Meiosis



 Discovery:

August Weismann is 1887 predicted on theoretical grounds  that the number of chromosomes must be reduced by one__ half during  gamete formation. Edouard Van Baneden demonstrated reduction division  in 1887.The term ' meiosis'was introduced by J.B. Farme and Moore in 1950
August Weismann was a German evolutionary biologist and geneticist who lived from 1834 to 1914. He is best known for his work on the theory of germ plasm, which proposed that hereditary information is carried only in the germ cells (eggs and sperm) and not in the somatic cells (body tissues) of an organism.

August Weismann 


Weismann was also interested in the mechanism of evolution, and he proposed that natural selection acted on variations in the germ plasm, rather than on the physical characteristics of the organism itself. This idea was influential in shaping modern evolutionary theory.

Weismann conducted experiments on inheritance in mice and other animals, and he developed the theory of genetic recombination, which explained how new combinations of genetic traits could arise in offspring. He also introduced the concept of the germ line, which refers to the cells that give rise to the eggs and sperm.

Today, Weismann is considered one of the founders of modern genetics and a pioneer in the study of evolution.

Edouard Van Beneden 


Edouard Van Beneden was a Belgian cytologist and embryologist who lived from 1846 to 1910. He is known for his work on cell division, particularly in observing and describing the process of meiosis. 

Van Beneden observed and described the behavior of chromosomes during meiosis, and he discovered that the number of chromosomes in sex cells (eggs and sperm) is reduced by half. This is known as the principle of reduction division, and it is a critical mechanism in sexual reproduction.

Van Beneden also studied the development of embryos, and he proposed the concept of "morphological polarity," which suggests that the orientation of the egg and sperm during fertilization determines the polarity of the resulting embryo.

Van Beneden's work on meiosis and sexual reproduction laid the foundation for modern genetics and greatly advanced our understanding of the mechanisms underlying heredity. Today, he is considered a pioneer in the field of cytogenetics and his work has had lasting impact on the study of genetics and embryology.

Occurrence:

Mitosis can occur in all kinds of  eukaryotic cells. Meiosis is confined to certain cells and takes place at a particular  time. Only the cells of sexually  reproducing organisms undergo meiosis, and only special cells in the multicellular organisms switch over from mitosis to meiosis at a specific  time in the life cycle.Meiosis produces gametes or gemetic  nuclei in animals,  some lower plants, and various  protists and fungi. Meiosis form asexual   reproductive bodies, the spores, in higher plants.The spores give rise to the gamete producing  structures  which produce  gametes by mitosis.

Duration

Meiosis takes days to complete  instead of hours or minute needed for mitosis. 

Meiocytes : The cells in which meiosis  takes place are called meiocytes. These are 13 types:oocytes, spermatocytes and sporocytes.The oocytes  gives rise to female gametes called ova or eggs; spermatochtes form male gametes termed spermatozoa or sperms; and sporocytes produce special cells known as spores.Oocytes and spermatocytes are formed in the sex  organs  or gonads.Sporocytes are formed in the organs called sporogonia.

Essential  ( special) Features of Meiosis:

These include  __ (i)  two successive  divisions  without DNA replication  between  them,

(ii) Synapsis of homologous chromosomes, 

(iii) Crossing  over of segments  between  nonsister chromatids of synapsed chromosomes, 

(iv) Segregation of homologous chromosomes, and

(v) Separation of sister chromatids.




Meiosis:

Meiosis is a  double which occurs in a diploid  cell and gives rise  to four haploid cells.The term meiosis was coined by Farmer and Moore. The cells undergoing  meiosis  are often called meiocytes.


 The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter  cells. This kind of division  is called meiosis .Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation  restores the diploid phase. We come across meiosis during gametogenesis in plants and animals. This leads to the formation  of haploid gametes. The key features  of meiosis are as follows: 




● Meiosis involves two sequential cycle of nuclear and cell division  called meiosis _I and  Meiosis _ II but only a single cycle of DNA replication . 


● Meiosis __ I is initiated after the parental   chromosomes have replicated to produce identical sister chromatids at the S phase.

● Meiosis involves pairing of homologous chromosomes and recombination between  non__ sister chromatids of homologous chromosomes. 


● Four haploid  cells are formed at the end of meiosis __ II .






