Rabu, 20 November 2013

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Meiosis I Homologous Chromosomes Separate

Meiosis  occurs in diploid  (2n) eukaryotes  that reproduce sexually. Meiosis produces  4 haploid  (n) daughter  cells, each of which carries half of genetic material of their parents.

Prior  to meiosis, cell has  interphase.  In  this phase, there occurs material synthesis  that  is required for cell growth and cell preparation  to generate division.  During  the phase, genetic information  of a cell in the form of DNA undergoes  replication.

in meiosis,  there occurs  2 cell divisions consecutively.  We call them as meiosis  I and meiosis  II. Each meiosis, whether  it is meiosis  I or meiosis II, occurs  in some phases  just  like  in mitosis. For better understanding,  let us pay attention  to the following explanation.

Meiosis I consists of prophase I, metaphase  I, anaphase  I, and Telophase  I.

Prophase  I
At the beginning  of prophase  I, chromosome  duplicates and coils, so  that it looks  shorter and thicker.  Each pair of chromosomes  has double arms (tetrad) or a pair of  twin chromatid.  hr the phase, there occurs crossing  over. Crossing  over is a process where chromosome arms crisscross  with the homologous chromosomes.  It can cause  gene exchange and produce  genetic recombination. The site where crossing  over occurs  is called  chiasma.

Meiosis I and its stages
Nucleolus  then disappears  in cytoplasm.  Spindle is formed  between two pairs of centrioles when  the two of them move to the opposite  pole. At the end of prophase  I, nucleus membrane  disappears.

metaphase  I
In metaphase  I, the  two pairs of centrioles  have already  been  in opposite pole. The pair of homologous  of chromosomes  are arranged  in metaphase plate (equator  plate) and each of the chromosomes  attaches  to spindle.

Anaphase  I
Anaphase I begins when  chromosome  pairs separate and start  to move to opposite pole.  In this process,  the  twin chromatid  still attach  to the centromere  and move simultaneously toward  the pole.

Telophase I
In this phase, each pole has one set of chromosomes. Each  chromosome still has two chromatid. Nucleus membrane  is formed, and  it surrounds each set of chromosome.  Spindle  disappears, and  it is followed  by cytokinesis.

source: biology of life 2006.

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