Outline of Meiosis I

Two pairs of chromosomes are illustrated
in a diploid (2n) animal cell. Homologous chromosomes are indicated in blue
and pink. Micrographs show meiosis in a lily plant cell (Lilium regale).

    Prophase I. Homologous chromosomes condense,
pair up, and swap segments. Spindle micro- tubules attach to them as the nuclear envelope breaks up.

     Metaphase I. The homologous chromosome pairs
are aligned midway between spindle poles.

      Anaphase I. The homologous chromosomes separate and begin heading toward the spindle poles.

     Telophase I. Two clusters of chromosomes reach the spindle poles. A new nuclear envelope forms around each cluster, so two haploid (n) nuclei form.

        Prophase II. The chromosomes condense. Spindle microtubules attach to each sister chromatid as the nuclear envelope breaks up.

        Metaphase II. The (still duplicated) chromosomes are aligned midway between poles of
the spindle.

       Anaphase II. All sister chromatids separate. The now unduplicated chromo -somes head to the spindle poles.

        Telophase II. A cluster of chromosomes reaches each spindle pole. A new nuclear envelope encloses each cluster, so four haploid (n) nuclei form.