Meiosis stages



 MEIOSIS:

Meiosis ( term ' meio'  means to reduce')  is a form of  nuclear division  in which the chromosome number is halved from the diploid  number (2n) to the haploid number (n) .Similar to mitosis, it involves  DNA  replication  during interphase in the parent cell, but this is followed by two cycle  of nuclear division and  cell division known as Meiosis I  and Meiosis II, respectively. Thus, a single diploid cell gives rise to four haploid  cells as shown in the basic characteristics of meiosis.Meiosis occurs during the formation  of sperm eggs ( gametogenesis) in animals and during spore formation  in plants. 

Basic Characteristics of meiosis  are listed as follows : 

■ During the nuclear division, the duplication  of one chromosome occurs. Note that, as for mitosis,chromosomes may be single or double structure .When double, the two parts are called chromatids. 


■ Similar to mitosis, meiosis  is a continuous  process but is conveniently divided into  prophase, metaphase, anaphase  and telophase. 

■  These stages occurs in Meiosis  I and Meiosis II.

Mechanism of Meiosis 





Meiosis consists of two division that take place in rapid succession, with the chromosomes replicating  only once. Thus, a parent cell produces four daughters cells, each having half the number of chromosomes and half of the nuclear DNA amount present in the parent cell. Meiosis is, therefore, also known as the reduction division. The two divisions of meiosis are called the first and the second meiotic divisions or meiosis _ I and meiosis _ II.

Similarly between  Mitosis and Meiosis 

The two meiotic divisions have many features  in common with the mitotic division, For instance, each meiotic division consists of the same four stages, viz., prophase, metaphase, anaphase and telophase. Also, formation of asters and spindle, behaviors of nucleoli and nuclear envelope, and division  of cytoplasm are as in mitosis. Both mitosis and meiosis follow interphase. 

1: First  Meiotic Division or Meiosis  I 

In this division, the two homologous chromosomes of each  pair separate from each other and go to separate daughter cells. This reduces the number of chromosomes from diploid to haploid condition. Meiosis __ I is, therefore, known as heterotypic division. The four phase of this division  are called prophase _ I, metaphase _I anaphase __I  and telophase __I. 

A): Prophase_I : 

The prophase of meiosis _I is more elaborate  and lasts longer than the prophase of mitosis. It takes more than 90% of the time required for meiosis . It is divide into five sub_ phase _ leptotene, zygotene, pacytene, diplotene and diakinesis.Another sub_ phase called preleptonema is sometimes seen prior to leptonema. In preleptonema chromosomes are not distinguishable because of their thinness but sex chromosomes may be seen as heterochromatin  ( heteropycnotic) bodies.

1): Lepotene or Leptonema:

Nucleus increases  in size and the chromosomes  become more apparent . Each chromosome is seen to produce a long thread. Frequently, these leptonemic chromosomes have a definite polarization and form loops whose both ends are attached to the  nuclear envelope towards the side having replicated centrosomes or centriole pairs.The chromosomes are attached to the nuclear envelope  through a specialised structure  called attachment  plate.This peculiar  arrangement of chromosomes is often called bouquet stage.

Attachment  of chromosomes with nuclear membrane is usually  seen in animals especially  insects. Absence of such polarization  of chromosome end in plants in perhaps due to absence of centrosome.





  The chromosomes  in leptonema are replicated but chromatids are  not distinguishable due to the presence  of nucleoprotein core between  them.


  The leptonemic chromosomes possess a string of bead like thickening  called    chromomers.Because these beads are characteristic in size, number and position for a particular  chromosome, they may be used as landmarks to identify  a specific  chromosome of an organism. With further contraction of chromosomes during zygonema and pachynema, chromosomes become larger and fewer  in number . Chromomers were once thought to represent  genes. But they are too few in number to include all   the  genes.  One of the centriole pairs begins to move to the opposite side. Both the centriole pairs develop astral rays. In cells undergoing  meiosis, there are two similar  chromosomes of each type. Such chromosomes are called homologous chromosomes. They are contributed by different  parents. One of them belongs  to the father and is called paternal chromosome.The other chromosome of homologous pair belongs to the mother and is called maternal chromosome.The homologous chromosomes resemble each other in the position of their centromeres, position of chromosomes, shape and size.

