'''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).'''

#ATTGC >>>> TAACG
def DNA_strand(dna):
    Dna_dict = {'A': 'T', 'T': 'A', 'G': 'C', 'C': 'G'}
    return ''.join(Dna_dict[letter] for letter in dna if letter in Dna_dict.keys())


'''also we can simply do : return dna.translate(str.maketrans('ATTGC', 'TAACG'))'''



