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())))
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())))