What is Meiosis? Phases, Importance, Function and Errors in Meiosis.

What is Meiosis? Phases, Importance, Function and Errors in Meiosis.

Meiosis is the mechanism of cell division in germ cells, it involves the production of sex cells (gametes) in all sexually reproducing eukaryotic organisms that is sperm or male gametophyte and egg or female gametophyte. Meosis is the reductional division because the chromosomes no in daughter cells reduce to half of the parent cell. This division occurs in two fission in a parent cell and at the end, four daughter cells are formed with half no of chromosomes which are called gametes. Now the question is whether are all four daughter cells are identical in meosis? The answer is no, because each of four daughter cells have half no of chromosomes and have new combination of DNA because of crossing over. The gametes are then fused during sexual reproduction and form a diploid zygote. Meiosis occurs in all eukaryotes, in plant and fungi meiosis occurs in spores, similarly meiosis occurs in human and animals germ cells to produce sperm and egg. As we know meiosis is the reductional division, the basic purpose of meiosis is to retain the no of chromosomes constant generation after generation in a specie. Every organism has a different level of polidy (no of chromosomes), humans have 46 chromosomes (23 pairs) meiosis occurs in the germ cells to reduce the number of chromosomes into half and produces the haploid gametes. Gametes after fusion produce the diploid zygote. Error in meiosis causes an abnormal no of chromosomes and genetic disabilities. 

Prior to meiosis the DNA in the germ cells (both paternal and maternal germ cells) replicated and formed copies of genetic material, after contraction and condensation of chromosomes it appears as an X-shaped structure having duplicated chromatids. The homologous chromosome pair up in the germ cells, and crossing over occurs, and form a new combination of DNA, the homologous chromosome separates, and after two rounds of division the four daughter cells are formed having half no of chromosomes.  

Definition of Meiosis

“Meiosis is a type of cell division in which parent cell divides into four daughter cells in two fusion, with half of the no of chromosomes to the parent cell.”

Meiosis is the division of germ cells, to produce the gametes, in animals, meosis takes place in organ i-e testes and ovaries during spermatogenesis and oogenesis, while in plants its result is the formation of spores. The basic purpose of meiosis is to maintain the no of chromosomes constant in a specie. The diploid no of chromosomes in the parent cell is reduced to the haploid no of chromosomes.

The division occurs into two phases i.e. meiosis I and meiosis II.

Fig 1: Meiosis and its phases meiosis I and meiosis II.

Meiosis I

Meiosis I begins as the interphase marks the end. Meiosis I is the separation of homologous chromosomes, at the end of meiosis I two daughter cells are formed with the reduction of the chromosome into half, so meiosis I is reductional division. The karyokinesis of the first meiotic division is divided into 4 stages:

Prophase I

Metaphase I 

Anaphase I

Telophase I

Prophase I

Meiosis prophase I,  is unique in way that it is long enough which usually accounts for 90% or more time in meiosis. It is the most complicated phase of meiosis and is different from mitotic prophase in a way that, in meiotic prophase I critical events occur which include, pairing of homologous chromosomes, synapsis, and crossing over. It is further divided into five distinct stages.

Leptotene

The leptotene phase is the first phase of prophase I, initially during this phase size of the nucleus increases, and the pair of centrioles starts moving to the opposite poles.

• During this phase, the chromosome starts appearing as a long, slender, thread-like structure.
• Each chromosome consists of small slender chromatids attached at the centromere.
• The homologous chromosome starts identifying, due to the similarities in size, structure, and position of the centromere. (the homologous chromosome are the chromosomes that are identical to each other length-wise, size-wise, and relative position of gene and loci)

Zygotene

• Leptotene is followed by zygotene, during this phase shortening of the chromosome by continuous coiling makes them distinctly visible.
• The homologous chromosome starts pairing. The paring is lengthwise, point to point, and locus to locus. The pairing of the chromosome is called synapsis, which occurs in zip-like fission, between the non-sister chromatids of homologous chromosomes.
• As a result of pairing a tetrad or bivalent is formed, tetrad is a combination of two chromosomes each having four chromatids attached at the centromere, while bivalent is the association of tetrad.