Meiotic events can be  grouped under the following  phases: 

Interphase:

Interphase of meiosis is similar  to that of mitosis except that S_ phase is prolonged. A distinct G2 phase is either short or absent. Like mitosis meiosis can be divided into two steps: Karyokinesis  and Cytokinesis. While mitosis is rather brief ( 1 or 2 hours), meiosis is a  long process. For example, in human male it may last 24 days, and in females it may extend for several years.

Karyokinesis: 

The nucleus undergoes two divisions. The first one is called heterotypic or reduction  division. During this  division  the number of chromosomes is reduced to half.The two chromatids of a chromosome become genetically  different due to crossing over. These chromatids get separated  in the second division remains the same as produced after the end of the first division. Like mitosis, meiosis  also involves indirect nuclear division. 



Meiosis _ I

It has four stages 
(a)  prophase, ( b) Metaphase   (c) Anaphase   (d) Telophase 




A): Prophase_ I:

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.





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.






Depending upon the place of origin of pairing, synapsis is procentric ( starting  from centromeres and proceeding towards ends), and intermediate or random ( at various  places in between  centromeres  and ends). It produces a complex known as synaptinemal complex.The  nucleoprotein core in synaptinemal complex has a tripartite structure,two  lateral  components or arms and a central or medial element. Central element  lies between  the two homologous chromosomes. Each lateral  element is shared by the two  sister chromatids of homologue.One function of this complex appears to be to stabilize the pairing  of homologues  and  another  is to facilitate  recombination.Because of synapsis,chromosomes  form pairs or bivalents. The  number of bivalents  is half the number of the total chromosomes. 

3): Panchytene or Pachynema:

Panchynema may last for days, weeks or even years, whereas leptonema and zygonema last only a few hours. The bivalents shorten. Each bivalent is made up of hour chromatids two of each chromosome. The two chromatids belonging to the same chromosome are called sister chromatids. Chromatids belonging to the two different  chromosomes of a homologous pair are termed as nonsister chromatids. In the presence  of enzyme  endonuclease breaks develop in the chromatids.  The process  is called nicking.In most of the cases,  the  nicks get healed but in one some gaps develop in the region of  nicks by the activity  of another enzyme exonuclease. Separation  of chromatid segments occurs in between  two gaps by U_ protein or enzyme unwindase.The separated segments of nonsister chromatids may exchange position  if they show the same  degree of nicking .The process is called crossing over. It is a process of exchange of corresponding chromatid segments between  non_ sister chromatids of homologous chromosomes. The separated segments soon get re_ united with the help of an enzyme known as R__ protein.The process is called re_ annealing. For the crossing  over , to occur, DNA fibers of both  homologous chromatids must reach a distance of about  1nm in the central component of the synaptinemal complex . Electron microscopic studies have revealed the presence of dense nodules in intimate association with the synaptinemal complex. The number of these nodules, called recombination nodules, and their location along the bivalents are related to the number and distribution of the genetic exchanges ( cross overs). This is a strong indication that the recombination nodule is the site at which crossing__ over takes place. These nodules probably contain enzymes needed for the process of crossing over.




4): Diplotene or Diplonema: 

The nucleoprotein complex of the chromosomes dissolves partially. Therefore, the homologous chromosomes separate except in the region of crossing over. The chromatids also become distinguishable (tetrad stage). The points of attachment between  the homologous chromosomes after dissolution of synaptinemal  complex are called chiasmata. Chiasmata may be terminal or interstitial.Shape of the bivalent may give  an indication of the number of chiasmata per bivalent like it is X __ shaped in  case of one chiasm; ring shaped in case of two chiasmata and a loop of many rings in case of more  than two chiasmata. Number if chiasmata  per bivalent per cell is called chiasma frequency. Diplonema is a long_ lasting and metabolically active period. In the fifth month of prenatal life, for example, human oocytes have reached  the stage of diplonema and remain in this stage unit many years later, when ovulation  occurs. At this stage the chromosomes decondense and engage in RNA synthesis. Lampbrush chromosomes found in amphibians are actually decondensed diplotene chromosomes. 


5): Diakinesis:

The chromosomes again contract. Chiasmata  shift towards the ends of the chromosomes. The process is called terminalisation.Thus the number of chiasmata diminishes. By  the end of diakinesis,  the homologues are held together only at their ends. The nucleolus degenerates.  Simultaneously,  nuclear envelope disintegrate.

