Meiosis I

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Meiosis I

Telaphase I

Many cells that undergo rapid meiosis do not decondense the chromosomes at the end of telophase I. Other cells do exhibit chromosome decondensation at this time; the chromosomes recondense in prophase II.

A nuclear envelope reforms around each chromosome set, the spindle disappears, and cytokinesis follows. In animal cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells. After cytokinesis, each of the two progeny cells has a nucleus with a haploid set of replicated chromosomes.

The homologous chromosome pairs complete their migration to the two poles as a result of the action of the spindle. Now a haploid set of chromosomes is at each pole, with each chromosome still having two chromatids.

The homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis follows to produce two cells.

Anaphase I

Notice that in anaphase I the sister chromatids remain attached at their centromeres and move together toward the poles. A key difference between mitosis and meiosis is that sister chromatids remain joined after metaphase in meiosis I, whereas in mitosis they separate.

Anaphase I begins when the two chromosomes of each bivalent (tetrad) separate and start moving toward opposite poles of the cell as a result of the action of the spindle.

The two chromosomes in each bivalent separate and migrate toward opposite poles.

Metaphase I

Spindle fibers from one pole of the cell attach to one chromosome of each pair (seen as sister chromatids), and spindle fibers from the opposite pole attach to the homologous chromosome (again, seen as sister chromatids).

The pairs of homologous chromosomes (the bivalents), now as tightly coiled and condensed as they will be in meiosis, become arranged on a plane equidistant from the poles called the metaphase plate.

The centrioles are at opposite poles of the cell.

The pairs of chromosomes (bivalents) become arranged on the metaphase plate and are attached to the now fully formed meiotic spindle.

Prophase I

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

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

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 nucleolus disappears during prophase I.

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).

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.

Chromosomes become visible, crossing-over occurs, the nucleolus disappears, the meiotic spindle forms, and the nuclear envelope disappears.