Returns the sequence of the `Polynucleotide` as a string.

        Returns
        -------
        sequence : str
            String of the monomer sequence of the `Polynucleotide`.

def sequence(self):
        """Returns the sequence of the `Polynucleotide` as a string.

        Returns
        -------
        sequence : str
            String of the monomer sequence of the `Polynucleotide`.
        """
        seq = [x.mol_code for x in self._monomers]
        return ' '.join(seq)