          OR 

Laptoten :

The centrioles, already duplicated, move apart in pairs. Astral Rays appear around each pair of centrioles to form an aster. Spindle begins to develop between  the centriole pairs. Chromosomes differentiate from chromatin fibres. They are very long, fine filaments ( G.leptos= fine ; tene = thread). They are double, each consisting of two identical chromatids due to DNA replication during premeiotic interphase.The  chromatids are, however,  very closely adhered together and are indistinct. 




  Each chromosome is attached by both of its 
ends to the nuclear matrix that lines the nuclear envelope with the help of a specialized structure termed the attachment plate.

Note: 

 ☆ There are 2 sets of chromosomes in a diploid cell undergoing meiosis, one set contributed by the male parent and the other by the femal parent. There are always two similar  chromosomes, having  the same size, form and structure. They are called homologous chromosomes. One of them is paternal chromosome and other maternal chromosome.

There are no centrioles in plant cells and no asters are formed.


2): Zygotene or Zygonema:

The two homologous  chromosomes  get attached other laterally due to development  of nucleopment of nucleoprotein between  them. It is similar  to nucleoprotein core present between  two chromatids of a chromosome. Pairing is such that the genes of the same character  present  on the two chromosomes come  to lie exactly opposite. The process  of attachment  of the homologous chromosomes is known as synapsis or syndesis.Pairing is so thorough that the corresponding  ends and all the corresponding  points ( genes) lie exactly  opposite to each other. A pair of homologous chromosomes lying together is called a bivalent.The chromatids are still not visible. A fibrillar, somewhat ladder_ like, organelle, called synaptonemal complex,developes between  the synapsed homologous chromosomes. It is though to stabilize the paired condition of chromosomes till crossing  over is completed.





Pairing of two homologous chromosomes begins when their corresponding ends come together on the nuclear matrix. Pairing may occur in one of the following  three ways __ 

(i) Proterminal Pairing  : It starts at the centromeres and progresses toward the ends.

(ii) Procentric Pairing: It begins at the centromeres and progresses toward the ends. 

(iii) Random ( Intermediate) Pairing: It commences at many points toward the ends.

3): Pachytene : 

The  synapsed  chromosomes  continue to become short and thick ( G. Pachus = thick; tene = thread). The chromatids of each synapsed chromosome slightly  separate and become visible. The two visible  chromatids of a chromosome are referred  to as a dyad. A group of four homologous chromatids (two dyads) is called a tetrad. The two chromatids  of the same chromosome are called sister chromatids and those of to homologous chromosomes ( bivalent) are termed nonsister chromatids. 

  Crossing  over ( recombination) occurs during pachytene. Recombination involves mutual exchange of  the corresponding  segments of nonsister chromatids of homologous chromosomes. It takes place by breakage and reunion of chromatid segments. Breakage, called nicking, is assisted by an enzyme  endonuclease and reunion, termed annealing ,is aided by an enzyme ligase.




It has been found crossing  over is a common event. Normally , each tetrad undergoes at least one recombination.



4): Diplotene:

Now the synaptic force keeping  the homologous chromosomes together come to an end. The homologous chromosomes start separating ( G.diplos =  double ;  tene =thread). This is called disjunction. It makes chromatids more distinct and the tetrads very clear. Separation of homologous chromosomes does not take place at the points called chiasmata ( singular, chiasma). The chiasmata  mark the sites where crossing  over occurred during pachhytene ( Gr.chiasma = crosspiece).They help hold homologous chromosomes together. 

   Normalization of Chromosomes:In  diplotene, the chromosomes may unfold to nearly  normal form and start  transcription of  mRNA and rRNAto build up food reserves in the cytoplasm. This process is most pronounced in the primary  oocytes of  amphibians,  reptiles and birds. In some species, the chromosomes enlarge greatly, assuming lampbrush form.





