Simply Science, Aug. 29, 2011

There are so many problems in the world today. Let me shed some light on one of them. The problem upon which I wish to shed light is sunburn ... and why would I want to shed light on sunburn? Thanks for asking.

Sunlight has a wide spectrum of wavelengths. Included are very short wavelengths called ultraviolet, or UV, light. UV light is a form of energy that can cause damage to cells. If too much damage is caused, the cell is killed and we call that a sunburn. When UV light reaches the DNA in skin cells, it forces two sides of the DNA molecule together causing a distortion of the DNA molecule. This creates a break on each side of the molecule.

However, in a cellular version of “If-you-break-it-you-fix-it,” many of the breaks are repaired by a mechanism called light repair, or photo-reactivation. While UV light is breaking the DNA, the visible light portion of the light spectrum causes your cells to produce an enzyme called photolyase.  Photolyase repairs DNA by tearing open the misshapen, damaged area of the DNA and reforming it into its original, undamaged shape. (It’s all imaginary, so just imagine little mechanics with wrenches running around in the cell fixing broken things. It’s easier for me.)

So even though you break DNA with sunlight, light also triggers the repair mechanism that fixes the breaks. The health of the cell becomes a race between breaking and repairing. Of course, a break has to occur before a repair can be initiated, so repair is always lagging a little behind.

Over short periods of time, the repair mechanism can keep up. But the longer the cell’s exposure to UV light, the more breaks occur, and the fewer breaks are repaired.

This process mirrors my truck. The truck breaks down. I repair it. Something else breaks again, but now I’m broke so the repairs have to wait. While waiting, something else breaks again, and again. Pretty soon it’s mostly broken. In the same way, over a period of years, exposure to sunlight is accumulative and the accumulated damage done by UV light induced breaks becomes mutant cells that are a form of cancer.

Recent research at Ohio State University by Professors Dongping Zhong and Robert Smith has shown that photolyase doesn’t repair the breaks on both sides at once. It’s a two-step process that sends an electron across the DNA molecule in a circular route from one breakup site to the other. This all happens very fast, over extremely short distances, but they were able to measure the time difference in the two-step process by using a strobe laser that can take measurements in time periods of 90-trillionths of a second.

Theoretically, the electron could jump straight across the DNA molecule to repair the cell. However, there is another molecule the electron follows that acts like a bridge. So even though it covers a slightly longer distance, the electron travels more quickly. It’s like the Riverside Parkway.

So the way you fix UV light-induced breaks in DNA is by shining even more light on the cell (albeit light waves of the proper wavelength).

And the way we measured the two-step process is by using a strobe light to capture changes over short periods of time.

“There are so many problems in the world today. Let me shed some light on one of them.” Get it? We get the breaks when we shed light on cells. We repair the breaks when we shed light on them. And we measure the repair process by shedding light on it.

It’s been said that the pun is the lowest form of humor. I suppose that makes this article an even lower form of humor than usual, for puns that is. Does this mean I get a trophy or something?

Gary McCallister is professor of biology at Colorado Mesa University.


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