Alice wondered.
Because, explained the Red Queen, here behind the looking glass, you have to run as fast as you can … just to stay in one place. If you want to actually get somewhere, you have to run twice as fast as that.
In this dog-eat-dog world, we are surrounded by bacteria who see our bodies as food … we are surrounded by viruses who see our bodies as machines for making more viruses. And as fast as our bodies invent a way to fight off the microbes, the bugs invent ways to get around those defenses, forcing us to invent yet more defenses. In biology, this is known as the Red Queen’s Dilemma.
To stand a chance of just keeping up in this crazy race, the ancestors of plants and animals invented sex – mixing up the genes of two parents, rather than one, to give their offspring a fighting chance against the bugs.
More than that, even before mating, the mom-to-be, when making her eggs … the dad-to-be, when making his sperm … s/he makes double-strand breaks in their DNA charm bracelets, trading pieces of one bracelet with pieces from another … really mixing up the instructions.
But they do this very carefully, keeping track of every charm on the bracelet, so nothing is lost.
There’s another extreme situation, in which we take the risks of double-strand breaks.
Fool me once, shame on you … fool me twice, shame on me. When a new bug invades our body, we “take a picture” of it – we remember it. Next time that bug comes by, we’re ready for it – we recognize it, and waste no time fighting it off. The “photographs” we make of the bugs are antibodies.
Wouldn’t it be cool to have antibodies, not just for bugs we’ve seen, but also for bugs we have yet to see … maybe for bugs that don’t even yet exist. Looking to the future, our immune systems do just that.
It takes immense creativity to make such a variety of different antibodies. Immune cells do it by breaking both strands of their DNA, then allowing these double-strand breaks to re-assemble every-which-way … this is the one part of our body where we want to create mutants: Mutants R us.
It might seem haphazard, but, often enough, it works.
It works, that is, if we keep our immune systems healthy. Which means we must eat enough … move enough … sleep enough.
IN THE SKY
Most constellations look nothing like the critters for which they’re named, but Leo is an exception. Tonight and the next several nights, the moon hangs out with Leo. Think of a library lion, resting, but alert. To the right of the moon the lion’s foreparts are drawn with a backwards question mark; to the right of the question mark a single star marks his forepaws. A triangle of stars traces out the lion’s haunch.
Al Stahler enjoys sharing science and nature with friends and neighbors on KVMR-FM, and can be reached at stahler@ kvmr.org.
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