code: def comp(seq):
      
      ## makes base to its small complement then makes upper
      return seq.replace("A", 't')\
                .replace('T', 'a')\
                .replace('C', 'g')\
                .replace('G', 'c')\
                .replace('n', 'Z')\
                .upper()\
                .replace("Z", "n")
docs: returns a seq with complement. Preserves little n's for splitters.