Deoxyribonucleic acid (DNA) is a chemical found in the nucleus of cells and carries the "instructions" for the development and functioning of living organisms.
If you want to know more http://en.wikipedia.org/wiki/DNA
In DNA strings, symbols "A" and "T" are complements of each other, as "C" and "G". You have function with one side of the DNA (string, except for Haskell); you need to get the other complementary side. DNA strand is never empty or there is no DNA at all (again, except for Haskell).
DNAStrand ("ATTGC") # return "TAACG"
DNAStrand ("GTAT") # return "CATA"