Meiosis 1and Meiosis 2



Mechanism of Meiosis:

Meiosis consists of two divisions that  take place in rapid succession, with the chromosomes replicating only once. Thus, a parent cell produces four  daughter 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 

Similarity 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, behaviour 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 phases of this division are called  prophase __ I, metaphase__ I, anaphase __ I and telophase __ I.



1): PROPHASE__ I: 

The prophase  of meiosis __ I is more elaborate  and last longer than the prophase of mitosis. It takes more than 90%  of the time required  for meiosis. It may be divided into 5 substages: leptotene, zygotene, pachytene, diplotene and diakinesis. The first four subphases  are also known as leptonema, zygonema, pachynema  and diplonema respectively. 

(a) Laptotene: 

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 chromose 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.

Notes: 
●  There are 2  sets of chromosomes in a diploid  cell undergoing meiosis, one set contributed by the male parent and the other by the female 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 the other maternal chromosome. 

In some organisms, the  chromosomes give beaded appearance due to having chromomeres.

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


(b) Zygotene:

The homologous chromosomes come to lie side__ by__ side in pairs ( G. zygon = yolk; tene= thread). This pairing of homologous chromosome 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, developedes 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 ends and  proceeds toward the middle.

(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.

(C) 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 two homologous ( 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 that crossing  over is a common  event. Normally , each tetrad undergoes at least one recombination. 





(d) Diplotene:

Now the synaptic forces keeking 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 pachytene ( 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 rRNA to 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.

(e) Diakinesis:

The chromosomes  condense 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. 


2): 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 is formed by   the       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 chromosomes has two kinetochores, one  for  each of its two chromatids. Both the kinetochores of a homologous chromosome connect to the same spindle pole. The two kinetochores of its homologue join the opposite  spindle pole.




3): ANAPHASE_I:

From each tetrad, two chromatids of a chromosome move as a unit  ( dyad) to one pole of a spindle, and the remaining  two chromatids of its homologue 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 pole 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 pairs 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. 

(4) 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 develop nucleolus, 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 and the prophase of meiosis __ II  is almost eliminated,  except the duplication of centrioles and the formation of two new spindle with asters at the poles to replace the telophase __ I spindle and asters. The telophase __ I  chromosomes pass over the new spindle and start metaphase __ 2

Significanc of Meiosis __I  :

Meiosis__ I involves many Significant 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 combinations 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.







(1) 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. 

2): METAPHASE _ II :

The chromosomes take up positions at the equator of the spindle, forming  a single metaphase 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. 

3):   ANAPHASE _ II :

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

4): 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 latter 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 into two haploid cells,  which  then produce four haploid cells by homotypic division. The four daughter cells may form a linear or isobilateral tetrad. Often Cytokinesis is  delayed unit both the nuclear  divisions 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.



Meiosis _ II is not Mitotic :

Meiosis _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  mitosi.

(ii) It is not preceded by an interphase. 

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

(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 in 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 meiosis _ II ?

 The chromosomes that separate in the anaphase  of meiosis _ I are still 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 _ II. Truely haploid nuclei in  terms of DNA content as well as chromosome number are formed in meiosis __ II , when the chromatids of each chromosome are separated into  different  nuclei.  Thus, meiosis __ II is necessary. 


Q : What does a kinetochore consist of ?
Ans A kinetochore consists of proteins and specific sections of chromosomal DNA at the centromore. It has motor proteins that help the chromosome/ chromatid to move along the shortening microtubules of the spindle. 

Significanc of Meiosis 

Meiosis has a six _ fold 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 chromosome number in the gametes so that fertilization  may restore the original 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 

(i) By crossing  over in prophase _ I and  by

(ii) 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) Mutation : 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, brings essentially similar in all sexually reproducing organisms,  offers further evidence of the basic relationship  of living  organisms. 















































































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