MEIOSISMeiosis involves two successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) sex cells, each with half of the genetic material of the original cell. Through the mechanisms by which paternal and maternal chromosomes segregate, and the process of crossing-over, genetic variation is produced in the haploid cells.

When does meiosis occur?• Meiosis occurs in diploid cells. The chromosomes duplicate once, and through two successive divisions, four haploid cells are produced, each with half the chromosome number of the parental cell.

• Meiosis occurs only in sexually reproducing organisms. Depending on the organism, it may produce haploid gametes, which do not divide further but instead fuse to produce a diploid zygote; or it may produce haploid spores, which divide by mitotic cell cycles and produce unicellular or multicellular organisms.

• In animals, where the somatic (body) cells are diploid, the products of meiosis are the gametes.

• In many fungi and some algae, meiosis occurs immediately after two haploid cells fuse, and mitosis then produces a haploid multicellular "adult" organism (e.g., filamentous fungi, algae) or haploid unicellular organisms (e.g., yeast, unicellular algae).

• Plants and some algae have both haploid and diploid multicellular stages. The multicellular diploid stage is the sporophyte. Meiosis in a sporophyte produces haploid spores. These spores alone are capable of generating a haploid multicellular stage called a gametophyte. The gametophyte produces gametes by mitotic cell cycles.

The process of meiosis:

Meiosis consists of two successive nuclear divisions, meiosis I and meiosis II. Each division consists of these stages: prophase, metaphase, anaphase, and telophase.

The chromosomes duplicate prior to meiosis

• Prior to meiosis, all chromosomes are duplicated in a process similar to chromosome duplication prior to mitosis.

• Outside the nucleus of animal cells are two centrosomes, each containing a pair of centrioles. The two centrosomes are produced by the duplication of a single centrosome during premeiotic interphase. The centrosomes serve as microtubule organizing centers (MTOCs). Microtubules extend radially from centrosomes, forming an aster.

• Plant cells do not have centrosomes. Different kinds of microtubule organizing centers serve as sites of spindle formation

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

• 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.

Metaphase IThe 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 IThe 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 IThe 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.

MEIOSIS IIProphase IIMeiosis 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