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Meiosis
Concept 4: Meiosis I: Prophase I
Concept 4: Meiosis I: Prophase I
Chromosomes become visible, crossing-over occurs, the nucleolus disappears, the meiotic spindle forms, and the nuclear envelope disappears.
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• At the start of prophase I, the chromosomes have already duplicated. During prophase I, they coil and become shorter and thicker and visible under the light microscope.

• The duplicated homologous chromosomes pair, and crossing-over (the physical exchange of chromosome parts) occurs. Crossing-over is the process that can give rise to genetic recombination. At this point, each homologous chromosome pair is visible as a bivalent (tetrad), a tight grouping of two chromosomes, each consisting of two sister chromatids. The sites of crossing-over are seen as crisscrossed nonsister chromatids and are called chiasmata (singular: chiasma).

• The nucleolus disappears during prophase I.

• In the cytoplasm, the meiotic spindle, consisting of microtubules and other proteins, forms between the two pairs of centrioles as they migrate to opposite poles of the cell.

• The nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus.

• Prophase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.