Mitosis and meiosis compared. Mitosis is shown on the left, meiosis on the right. (Note that numbering of the phases as prophase I, metaphase I, and so on, is necessary only for meiosis, as explained in the text.) For clarity, only two pairs of homologous chromosomes are shown. Cells double their chromosome number in S phase, producing four pairs of sister chromatids. The centrosomes (green) give rise to the spindle apparatus from each cell pole, and spindle fibers eventually connect to each sister chromatid pair (or tetrad in meiosis). The nucleus is shown as a blue-