Meiosis 1,prophase 1

1.	Meiosis 1,prophase 1
Answer» \tAt 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.\tThe 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).\tThe nucleolus disappears during prophase I.\tIn 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.\tThe nuclear envelope disappears at the end of prophase I, allowing the spindle to enter the nucleus.\tProphase I is the longest phase of meiosis, typically consuming 90% of the time for the two divisions.

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