DNA has a double helical structure with the backbone of each helix being
made up of pentose sugars alternating with phosphates. Inside the chains,
and bonded to each sugar is one of 4 'bases' (cytosine, guanine, adenine, 
or thymine) with the bases on the two chains attract to one another by 
hydrogen bonds in a a very specific way - adenine always pairs with 
thymine and cytosine always pairs with guanine.