Process of Meiosis

- Occurs in two steps; meiosis I which is termed the reduction division of meiosis and meiosis II which is much more like mitotic division and is thus called the equation of division;
- Meiosis I has 4 discrete stages: Prophase I, Metaphase I, Anaphase I and Telophase I;
- Meiosis I is preceded by (occurs after) interphase I during which the chromosomes are not visible as distinct bodies; the genetic material replicates so that sufficient DNA is made available for the two daughter cells; there is the formation of new cytoplasmic organelles; the cell builds up a sufficient amount of energy reservoir for the successive stages of mitosis to occur; the cell then enters prophase I.
- During Prophase I the nucleolus disappears; the centrioles (if present) become arranged at opposite sides of the nucleus; homologous chromosomes pair up and lie side by side, each pair of which is called a bivalent i.e. there is the pairing of homologous chromosomes; each chromosome splits into 2 chromatids except at the centromeres; the chromosomes shorten and thicken and coil around each other i.e. crossing over occurs; contact between the bivalent loosens and the chromosomes move away from each other; however, they still retain contact at the chiasmata; chromatid portions then break and reunite at cross-over points i.e. terminalization; the cell then enters metaphase I.


- Metaphase I is the stage of independent assortment of homologous chromosomes; the homologous chromosomes align at the equator of the cell in two lines i.e. the bivalents move to the equator of the spindle and arrange themselves in pairs;
- During anaphase I there is the separation of the bivalents through the shortening (contraction) of spindle fibres; centromeres don't separate; each univalent (a single member of the original homologous pair) moves to the poles; upon arrival at the poles each chromosome is replicated to form chromatids;
- Anaphase I is the reduction stage (chromosome number is halved) and the number of chromosomes in each daughter cell is half the number in the mother cells; the cell then enters telophase I;
- Telophase I is characterized by the arrival of univalent at the poles: the chromosomes uncoil and hence move and thin; spindle fibres disappear; nucleus and nuclear membrane reappear; the nuclear membrane may sometimes however not reform around the two sets of chromosomes and prophase Il follows immediately; cytokinesis (a division of the cytoplasm) may or may not occur;
- Meiosis I is thus complete;
- Meiosis II is much more like mitotic cell division; in most cases, meiosis I does not reach telophase and hence second meiosis follows anaphase I immediately; thus a nuclear membrane does not form around the paired chromatids by the end of Anaphase I; instead, they immediately undergo a second division at right angles to the first, only this time the chromatids are separated at the centromeres Oust like in mitosis).
- Interphase II mainly occurs in animal cells; and is much more like interphase I and there is duplication of centrioles, except for one major difference: there is no replication of nucleotides (DNA) since all the genetic material was formed during interphase I;
- During prophase I the chromosome has already split into 2 chromatids; new spindle fibres form, at right angles to those formed during meiosis I; nucleolus disintegrates and the nuclear membrane dissolves; however in plant cells the nucleolus persists and does not disintegrate; In Metaphase II chromosomes move and align at the equator of the cell in a single line; spindle fibres attach to the chromatids, at the centromeres;
- At anaphase Il there is the separation of chromatids at the centromeres and movement of each member to the opposite poles; the spindle fibres contract and pull the sister chromatids to the opposite pole
- The cell then enters telophase Il where the chromosomes lengthen and become thin; the spindle fibres disintegrate; the nuclear membrane and nucleolus re-appear at each pole; cytokinesis occurs resulting in 2 daughter cells from each of the two formed at the end of meiosis I; hence a total of four haploid daughter cells; in animal cells, cytokinesis (a division of the cytoplasm) occurs by constriction of the cell membrane; while in plant cells cytokinesis is by deposition of a cell plate along the equator of the spindles;