def rev_comp(item):
    """
        Retuns the base on the opposite strand
    """
    if item == 'A':
        return 'T'
    elif item == 'G':
        return 'C'
    elif item == 'T':
        return 'A'
    elif item == 'C':
        return 'T'
    else:
        return 'N'