Page 113 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.4 Meiosis: Cell Divisions That Halve Chromosome Number 105

Figure 4.15 To aid visualization
of the chromosomes, the figure is
simplified in two ways: (1) The nuclear
envelope is not shown during prophase
of either meiotic division. (2) The
chromosomes are shown as fully
condensed at zygotene; in reality, full
condensation is not achieved until
diakinesis.

Prophase I: Diplotene Prophase I: Diakinesis
1. Synaptonemal complex dissolves. 1. Chromatids thicken and shorten.
2. A tetrad of four chromatids is visible. 2. At the end of prophase I, the nuclear
3. Crossover points appear as chiasmata, membrane (not shown earlier) breaks
holding nonsister chromatids together. down, and the spindle begins to form.
4. Meiotic arrest occurs at this time in many
species.

Telophase I Interkinesis
1. The nuclear envelope re-forms. 1. This is similar to interphase with one
2. Resultant cells have half the number of important exception: No chromosomal
chromosomes, each consisting of two duplication takes place.
sister chromatids. 2. In some species, the chromosomes
3. Cytokinesis separates the daughter decondense; in others, they do not.
cells (not shown).

Telophase II Cytokinesis
1. Chromosomes begin to uncoil. 1. The cytoplasm divides, forming four
2. Nuclear envelopes and nucleoli (not new haploid cells.
shown) re-form.