(Intro)

You've probably been warned that getting too much sun can do some serious harm to your skin because sunlight can damage your DNA. Even a garden variety sunburn is no picnic, but if the damage is bad enough, it can lead to skin cancer. And here's the thing, this DNA damage doesn't just happen in the summer. That's because ultraviolet radiation bombards the whole earth year-round, even now when the northern hemisphere is all chilly and cloudy.

When UV radiation interacts with your DNA, a precise chemical reaction happens and your body tries to fix the damage as much as it can. We divide the UV radiation in sunlight into three main types, based on their wavelength. Low-energy UVA is mostly absorbed by the skin pigment called melanin, and high-energy UVC is absorbed by the Earth's ozone layer. The main DNA-damaging culprit is the medium-energy UVB, which you encounter pretty much any time you go outside.

But it's not like sunlight takes a baseball bat to your genome and bashes things at random. See, DNA is made up of four different building blocks called "bases": adenine, cytosine, guanine, and thymine. These bases are linked together by a backbone made of sugars and phosphates to form a DNA strand. To make that iconic double helix structure, the bases of one strand pair up with the bases of another strand. T pairs with A, and C pairs with G. And when a photon of UVB light strikes a specific combination of DNA bases, usually when two T's or two C's are next to each other, a very specific chemical reaction takes place. The energy from the UVB photon links side-by-side T's or C's together. When bases are linked to their next-door neighbors instead of being paired with bases on the opposite strand, it causes a bulge in the DNA double helix. This funky shape gives the cell trouble when it comes to reading or copying the information contained in your DNA, which, obviously, is not so good for the cell.

Fortunately, because this is such a specific kind of damage, our bodies are able to spot and fix it without much fuss. But, if you expose your cells to too much UV radiation, DNA damage can build up faster than it can be fixed. That's what leads to permanent mutations in your DNA, and down the line, possibly cancerous cells. So, even though our cells are awesome at spotting problems, and even though it's winter time here, you might want to slather on a bit of sunscreen if you're spending a lot of time outside.

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