 

Pachytene 

• Pachytene begins as the synapsis is completed, synapsis of homologous chromosomes which started in the Zygotene stage nearly completed in the pachytene stage, and all the autosomal chromosomes have completely synapsed, while sex chromosome exchange segments at some regions called pseudo-autosomal regions.  
• The chromosome becomes thicker and clearer by constant condensation and nucleoli become more evident.
• The homologous chromosome forms the loop called chiasmata, where crossing takes place.
• Non-sister chromatids of homologous chromosomes exchange their segments due to chiasmata formation and this process is called crossing over. 
• As a result of crossing over an exchange of genetic information occurs and new recombination of DNA is formed.
• Pachytene may last for days or even years.

Fig 2: meiosis crossing over occurs  in which chromosomes exchange their segments due to chiasmata formation.

Diplotene 

• The pachytene is followed by diplotene, in the diplotene stage the paired chromosome begin to separate from each other, but a bivalent is tightly held together at chiasmata.
• The contraction of chromosomes continues even at diplotene stage.

Diakinesis 

• During Diakinesis nucleoli become disappeared and the nuclear envelope gradually starts disintegrating.
• Chiasmata also starts disappearing and paired chromosome are only held at one point.
• Chromosomes achieved their maximum condensation, spindle fibers start appearing and the nuclear envelope disintegrated to liberate the chromosomes.

Metaphase I 

• The meiosis metaphase I stage begins soon after, as the nuclear envelope and nucleoli vanished, the spindle apparatus formation and movement of bivalent in midway.
• The centromeres of each homologous chromosome or bivalent start connected with the kinetochore spindle fibers at their respective poles.
• The homologous chromosomes arrange themselves midway at the equator between two poles and form a metaphase plate.
• The cohesion protein prevents the sister chromatids from separating apart.

Fig 3: Centromeres of each homologous chromosome connected with the kinetochore spindle fibers at their respective poles.

Anaphase I

• The anaphase started as the homologous chromosomes starts moving apart to their respective pole.
• Meiotic spindle fibers start contracting and pulling the whole chromosome at their respective pole, one set of chromosomes starts moving to one pole, and the other set of chromosomes to the opposite pole.
• Meiotic anaphase I is different from mitotic anaphase in a way that, in meiotic anaphase I the centromeres do not divide and chromatids do not separate, while the whole chromosome (a tetrad) of homologous chromosome starts moving to one pole and another set is moving to the opposite pole. 

Telophase I and cytokinesis I

• Telophase I begins with the arrival of homologous chromosomes at their respective pole.
• The nucleoli and nuclear membrane start reappearing, and the chromosomes start uncoiling and become invisible. The spindle apparatus starts vanishing and centrioles persist. Thus a complete nucleus is formed at each pole.
• At the end of telophase I cytokinesis divides the cell's cytoplasm, which results in the division of a single parent cell into two daughter cells with half no of the chromosomes to their parent cell, this marks the end of meiosis I.

Meiosis II

After the end of meiosis I, the cell undergoes for a second round of division called Meiosis II. Which is simple and shorter as compared to meiosis I. The two daughter cells which were the product of the first meiotic division undergo for meiosis II. These cells are haploid cells and have one set of chromosomes lesser than the parent cell. The DNA does not replicate for meiosis II but rather chromosome (a tetrad)

Still consist of four chromatids and a centromere. In simple meiosis II is like mitosis, we can say that is is just the mitotic division of a haploid cell. The stage is further divided into four phases.

Prophase II

Metaphase II 

Anaphase II

Telophase II

Prophase II

In prophase II the centrioles start moving to the opposite side, nuclei start disappearing and holes start appearing in the nuclear envelope, as a result, the nuclear envelope starts disintegrating liberating the chromosome.  