5): Diakinesis:

The chromosomes condense again to acquire their characteristic size and form. The chiasmata disappear by sliding  toward the tips of the chromosomes  due to tight condensation ( G.dia =across ; kinesis = movement). This process is called terminalization. The nucleoli disappear, nuclear envelope breaks down into vesicles, setting the tetrads free in the cytoplasm. 

B) : METAPHASE __ I :

The spindle shifts to  the position formerly occupied by the nucleus. The tetrads move to the equator  of the spindle and come to lie in two parallel metaphase plates. One of these plates in formed by the n chromosomes, and the other by those of their  homologous.  The attachment of tetrads to the spindle fibres in metaphase _ I is different  from that of mitotic metaphase  chromosomes. Each homologous chromosome has  two kinetochores, of a homologous chromosome connect to the same spindle pole.The two kinetochores of its homologue join the opposite spindle  pole.





 C) : ANAPHASE _ I : 

From each tetrads, two chromatids of a chromosome move as a unit ( dyad) to one pole of a spindle, and the remaining  two  chromatids of its homlogue migrate to the opposite  pole.

   Thus, the homologous chromosomes of each pair, rather than the chromatids of a chromosome, are separated.With the result, half of the chromosomes,  which appear in early prophase, go to each pole. It is here in the anaphase __ I that the real reduction in the number  of chromosomes occurs. Each chromosome at the poles is still double and consists of two chromatids. This is in contrast to the single _  stranded chromosomes of mitotic anaphase. 


    Independent Assortment of Chromosomes : The paternal and maternal chromosomes of each homologous pair segregate during anaphase __ I independently of the other  chromosomes. Anaphase__ I is  the cytological event that corresponds to Mendel's  law of independent assortment. Although  the paternal and maternal chromosomes of a homologous pair have the genes for the same traits, either chromosome of a pair may carry different  alleles of the same genes. Therefore, independent assortment of homologous chromosomes in anaphase __ I introduces genetic variability. 

D): TELOPHASE __ I  :

The chromosomes at each pole of  the spindle now uncoil and elongate, but remain straight and  often do not  assume interphase form. The satellite chromosome develops  nucleus, nucleoplasm appears, and nuclear envelope forms around the chromosomes and nucleoli. The spindle and the astral rays gradually  disappear. 

     The cytoplasm divides at its middle by cleavage ( constriction)  in an animal cell and by cell plate formation in a plant cell. This produces two daughter  cells, each with one nucleus.  The nucleus   of each daughter  cell has received  only one  chromosome from each homologous pair. Thus, it has half the number of chromosomes, but double the   amount of  nuclear DNA as each chromosome is still double.

    The daughter cells formed by meiosis  _ I are called secondary , spermatocytes or secondary  oocytes in male and female animals. 

INTERPHASE : 

Generally  there is no interphase between  meiosis __ I  and  meiosis __ II.A brief interphase, called interkinesis, or intrameiotic interphase,may occur in  some cases. There is no replication of chromosomes during this interphase. When there is no interphase,  the telophase of meiosis__ I is not completed an the prophase of meiosis __ II is almost eliminated, except the duplication of  centrioles and the formation of two new spindle with asters at the pole to replace the telophase __ I spindle and asters. The telophase __ I chromosomes   pass  over  the new spindles  and start metaphase __ 2.

Significanc of Meiosis _I :

Meiosis__ I involves many Significanc events__ 

(i) It separates the homologous chromosomes to reduce the chromosome number to the haploid state, a necessity for sexual  reproduction. 

(ii) It introduces variation by forming new gene combination through  crossing over and randon assortment of paternal and maternal chromosomes.

(iii) It may  at  times cause  chromosomal mutation by abnormal disjunction. 

(iv)  It induces the cells to  produce  gametes for sexual reproduction  or spores for asexual  reproduction.


