Re: How is DNA affected by ultraviolet light?

Hello!

In order to answer your question, let's start with what DNA looks like.

DNA is a molecule that resembles a long string with differently shaped 
tabs attached along the length of it.  There are four shapes to these 
tabs, called adenosine, guanine, cytosine and thymine.  The proper name 
for these four tabs is "bases".

Think of the order of bases along the DNA string as the words to the 
instruction manual for the cell.  A molecule (called RNA polymerase) rolls 
along the DNA and reads the bases, then makes a copy which gets sent out 
to the rest of the cell.

Ultraviolet light damages DNA by changing the shape of the thymine base.  
If two thymines are attached side by side along the DNA molecule, 
ultraviolet light will stick the two bases together and warp the DNA 
strand.  This makes it impossible for the RNA polymerase to read the 
instructions so it makes a bad copy that gets sent out to the cell, and as 
a result, the cell acts abnormally, or it may even die.  There are repair 
systems in every cell to fix damaged DNA, but it can get overpowered if 
there is too much damage.  

Any cell can have its DNA damaged by ultraviolet light, be it from your 
heart, your liver or your lungs.  Fortunately for us, the skin absorbs 
ultraviolet light and so protects our inside organs.  Aside from the skin, 
our eyes and hair get damaged by ultraviolet light.

It isn't the damage to the DNA that is important with our eyes and hair- 
in this case, the ultraviolet light creates very energetic molecules 
called free radicals which destroy the proteins in the lens of your eye 
(where you see out from), and in your hair, which is why you get streaks 
in your hair from being out in the sun.

All in all, enjoy going out in the sun, but keep in mind the ultraviolet 
light and dress appropriately!

Evelyne

here are my references:
1. Molecular Cell Biology. 2nd ed. 1990. J. Darnell, H. Lodish, and D. 
      Baltimore.  pub: Scientific American Books, Inc., New York.
2. Solar Protection. 1998. R. Souccar.  pub: Actualité Santé, Paris.