2 successive divisions of a diploid (2N) eukaryotic cell of a sexually reproducing organism that result in four haploid (N) progeny cells, each with half of the genetic material of the original cell.
• 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.
• 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 (synapsis), 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 (pair is called tetrad). 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.