Question

Identify the stages of meiosis on the diagram.

Answer Bank:

A. in anaphase I, homologous pairs split up. B. Telophase II and cytokinesis produces four cells. C.Interphase is the stage in which the chromosomes replicate. D. sister chromatids line up in the center of the cell during metaphase II. E. Crossing over happens between homologous pairs in prophase I. F. Homologous pairs of chromosomes line up in metaphase I. G. Spindle fibers reform in prophase II. H. Sister chromatids separate in anaphase II. I. Telophase I and cytokinesis forms two cells.

Answer 1

INTRODUCTION

• Cell cycle is defined as the series of changes that occurs in a cell,which leads to division of the cell into 2 daughter cells.
• cell cycle divided into 2 phases ; interphase and m - phase.
• Interphase (phase - in which cell prepares before the cell division )  is further divided into ; G1 (cell synthesizes more proteins,increase the number of ribosomes,mitochondria) ,S (DNA is replicated ) and G2 phase ( cell prepare itself before the m - phase).
• This m phase can be of 2 types - depending on the cells that undergoing cell division - meiosis and mitosis.
• Mitosis - also known as equational division is much more simple compare to Meiosis (reduction division).

MEIOSIS / REDUCTIONAL DIVISION

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

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

Stages are described in detail below :

MEIOSIS I :

C. INTERPHASE - is the stage in which chromosomes replicate

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

E. PROPHASE I -  

CROSSING OVER HAPPENS BETWEEN HOMOLOGOUS CHROMOSOMES IN PROPHASE I ;

Longest phase of meiosis I , condensed chromosomes becomes X shaped structures, during this phase homologous chromosomes pairs and DNA segments are exchanged (recombination / crossing over),at the end of this stage - the membrane around the Nucleus dissolves away - releasing chromosomes - meiotic spindle consisting of microtubules and other proteins - extend across the cell between the centrioles.

The above processes occurs in 5 stages of prophase I - they are leptotene,zygotene,pachytene,diplotene,diakinesis.

F. METAPHASE I - HOMOLOGOUS PAIR OF CHROMOSOMES LINE UP IN THIS STAGE ;

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

A. ANAPHASE I - HOMOLOGOUS PAIRS SPLIT UP ;

Pair of chromosomes are pull apart by the meiotic spindle,which pulls one chromosome to one pole and the other to the opposite.(In meiosis I ,sister chromatids stay together - unlike in mitosis and meiosis II).

I.TELOPHASE I AND CYTOKINESIS PRODUCES 2 CELLS ;

Chromosomes complete their move to the opposite poles,at each pole a full set of chromosomes gather together,a membrane forms around each set to form a new nuclei. The single cells after that pinches from the middle to form 2 daughter cells separately,each containing 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 in each daughter cells - the chromosomes condense again into visible x shaped structures - membrane around the Nucleus dissolves away releasing chromosomes - centrioles duplicate and meiotic spindle forms again.

D.METAPHASE II - SISTER CHROMATIDS LINE UP IN THE CENTRE OF THE CELL IN THIS PHASE;

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

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

Sister chromatids are pulled to opposite poles due to the action of the meiotic spindle,the separated chromatids are now individual chromosomes.

B.TELOPHASE II AND CYTOKINESIS PRODUCES 4 CELLS ;

In TELOPHASE II ; the movement of chromosomes towards the opposite poles are completed and at each pole of the cell a complete set of chromosomes gather together,a membrane forms around each set of chromosome to create 2 new cell nuclei.After that again another round of cytokinesis takes place ;thereby creating 4 grand daughter cells,each with haploid / half set of chromosomes.(In males these 4 cells are called sperms ,in females - among these 1 is the egg cell and other 3 are polar bodies ) .