Cell Division-II :-MEIOSIS

 

Meiosis :-      (reduction division)   

·       Production of offspring by sextual reproduction includes the fusion of gametes

·       Gametes are formed from specialised diploid cells . this specialised  kind of cell division that reduces the chromosome number by half.

·       significance of meiosis:

v  An essential process in all sexually reproducing animals.

v  Maintain diploidity of zygote (i.e. maintenance of chromosomal

v  number in zygote)

v  Meiosis show crossing over, resulting in genetic recombination.

v  Meiosis show increased incidences of mutation (sudden changes in

v  genetic material)

Meiosis involves two rounds of cell division giving rise to four daughter cell. these two rounds of cell division are called:-

·       MEIOSIS-I

Prophase-I

Metaphase-I

Anaphase-I

Telophase-I

·       MEIOSIS-II

Prophase-II

Metaphase-II

Anaphase-II

Telophase-II

 

Ø  MEIOSIS-I:-Reduction Division/ Heterotypic  Division

·       Prophase -I

·       Longest and most complicated prophase.

·       Divided into five sub-stages:

1.     Leptotene  

2.     Zygotene  

3.     Pachytene   

4.     Diplotene  

5.     Diakinesis

1.     Leptotene: 

·       Shortest phase of prophase-I

·       Condensation of chromosome begins.

·       Each chromosome consist two chromatids, but appear as single thread (monad ) under, light microscope.

·       Bouquet stage observe (meiosis specific telomer clustering on the nuclear envelope)

2.      Zygotene:   

·       Pairing between two homologous chromosome called synapsis.

·       Synapsis caused by nucleoprotein called synaptonemal complex protein.

·       Bivalent formation (paired chromosome is bivalent).

·       Initiation of tetrad formation.

3.      Pachytene:

·       Longest phase.

·       Tetrad chromosome clearly visible.

·       Crossing over or recombination (i.e exchange of genetic material between two non-sister chromatids of homologous chromosomes ) occurs.

·       Recombinase enzyme mediated process.

·       Crossing over site Recombination nodules appear.

·       Produces genetic recombination( variations ) in offspring.

4.     Diplotene:  

·       Chiasmata (chiasma) formation i.e recombine homologous chromosome of bivalent separation, except crossing over site.

·       In oocytes of some mammals diplotene stay for years.

·       Synaptonemal complex protein dissolution occur.

·       Homologous chromosome repels to each other.

·       Terminalisation process begins.

·       Disappearance of nuclear envelope, ER, nucleolus starts.

5.     Diakinesis :

·       It is marked by “Terminalization of chiasmata’’ (shifting of chiasmata towards end of chromosome )

·       At the end of, Diakinesis chromosome fully condensed, Nuclear envelop & nucleolus disappear.