Identify the stages of meiosis

INTRODUCTION

• Cell cycle is defined as the series of changes that occurs in acell,which leads to division of the cell into 2 daughtercells.
• cell cycle divided into 2 phases ; interphase and m -phase.
• Interphase (phase – in which cell prepares before the celldivision )  is further divided into ; G1 (cellsynthesizes more proteins,increase the number ofribosomes,mitochondria) ,S (DNA is replicated ) and G2 phase ( cellprepare itself before the m – phase).
• This m phase can be of 2 types – depending on the cells thatundergoing cell division – meiosis and mitosis.
• Mitosis – also known as equational division is much more simplecompare to Meiosis (reduction division).

MEIOSIS / REDUCTIONAL DIVISION

• Process in which a single cell divides twice to produce 4daughter cells,containing half the amount of parent geneticinformation(hence known as reductional division).
• Occurs in sex cells – sperm in males,eggs in females.
• Divided into 9 stages ; divided between first time the celldivides (Meiosis I ) and the second time it divides – MeiosisII

Following diagram represents different stages of bothMeiosis I and Meiosis II ;

Stages are described in detail below :

MEIOSIS I :

C. INTERPHASE – is the stage in whichchromosomes replicate

(DNA in the cell is copied resulting in 2identical full sets of chromosomes,outside Nucleus there are 2centrosomes – each containing a pair of centrioles – structurescritical for the process of cell division, during this phase -microtubules arise from these centrosomes.)

E. PROPHASE I –  

CROSSING OVER HAPPENS BETWEEN HOMOLOGOUS CHROMOSOMES INPROPHASE I ;

Longest phase of meiosis I , condensed chromosomes becomes Xshaped structures, during this phase homologous chromosomes pairsand DNA segments are exchanged (recombination / crossing over),atthe end of this stage – the membrane around the Nucleus dissolvesaway – releasing chromosomes – meiotic spindle consisting ofmicrotubules and other proteins – extend across the cell betweenthe centrioles.

The above processes occurs in 5 stages of prophase I – they areleptotene,zygotene,pachytene,diplotene,diakinesis.

F. METAPHASE I – HOMOLOGOUS PAIR OF CHROMOSOMES LINE UPIN THIS STAGE ;

Centrioles are now at opposite poles of the cells,with meioticspindles extending from them,the meiotic spindle Fibres attach toone chromosome of each pair.

A. ANAPHASE I – HOMOLOGOUS PAIRS SPLIT UP ;

Pair of chromosomes are pull apart by the meiotic spindle,whichpulls one chromosome to one pole and the other to the opposite.(Inmeiosis I ,sister chromatids stay together – unlike in mitosis andmeiosis II).

I.TELOPHASE I AND CYTOKINESIS PRODUCES 2 CELLS ;

Chromosomes complete their move to the opposite poles,at eachpole a full set of chromosomes gather together,a membrane formsaround each set to form a new nuclei. The single cells after thatpinches from the middle to form 2 daughter cells separately,eachcontaining a full set of Nucleus – this process is cytokinesis.

G.SPINDLE FIBERS REFORM IN PROPHASE II ;

The 2 daughter cells thus formed,each with 23 chromosomes and ineach daughter cells – the chromosomes condense again into visible xshaped structures – membrane around the Nucleus dissolves awayreleasing chromosomes – centrioles duplicate and meiotic spindleforms again.

D.METAPHASE II – SISTER CHROMATIDS LINE UP IN THE CENTRE OF THECELL IN THIS PHASE;

The centrioles are now at opposite poles in each of daughtercells,meiotic spindle Fibres at each pole of cell attach to each ofsister chromatids.

H. ANAPHASE II – SISTER CHROMATIDS SEPARATE IN THIS PHASE ;

Sister chromatids are pulled to opposite poles due to the actionof the meiotic spindle,the separated chromatids are now individualchromosomes.

B.TELOPHASE II AND CYTOKINESIS PRODUCES 4 CELLS ;

In TELOPHASE II ; the movement of chromosomes towards theopposite poles are completed and at each pole of the cell acomplete set of chromosomes gather together,a membrane forms aroundeach set of chromosome to create 2 new cell nuclei.After that againanother round of cytokinesis takes place ;thereby creating 4 granddaughter cells,each with haploid / half set of chromosomes.(Inmales these 4 cells are called sperms ,in females – among these 1is the egg cell and other 3 are polar bodies ) .