def get_backbones(nts):
    """ Get backbone pairs.
    Args:
    ___
    nts (dict): DSSR nucleotide info.

    Returns:
    ---
    bb (list): list of tuples (5' base, 3' base)
    """
    bb = []
    for i, three_p in enumerate(nts):
        if i == 0:
            continue
        five_p = nts[i-1]
        if five_p['chain_name'] != three_p['chain_name']:
            continue
        if three_p['nt_type'] != 'RNA' or five_p['nt_type'] != 'RNA':
            continue
        if 'break' not in three_p['summary']:
            bb.append((five_p, three_p))
    return bb