B): Metaphase_I :

A   bipolar spindle apparatus appears in this phase. The fibres  of spindle  apparatus  converge  toward the poles. The bivalents arrange themselves on the equator of the spindle. The limbs of the chromosomes lie horizontally on the equator. The centromeres slightly project towards the periphery. Since, there are two centromeres in each bivalents,  the centromeres of all the bivalents produce a  double metaphasics plate. Meiotic metaphase_ I can be distinguished  by the fact that the homologues are still  attached by chiasmata at their ends.

The distribution of bivalents is at random so that the individual  paternal and maternal chromosomes can face either  of two poles of the spindle. Each chromosome gets attached to the spindle pole of its side by means of one chromosome fibre in the region of the centromere. ( This is in contrast to the development  of two tractile fibrils from the same centromere in mitosis). Thus in metaphase of meiosis__ I, the two sister chromatids behave as a functional unit. The fibres of the homologous chromosomes are always in the opposite directions.




C): Anaphase__ I: 

The homologous chromosomes break their connections and separate out. The process of separation is named as disjunction. The separated chromosomes, i.e., univalents are also known as dyads because each of them consists of two chromatids. The double stranded chromosomes of anaphase__ I  are in contrast to their single_ stranded nature in anaphase of mitosis. 

The chromosomes move towards the opposite poles. At the end of anaphase I, two groups of  chromosomes are produced, with each group having half the number  of chromosomes present in the parent nucleus.

(D): Telophase__ I:

The chromosomes at the poles arrange themselves  into haploid or dyad nuclei. Although  in many cases the chromosomes do undergo some dispersion, they do not  reach the extremely  extends state of the interphase  nucleus. A nucleolus is formed by the satellite chromosome. The nucleoplasm and nuclear envelope reappear. The elongated chromosomes remain straight and do not enter the interphase. In some cases telophase is completely  omitted when the anaphase chromosomes directly  enter the metaphase of next division ( e.g., Trillium).Cytokinesis may or may not follow first meiotic division

Significanc of Meiosis_ I:

1): It separates  the homologous chromosomes and reduces the chromosome number to half. This reduction is essential for sexual reproduction. 

2): Crossing  over introduces new combinations of genes. This results in variations.

3): There is random distribution of paternal and maternal chromosomes into daughter cells. This independent assortment of chromosomes produces variations. 

4): Due to some defect in disjunction,  chromosomal and genomatic mutations take place.


5): It induces the cells to form spores or gametes.



Meiosis_II:

It is shorter than mitosis because of the shortening  of prophase. It maintains the number of chromosomes produced at the end of meiosis__ I. It is hence called homotypic or equational division. Though  it is similar to mitosis, meiosis_ II is not mitosis because 

(i) It occurs in haploid  cells,

(ii) It is not preceded by interphase, 

(iii) Chromatids of a chromosome are often dissimilar, and 


(iv) The daughter  cells formed after meiosis __ II  are neither similar  to each other nor similar  to the parent cell. Meiosis  __ II  is divisible  into prophase, metaphase, anaphase and telophase.  The state of nucleus having  elongated chromosomes is called  interkinesis as interphase is absent. In animal cells, centriole pairs undergo replication during interkinesis or early  prophase __ II. Protein and RNA synthesis  may also occur. DNA  replication is not seen during interkinesis. 



A2): Prophase__ II :

It is very short.It takes place simultaneously in the two nuclei.In animal cells, the centriole pairs develop asters and move to the regions of future spindle poles. The dyad chromosomes shorter a little.Nucleolus and nuclear envelope degenerate.The chromatids are separated widely and are hence much looser than the ones in mitosis. In case where telophase __ I is omitted,  the prophase__ II is also absent.

(B2): Metaphase __ II : 

Achromatic  spindles  are formed in the areas of dividing nucleic. The chromosomes reach the respective spindle and arrange themselves in such a fashion that their centromeres  come to lie at the equator. Each chromosome gets connected with both the spindle poles by means of chromosome fibres.

(C2) Anaphase _ II  : 

The centromere of each chromosome divides into two so that there is one centromere for each chromatid. The two chromatids of a chromosome separate and form daughter  or new chromosomes. The daughter  chromosomes move towards the spindle poles along the path of their fibres Thus, four groups of chromosomes are produced, each group having haploid number.


(D2) Telophase __ II  :

The four groups of chromosomes arrange themselves into haploid nuclei. The  chromosomes elongate to form chromatin. A nucleolus  is produced .Then there is formation  of nucleoplasm and a number nuclear envelope. The spindle fibres usually degenerate during telophase __ II.

