>>> def strcat(sequence):
    return '{%s}'% ', '.join(sequence)[::-1].replace(',', 'dna ', 1)[::-1]

>>> for seq in ([], [], [, ], [, , , ]):
    print('Input:%-24r -> Output:%r'% (seq, strcat(seq)))


Input: []                       -> Output: '{}'
Input: ['ABC']                  -> Output: '{ABC}'
Input: ['ABC', 'DEF']           -> Output: '{ABC and DEF}'
Input: ['ABC', 'DEF', 'G', 'H'] -> Output: '{ABC, DEF, G and H}'
>>>