def rev_c(read):
    """
    return reverse completment of read
    """
    rc = []
    rc_nucs = {'A': 'T', 'T': 'A', 'G': 'C', 'C': 'G', 'N': 'N'}
    for base in read:
        rc.extend(rc_nucs[base.upper()])
    return rc[::-1]