Cytokinesis:

Cytokinesis can be of two types, successive and simultanous. In successive type, the cytoplasm divides after every nuclear division.It produces two cells ( diad of  cells) after meiosis __ I. Both these cells undergo  meiosis __ II  and then form four cells ( tetrad   of   cells).
  

  In the simultanous type, cytokinesis takes place only  at the end of meiosis II. The nuclei are arranged in the form of a tetrahedron. Cytoplasm cleaves or  constricts in between  thr nuclei. Four furrows are formed.They deepen and meet in the centre.In plants, wall material is deposited in the furrows.It forms four haploid cells. These are arranged  tetrahedrally and are collectively called tetrahedral tetrad.

Significanc of Meiosis:

(1): Meiosis forms gametes.

2): It switches on the genetic information for the development of gametes or gametophytes and switches off the sporophytic  information. 


(3): It maintains the fixed  number of chromosomes in sexually reproducing organisms. It is essential,  as chromosome number would become double after fertilization. 


4): In meiosis paternal and maternal chromosomes assert independently.It causes reshuffling of chromosomes and the traits controlled by them.

5): It introduces variations because  of crossing  over.


6): Chromosomal and genomatic mutations can take place by irregularities in meiosis.

7): Details of meiosis are essentially  similar in the majority  of organisms showing their basic similarity and relationship. 


Types of Meiosis:

Meiosis  is essentially  similar in all animals and plants. However,  it is  of  3 types  with regard to time and place it occurs in the life cycle of an organism:  gametic , zygotic and sporogenetic.

1): Gametic or Terminal Meiosis: 

In many protozoans, all animals and some lower plants, meiosis takes place before fertilization during the formation of gametes. Such  a meiosis is described as gametic, or  terminal. The gametes formed by meiosis are haploid. When two gametes unite in fertilization, the resulting zygote is diploid.This diploid condition persists during the development  therefrom. The adult produces haploid  gametes  by meiosis in the  gonads. This type of life cycle  with diploid  adult and gametic meiosis is known as the diplontic cycle. 





2): Zygotic or  Initial Meiosis:

In fungi, certain  protozoan groups,  and some algae fertilization is immediately followed by meiosis in the zygote, and the resulting adult organisms are haploid. Such a meiosis is said to be zygotic, or initial.This type of life cycle with haploid  adult and zygotic meiosis  is termed the haplontic cycle.


     The diplontic life cycle seems to have been derived from th haplontic life cycle by postponement  of  meiosis , and has become almost  universal in the animals. The diplontic life  cycle is advantageous as it introduces variations at the time  of gametic formation  also besides  at the time of fertilization. 

3): Sporogenetic or Intermediate Meiosis:

In all higher and some lower plants, gametes are formed by mitosis and meiosis occurs at the time of spore formation . Spores are formed the zygote in primitive  forms, such as algae, and from the spore mother  cells in others. Meiosis  is zygotic in the former case and sporogenetic in the latter case.

  Spore may produce  a multicellular haploid  gametophyte phase,which forms the gametes. Life cycle of plants is said to be diplohaplontic  because  of having diploid and haploid  multicellular phases.


  In sexually   reproducing organisms, gametic fusion and meiosis occur alternately in a regular manner, and together  the two processes  maintain the chromosomes number constant through successive generations. 


QNo : Find examples where the  four  daughter ces from meiosis are equal in size and where they are found unequal in size?

During formation  of male gametes ( e.g., spermatozoa) in a typical mammal ( e.g.  human being),thr four daughter  cells from meiosis are equal in size.On the other hand, during  formation of female gametes ( eg., ovum) in a typical mammal ( e.g., human being), the daughter cells are unequal in size.

QNo: Describe the following: ( a) Synapsis (b) Bivalent ( c) Chiasmata


Ans: ( a) In zygotene stage of meiosis __ I, the homologous  chromosomes come to lie side_ by _ side in pairs . This pairing of homologous chromosomes is known  as synapsis.

(b)  A pair of homologous chromosomes lying together  is called a bivalent.

(c) Chiasmata are the sites where crossing  over between  two non_ sister chromatids occurs during  pachytene stage of prophase in mitosis __ I.



 QNo: Give an  alternative term for meiosis?

Ans: Reduction division 


 What is the life cycle with diploid adult and gametic meiosis called ?


Ans: Diplontic 

How many division  take place in meiosis? How many times, the chromosomes replicate during meiotic division?

Meiosis  consists  of two divisions: 
Meiosis __ I  and Meiosis __ II.Thesr occur in rapid succession. The chromosomes replicate only once.



















































































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