Meiosis - Cell cycle

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Meiosis
a two-stage type of cell division in sexually reproducing organisms that results in cells with half the chromosome number of the original cell
Prophase I
Early in this phase, the chromatin coils up, so that individual chromosomes become visible with the microscope. In a process called synapsis, homologous chromosomes, each composed of two sister chromatids come together as pairs. The resulting structure, consisting of four chromatids is called a tetrad. The prophase of meiosis occurring during the first meiotic division of the cell is usually called prophase I. In prophase I of meiosis, pairs of homologous chromosomes intertwine and the process called crossing over occurs as chromatids from homologous pairs of chromosomes swap genetic information.
Metaphase I
The pairs of chromosomes (bivalents) become arranged on the metaphase plate and are attached to the now fully formed meiotic spindle.The centrioles are at opposite poles of the cell.

• 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.

• 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).
Anaphase I
The two chromosomes in each bivalent separate and migrate toward opposite poles.• 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.

• 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.
Telophase and Cytokinesis
The homologous chromosome pairs reach the poles of the cell, nuclear envelopes form around them, and cytokinesis follows to produce two cells.• 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.
Prophase II
Meiosis II begins without any further replication of the chromosomes. In prophase II, the nuclear envelope breaks down and the spindle apparatus forms.• While chromosome duplication took place prior to meiosis I, no new chromosome replication occurs before meiosis II.

• The centrioles duplicate. This occurs by separation of the two members of the pair, and then the formation of a daughter centriole perpendicular to each original centriole. The two pairs of centrioles separate into two centrosomes.

• The nuclear envelope breaks down, and the spindle apparatus forms.
Metaphase II
The chromosomes become arranged on the metaphase plate, much as the chromosomes do in mitosis, and are attached to the now fully formed spindle. Each of the daughter cells completes the formation of a spindle apparatus.

• Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis. This is in contrast to metaphase I, in which homologous pairs of chromosomes align on the metaphase plate.

• For each chromosome, the kinetochores of the sister chromatids face the opposite poles, and each is attached to a kinetochore microtubule coming from that pole.
Anaphase II
The centromeres separate and the sister chromatids—now individual chromosomes—move toward the opposite poles of the cell. The centromeres separate, and the two chromatids of each chromosome move to opposite poles on the spindle. The separated chromatids are now called chromosomes in their own right.
Telophase II
A nuclear envelope forms around each set of chromosomes and cytokinesis occurs, producing four daughter cells, each with a haploid set of chromosomes. A nuclear envelope forms around each set of chromosomes.

• Cytokinesis takes place, producing four daughter cells (gametes, in animals), each with a haploid set of chromosomes.

• Because of crossing-over, some chromosomes are seen to have recombined segments of the original parental chromosomes.