def acnucseq_from_pdb(input_chain):
    """ Given an input DNA chain, returns a string with its nucleotide sequence. """

    acnucseq = ""
    for res in input_chain:
        if len(res.get_resname()) > 1:
            acnucseq += str(res.get_resname().rstrip()[2:])
        else:
            acnucseq += str(res.get_resname()).rstrip()

    return acnucseq