2 : Second Meiotic Division  or Meiosis __II 

In this division,  the two chromatids of each chromosome separate from each other and go to separate  daughter  cells. With the result, the number of chromosomes remains the same as produced by meiosis __ I. Meiosis  __ II is, therefore, known as homotypic division. The four stages of this division are called prophase __ II, metaphase__ II, anaphase __ II,and telophase __II.

A): PROPHASE __ II : 

Prophase __ II is very short. It occurs simultaneously in both the nuclei  formed by meiosis __ I .Centrioles, already duplicated, move apart in pairs. Each pair develops astral rays round it to form an aster. Spindle is  laid down between the 2 pairs of centrioles. The chromosomes, each comprising two chromatids, become visible  in the nucleus. They are set free in  the cytoplasm by breakdown  of the  nuclear  envelope.  Nucleoli   disappear.


B): METAPHASE __ II :

The chromosomes take up positions  at the equator of the spindle, forming a single metaphse plate.The chromatids of each  chromosome are joined  at their kinetochores by spindle fibres extending  from the opposite poles of the spindle as in mitosis. 

C): ANAPHASE __ II :

The two chromatids of each chromosome start moving away  from  each other. Finally, they reach the poles of the spindle.Here, they called th chromosomes. Each pole has haploid number of chromosomes and haploid amount of DNA. This amount is one__ forth of the DNA present in th original cell that started meiosis. 


D): TELOPHASE __II  :

The group of chromosomes at each pole of the spindle gets enclosed by a nuclear envelope. Nucleoli are laid down. Astral rays and spindle are lost. 


Cytokinesis: 

 Cytoplasm divides at its middle by furrowing in an animal cell and by cell plate formation  in a plant cell. This produces two daughter cells. The letter have half the number of chromosomes, and half the amount  of  nuclear  DNA. These cells are mature gametes in animals and spores in plants. 


  Cytokinesis may occur after each nuclear division. In such cases, it is said to be  of successive  type.First the diploid parent cell divides by heterotypic division. The four daughter cells may form a linear or isobilaternal tetrad. Often Cytokinesis is delayed unit both the nuclear division are completed, so that four cells are simultaneously formed, each with a haploid nucleus. The  cytoplasmic division  in such cases is said to be of simultanous type.

What does a kinetochoren  consist  of ?

Meiosis_II  is not  Mitotic : Meiois_II is often  described  as a  mitotic division  because it distributes chromatids to  the daughter  cells  like  mitosis. However , it is not mitosis as 

(i) It occurs  with haploid  number of chromosomes  unlike mitosis.  

(ii) It is not preceded by an interphase 

(iii) The two chromatids of chromosomes are often not similar  due to crossing  over in Meiosis_II 

(iv) The daughter cells formed by meiosis  _ II differ from each other as well as from the parent cell in the matter of gene combinations. This is not so on mitosis.

Necessity of Meiosis  __II  : The aim of meiosis  is to reduce the number of chromosomes to half.This is achieved in meiosis _ I. What is then the necessity of Meiois_II?

 The chromosomes that separate in the anaphase of meiosis _ I are sti double.Each consists of two chromatids and has 2X amount of DNA. Thus, reduction to haploidy in terms of  DNA content does not occur in  meiosis __I .Truely  haploid  nuclei in terms of DNA  content ad we as chromosome  number are formed in meiosis __II, when the chromatids of each chromosome  are separated  into different  nuclei. Thus, meiosis __II  is necessary. 

Significanc of Meiosis:

Meiosi has six_ foled Significanc. 

(i) Formation of Gametes : Meiosis  produces gametes for sexual  reproduction. 

(ii) Formation  of SPores  : Meiosis  produces  spores  in plants for asexual  reproduction. 

(iii) Maintenance  of Chromosome Number: Meiosis halves the chromosomes number in the gametes so that fertilization  may restore the orginal diploid  number in the zygote. 

