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Imagine that it's summer and you're at the beach, enjoying the pleasant June sun. It's not too hot, you have a relaxing book and a nice cool drink, and your tan is looking good.

What's wrong with this picture?From the point of view of a geneticist, a lot. This is not the peaceful scene that it appears to be. At a microscopic level, there's a relentless battle going on between ultra violet rays from the sun and the chromosomes inside your skin cells.

According to Daniel Hartl, a geneticist from Harvard University, the ultraviolet rays that get past our thinning ozone layer can easily penetrate skin cells. If those rays miss the nucleus of the cells, they'll probably do little damage. However, when one of these rays hurtles into the nucleus of a cell and then collides with one of the chromosomes inside the nucleus, that ray can cause a mutation.

It's like what would happen if you took an icepick and poked it through the stack of blueprints needed for building a skyscraper. Maybe the icepick wouldn't hit any important information. On the other hand, it might corrupt the information for constructing the electrical system or some other vital part of the building's architecture.

How does this relate to the skin cells? The odds of any mutation proving beneficial are close to zero. Geneticists agree that the chance of a new mutation doing you any good is comparable to the chances of a drunken monkey with a screwdriver making your watch keep better time.

So if the odds of the new mutation doing you any good are virtually non-existent, what are the odds that a mutation in one of your skin cells will harm you?

The good news is that in most cases the mutation will not be a problem. Normal cells can detect damaged chromosomes, and once they've detected the damage they have a remarkable capacity for repairing themselves. If the repair mechanisms are working right, the cell will stop reproducing as the chromosome cuts out its own damaged part and then manufactures a new copy of the correct part. This done, the cell will go back to reproducing itself.

Your skin cells naturally repair themselves countless times. The process works well, but it's not perfect. The more ultra violet exposure you receive, the more likely the process is to fail. Even when the process fails, in the case of skin cells, it usually won't matter too much. Perhaps a cell will not be able to reproduce and will die. But you have roughly 100 trillion cells, and the death of one doesn't matter a great deal.

However, there are cases where it does matter. If the mutation damages the cell's ability to control its own reproduction, it may start reproducing uncontrollably. In that case, it has become cancerous. Untreated, the cancer cells could multiply further, not only killing the nearby tissues but eventually destroying your vital organs as well.

If the ozone layer truly is thinning, and the vast majority of scientists say that it is, protecting our cells from ultraviolet rays will become more and more important in the future. Ultraviolet rays are not something to panic about, but they're serious enough so that it's worth taking a few easy precautions against them.

When you're sunbathing, use an effective sunscreen, wear sun glasses and avoid staying out in the sun so long that you get a sunburn.