Even if you from time to time think about your neurons, those little chemical-electrical switches that dictate your mental and physical activity, you probably don't give much thought to your transposons. And yet transposons don't just play a crucial role in neural application; in a very real sense, they define who you are.
A transposon is a fragment of DNA that inserts itself into another cell. Research suggests that about half our DNA sequence is made up of these fragments, these interlopers.
In the cells of, say, your lungs, heart, or kidneys, transposons have no real effect. They don't behave like viruses, which sneak into cells and multiply like crazy. They're more like very mellow hitchhikers: once they've found their way in, they're usually content to fall asleep and enjoy the ride.
The exception is the brain. Once transposons get inside neurons, they can alter the very nature of the cell. It's like a troupe of improv actors that show up unexpectedly at your birthday party: suddenly you're at a very different party. Transposons can influence a neuron's firing sequence, or turn it off or on, or even reconfigure the operating code of the whole chemical-electrical switch.
This means they can change the entire identity and purpose of a neuron. And like any good improv actor, they can shift into a variety of roles and characters.
It's a Darwinian parable playing out on a cellular level, what Kelly Clancy in her New Yorker piece "The Stranger in Your Brain" calls "a kind of evolution in miniature."
The result? Even among twins, no two brains are exactly alike. Identical twins begin with identical DNA, but the arrival of those improvising transposons makes neural activity wholly unique. And since transposons aren't passed down, your brain is truly a once-in-a-lifetime show.
Clancy explains that this is in part why it's so difficult to find the underpinning of neurological diseases: with each transposon doing its own thing, there may be no single static genome with regions that can be identified, isolated, and acted upon.
If all this sounds a little, well, scary (rogue DNA wreaks havoc on unsuspecting brain!), Clancy says there's no reason to panic just yet. After all, she writes, "it is our mosaic brains that may deepen our capacity for individual invention and imagination."
Besides, who doesn't love a little improv? More than half a century since its premier in 1959, Second City continues to thrive. And you'll be thriving for as long as your transposons continue to act.