meiosis+I

= MEIOSIS I = == 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 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.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. == == The centrioles are at opposite poles of the cell.The pairs of homologous chromosomes, 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. Spindle fibers from one pole of the cell attach to one chromosome of each pair ,and spindle fibers from the opposite pole attach to the homologous chromosomes. ==

[[image:http://www.phschool.com/science/biology_place/biocoach/images/meiosis/meana1.gif width="400" height="192"]]
==  Anaphase I begins when the two chromosomes of each bivalent separate and start moving toward opposite poles of the cell as a result of the action of the spindle.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. 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. 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. 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. ==