MODULE 11 (MEIOSIS)

• Meiosis employs many of the same mechanisms as mitosis.
• The starting nucleus is always diploid and the nuclei that results at the end of meiotic cell division are haploid.
• Meiosis is a process in which the chromosomes number of the cells is reduced to half, from diploid (2n=46 chromosomes) to haploid (n=23 chromosomes).
• Meiosis is a type of cell division occurs in sexually reproducing organisms and is restricted to the reproductive cells only.
• The importance of meiosis is to maintain the chromosome number of organisms from generation to generation.
• Meiosis follows phases of similar to those mitosis, but the outcome process is very different.
• The DNA of the parent cells is replicated in interphase preceding both mitosis and meiosis. However, in meiosis, replication is followed by two divisions. These two divisions are the nuclear division.
• Meiosis I is the reductional cell division, the chromosomes number is reduced by half (from 46 to 23 chromosomes).
• On the other hand, meiosis II is the equational cell division similar to mitosis which maintains the chromosomes number of organism (maintain 23 chromosomes).
• Meiosis reduces the chromosomes number of the parent cell so that the four daughter cells are produced. Each with one-half of the chromosome number of the parent cell.
• Each daughter cell contains only one chromosome from each homologous.
• At prophase I, the chromosomes have already duplicated.
• Prophase I is subdivided into five substages: Leptonema, Zygonema, parenchyma, diplonema, and diakinesis.
• Leptonema where replicated chromosomes have coiled and are already visible.
• The number of chromosomes present is the same at the number in the diploid cell.
• Zygonema, the homologous chromosomes begin to pair and twist each other in highly specific manner.
• Homologous chromosomes are identical in terms of visible structures.
• The pairing is called synapsis and because the pair consist of four chromatids it is referred to as bivalent.
• Parenchyma, chromosome become much shorter and thicker.
• A form of process of physical exchange of a chromosome region is called crossing-over.
• Crossing-over is the exchange of chromosome segment between homologous chromosomes.
• The importance of crossing-over is to ensure genetic variability.
• Diplonema, where two pairs of sister chromatids begin to separate from each other.
• It is at this point (diplonema) where crossing-over is shown to have taken place and the area of contact between two non-sister, called chiasma.
• Diakinesis, where the four chromatids of each tetrad are even more condensed and the chiasma often terminalize or move down the chromatids to the ends.
• At the end of prophase I, the nuclear envelope disappears allowing the spindle to enter the nucleus.
• Metaphase I, the centrioles are at opposite poles of the cell.
• The pair of homologous chromosomes (the bivalents), now is tightly coiled and condensed as they will be meiosis, become arranged on a plane equidistant from the poles called metaphase plate. (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).
• Anaphase I begins when two chromosomes of each bivalent (tetrad) separate and start moving toward the opposite poles of the cell as a result of the action of the spindle.
• In anaphase I, the sister chromatids remain attached at their centromeres and move together toward the poles.
• The difference between mitosis and meiosis in anaphase stage is that sister chromatids remained joined after the metaphase in meiosis I, whereas in mitosis they separate.
• Telophase I, at this stage homologous chromosomes pairs complete their migration to the two poles as result of the action of the spindle. Now a haploid set of chromosomes is at each type, with each having two chromatids.
• A nuclear envelope reforms around each chromosomes set, the spindle fiber disappears, and cytokinesis follows.
• In animal's cells, cytokinesis involves the formation of a cleavage furrow, resulting in the pinching of the cell into two cells.
• After cytokinesis, each of the progeny cell has a nucleus with a haploid set of replicated chromosomes.
• Many cells undergo rapid meiosis do not decondense the chromosome at the end of telophase I while the other cells do exhibit chromosome de-condensation. At this time, the chromosomes re-condense in prophase II.
• In prophase II, there is formation of spindle apparatus, and the chromosomes progresses toward the metaphase plate.
• Metaphase II, each of daughter cells complete the formation of a spindle apparatus.
• Single chromosomes align on the metaphase plate, much as chromosomes do in mitosis.