Describe the process of meiosis. (20marks)

  -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 1 has 4 discrete stages:

Prophase l,

 Metaphase l,

Anaphase I

and

Telophase I;

-Meiosis 1 is preceded by (occurs after) interphase 1 during which the chromosomes are not visible as distinct bodies; in which the genetic material replicates so that sufficient DNA is made available for the two daughter cells; there is formation of new cytoplasmic organelles; the cell builds up sufficient amount of energy reservoir for the successive stages of mitosis to occur;

-The cell then enters prophase I

-During Prophase 1 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 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 re-unite 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 1 there is separation of the bivalents through shortening (contraction) of spindle fibres; centromeres don't separate; each univalent (a single member of the original homologous 

nuclear membrane may sometimes however not reform around the two sets of chromosomes and prophase Il follows immediately; cytokinesis (division of the cytoplasm) may or may not occur; Meiosis 1 is thus complete;

Meiosis 1 is much more like mitotic cell division; in most cases, meiosis 1 does not reach telophase and hence second meiosis follows anaphase 1 immediately; thus a nuclear membrane does not form round the paired chromatids by the end of Anaphase 1; 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 Il mainly occurs in animal cells; and is much more like interphase 1 and there is duplication of centrioles, except for one major difference: there is no replication of nucleotides (DNA) since ail the genetic material was formed during interphase 1;

During prophase 1 the chromosome has already split into 2 chromatids; new spindle fibres form, at right angles to those formed during meiosis i; nucleolus disintegrates and 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 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 into 2 daughter cells from each of the two formed at the end of

meiosis 1; hence a total of four haploid daughter ceils; in animal cells cytokinesis (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;