def extract_start_stop(exon):
    """Extract start and stop from a chr1,100-200,- tuple

    Parameters
    ----------
    exon : tuple
        (chrom, startstop, strand) tuple of strings, e.g.
        ('chr1', '100-200', '-')

    Returns
    -------
    extracted : tuple
        (chrom, start, stop, strand) tuple of strings, with start and stop
        extracted

    >>> extract_start_stop(('chr1', '100-200', '-'))
    ('chr1', '100', '200', '-')
    """
    chrom, startstop, strand = exon
    start, stop = startstop.split('-')
    return chrom, start, stop, strand