• The condensation and contraction of chromatin material of the nucleus occur and the thread-like chromosome appears.
• The meiotic spindle apparatus starts appearing. Spindle fibers radiating at two poles are of three types: (the aster microtubules, the polar microtubules, and the kinetochore microtubules).
• With continuous contraction and condensation of the chromosome, it appears as an X-shaped structure, each chromosome consists of two sister chromatids that have identical genetic information and a centromere.

Metaphase II 

• The metaphase II of meiosis II starts as the chromosome arranged themselves midway between two poles which are called the metaphase plate or equatorial plate.
• Chromosomes are at maximum contraction and condensation during metaphase II.
• Centromeres of chromosomes attached fully to the Kinetochore microtubules of both poles, and spindle fibers of both poles start pulling the chromosomes at their respective pole, each chromosome consist of a pair of chromatids and a centromere.

 

Fig 4: Difference between metaphase I and metaphase II.

Anaphase II

• Anaphase II begins as the centromeres split into two by the contraction of spindle fibers and start pulling the chromosomes at their respective poles.
• After the division of centromeres chromatids start separating too, which becomes the chromosomes of daughter nuclei later.
• In anaphase II of meiosis II chromosomes are segregated equally and continue their journey at their respective poles. There is no division in the polidy at meiosis II.
• The microtubules start contracting and becoming shorter and moving the sets of chromosomes at their respective poles.

Telophase II

• Telophase II begins as the set of daughter chromosomes reached the opposite poles.
• The nuclear membrane starts reappearing around two sets of daughter chromosomes and nucleoli also start reappearing. The spindle apparatus also start disappearing.
• Chromosomes start uncoiling, become thinner and thinner, and eventually vanished.
• Meiosis II is completed and two sets of daughter nuclei are formed at opposite poles in single-parent cells. 
• Cytokinesis II also begins during telophase II, and after completion of cytokinesis as a whole four daughter cells are formed in two rounds of meiotic division from a single diploid meiocyte with exactly half the number of chromosomes to their parent cell.

Importance of meiosis

Why meosis is important?:

Meosis is impotant because: Each specie has a specific no of chromosomes, meiosis helps in maintaining the number of chromosomes constant generation after generation.

The pairing of homologous chromosomes occurs in meiosis, in this process crossing over takes place between non-sister chromatids which result in variation in nature. The inheritance of such variation in nature is important for the formation of new species during the course of evolution.

It is also important for the transfer of hereditary characteristics from parents to offspring during meiosis.

Fig 5: Microscopic view of meotic cell division.

Function of meiosis 

Meiosis helps in the formation of gametes in all sexually reproducing organisms, as at the end of meiosis four daughter cell are formed that form gamete during gametogenesis, these gametes after fertilization produces a diploid zygote for continuity of life.

 Meiosis involves in segregation of chromosomes, the homologous chromosome independently assorts during metaphase I and anaphase I which helps in keeping the polidy level constant.

Meiosis genetic variation also helpful in the formation of new recombination of DNA during chiasmata formation where non-sister chromatids exchange their segments and form a new combination of genetic information.

Errors in meiosis

As meiosis is an important process for the production of gametes but errors do occurs at some time during the process and cause abnormalities and genetic diseases. These errors are of two types: 1) error in chromosome number and 2) error in recombination or mutation    

Error in number occurs when homologous chromosomes are unable to separate and there is an error in segregation as result one cell may take an extra copy of a chromosome or it may have a depletion of a chromosome which may cause many diseases like Down syndrome, turner syndrome in females and others. While errors in recombination may results in mutation although mutation is very important during evolution, but some abnormal mutation or harmful mutation during meiosis causes several genetical diseases like sickle cell anemia, phenylketonuria, and a variety of others.

Fig 6: Meiosis labeled diagram in which single diploid parent cell divides in two fission and four daughter cells are forme with haploid no of chromosomes.