(iv) Introduction  of Variations: Meiosis  provides a chance for the.formation of new combinations of chromosomes . This brings about variations. The new combinations are produced 

(a)  By crossing  over  in Prophase_I   and by 

(b) Random distribution  of paternal and maternal ( homologous) chromosomes between  the daughter  cells in anaphase _ I. Variations  help animal and plant  breeders to improve the races of useful animals  and plants. 

(v) Mution : Abnormalities in meiosis may bring about chromosomal mutations, some of which may be advantageous  for the organism.


(vi)  Evidence  of Basic Relationship of Organisms: Meiosis, being essentially similar in a sexually  reproducing organism, offers further evidence of the basic relationship  of  living .


POLYPLOIDY

Colchicine, an alkaloid obtained from the corms of the autumn crocus plant, Colchicum autumnale, acts as a poison for cell division. It checks the formation of spindle by preventing the assembly of microtubules and so blocks mitosis and cell division. However, colchicine allows chromosomal  replication and separation of chromatids, thus doubling the number of chromosomes in the cell.The duplicated chromosomes remain in the same cell. Such  an increase  in the number  of full chromosome sets is called polyploidy.Polyploidy may produce 

(a) More vigorous offerspring, 

(b) Even new varieties  and species. The plant breeders  have used colchicine__ induced polyploidy for producing new vigorous  varieties of useful crops, fruit trees and ornamental plants.

  Colchicine is also used  to stop cell division  at metaphase so that condensed chromosomes may be obtained for studying  their morphology  and  prepare a karyotype . Colchicine is sometimes  used to stop division of cancer cells.


SUMMARY:

According  to the cell theory, cells arise from preexisting cells.  The process  by  which  this occurs  is  called division.  Any sexually  reproducing  organism starts  its life cycle from  a single  __ celled  zygote. Cell division  does not stop with the  formation  of the  mature  organism  but continues  throughout its life cycle. The  stages  through which  a cell passes from  one division  to the next is called  the cell cycle.   Cell  cycle  is divided into two phases called(i) Interphase  _ a period of preparation  for  cell division, and 
(ii) Mitosis  ( M phase ) _ the actual period of cell division. Interphase is further subdivided  into G1, S and G2, G phase is the period when the cell  growth and carries out normal Metabolism. Most of the organelles duplication also occurs  during this phase. S phasemarks the phase of DNA replication and chromosome  duplication.  G2 phase is  the  period  of cytoplasmic growth. Mitosis  is also divided  into four  stages namely prophase, metaphase, anaphase and telophase. Chromosome  condensation occurs during  prophase. Simultaneously, the centrioles move to the opposite poles.The nuclear  envelope and the nucleolus disappear and the spindle fibres start appearing.  Metaphase is marked  by the alignment of chromosomes at the equatorial  plate. During  anaphase  the centromere divide  and the chromatids  start  moving  towards the two opposite poles. Once  the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear  membrane reappear. This stage is called  the telophase.  Nuclear division  is then followed  by the  cytoplasmic  division  and is called  Cytokinesis. Mitosis  thus, is  the equational division  in which  the chromosome number of  the parent  is conserved  in the daughter  cell.

    In  contrast  to mitosis , meiosis  occurs in the diploid  cells,  which  are destined  to form gametes. It is called the reduction division  since  it reduces  the chromosome number by half while making the gametes. In sexual  reproduction when the two gametes  fuse  the chromosomes  number  is restored  to the value  in  the parent . Meiosis  is divided into two phases __ meiosis __ I and meiosis  __ II . In the first meiotic division  the homologous  chromosomes pair to form bivalents, and undergo crossing  over. Meiosis I has a long prophase, which is divided  further into five  phases. These are leptotene, zygotene , Pachytene,  diplotene and diakinesis.  During  metaphase I the bivalents arrange  on  the equatorial  plate. This is followed  by anaphase I in  which homologous chromosomes move to the opposite  number  of the parent  cell.In  telophase I, the nuclear  membrane  and  nucleolus  reappear . Meiosis  II is similar  to mitosis.  During  anaphase  II the sister chromatids  separate. This at the end of  meiosis  four  haploid  cells are formed.










































































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