def bin_to_nucleotide(bin_str):
    """
    Convert binary bits to nucleotide
    :param bin_str: pair of binary bits.
    :return: nulceotide A, G, C, T
    """
    if bin_str == '00':
        return 'A'
    elif bin_str == '01':
        return 'C'
    elif bin_str == '10':
        return 'G'
    elif bin_str == '11':
        return 'T'
    else:
        return None