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". Your function receives one side of the DNA (string, except for Haskell); you need to return the other complementary side. DNA strand is never empty or there is no DNA at all (again, except for Haskell).
More similar exercise are found here: http://rosalind.info/problems/list-view/ (source)

"ATTGC" --> "TAACG"
"GTAT" --> "CATA"
Copy

def DNA_strand(dna):
    comp_strand = ""
    pairs = {"A": "T",
            "C": "G",
            "G": "C",
            "T": "A"}
    for char in dna: 
        comp_strand += pairs[char]
    return comp_strand
Copy

pairs = {'A':'T','T':'A','C':'G','G':'C'}
def DNA_strand(dna):
    return ''.join([pairs[x] for x in dna])
Copy

def DNA_strand(dna):
    reference = { "A":"T",
                  "T":"A",
                  "C":"G",
                  "G":"C"
                  }
    return "".join([reference[x] for x in dna])

Copy