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Meiosis

Every living eukaryote organism is or has been a single cell. New cells are made by division of existing cells, which involves the division of both nucleus and cytoplasm. In eukarytots, two types of division takes place mitosis and meiosis. Meiosis is unique form of cellular differentiation and it is initiated usually only once in the life cycle of a eukaryote. Meiosis is a unique and distinctive event in the life of an organism. It is a event that normally involves an accurate and quantitative reduction in chromosome number and also precise partitioning of genetic material.

Meiosis fulfills two interrelated functions which are connected with the sexual reproduction process. It produces a haploid phase in the life cycle of an organism, also known as reduction and also provides the production of genetically distinct progeny, also referred as recombination. Deviations from this behavior are usually lethal or sublethal to the organism, as the a proper functional chromosome is an essential requirement for the basic function of cell during development. As meiosis is a ordered process, genes and proteins control the process and conserve the event throughout eukaryotes.

 

What is Meiosis

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Meiosis is a unique type of cell division, it is necessary to sexual reproduction in eukaryotic organisms. The cells that are produced by the process of meiosis are referred to as gametes or spores. Meiosis shuffles the genes between the chromosomes in a pair, which are received from each parent. It produces chromosomes with new genetic combinations in every gamete the process generates. Meiosis division produces genetically unique four cells, the chromosome number is half as that is in the parent cell.

Meiosis Chart

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Meiosis

Stages of Meiosis

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Meiosis is a one way process, unlike mitosis is a cell cycle. The preparatory phase to meiosis is identical in pattern and name to the interphase of the mitotic cell cycle.

Meiosis 1

Interphase

Meiosis interphase is divided into three phases
  • G1 phase or Growth 1 phase is a very active period. In this period the cell synthesizes vast range of proteins which includes the enzymes and structural proteins necessary for growth of the cell. In the G1 phase the chromosome are made of single molecule of DNA, at this point, in humans, the number of chromosomes per cell is 46 which is 2N and identical to the somatic cells. 
  • S phase or the Synthesis phase - There is replication of genetic material in this phase. Chromosomes duplicate, each of the 46 chromosomes become a complex of  two sister, identical chromatids. 
  • G2 phase or Growth phase is not present in meiosis. 
The interphase stage is followed by meiosis I and meiosis II. 

Meisois is divided into meiosis I and meiosis II stages. It is further divided into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. 

Meiosis I 

  • The pairs of homologous chromosomes, made up of two sister chromatids are split into two cells.
  • The resulting daughter cells contains one entire haploid set of chromosomes. 
  • The first meiotic division reduces the ploidy of original cell by a factor of two. 
  • It produces two haploid cells (N chromosomes, 23 in humans). 
  • Hence meiosis I is referred to as a reductional division. 
  • A diploid human cell contains 46 chromosomes and is said to be 2N because it contains 23 pairs of homologous chromosomes. 
  • Meiosis II is an equational division similar to mitosis, where the sister chromatids split and creating 4 haploid cells, two from each daughter cells from meiosis I.   

Prophase I 

  • Prophase I is the longest phase of meiosis I. 
  • During this phase, there is exhange of DNA between homologous chromosomes, this process is known as homologous recombination. This process often results in chromosomal crossover. 
  • The DNA created are of new combinations, and during crossover they are a significant source of genetic variation. This may result in beneficial new combinations of alleles.
  • The chromosomes that are paired and replicated are called bivalents or tetrads. 
  • They have two chromosomes and four chromatids, each chromosome comes from each parent. 
  • Pairing of homologous chromosomes is called synapsis. At the stage of synapsis formation, the non-sister chromatids may cross-over at points called chiasmata. 

Leptotene 
  • Leptotene is the first stage of prophase I and is also known as leptonema, which is derived from a Greek word which means "thin threads". 
  • In this stage, individual chromosomes consists of two sister chromatids. 
  • The chromosomes condense into visible strands within the nucleus. 
  • The two sister chromatids are tightly bound, that they are not distinguishable from one other. 
  • During this phase the lateral elements of the synaptonemal complex assemble. 
  • This stage is of very short duration and progressive condensation and coiling of chromosome takes place. 

Zygotene
  • Zygotene is also known as zygonema, it is derived from Greek word which means 'paired threads'. 
  • The chromosomes in this line up with each other into homologous chromosome pairs. 
  • This stage is known as bouquet stage, due to the way the telomeres cluster at on end of the nucleus.
  • Synapsis of homologous chromosomes takes place in this stage, it is facilitated by the assembly of central element of the synaptonemal complex. 
  • Pairing of chromosomes happens in a zipper like fashion and starts at the centromere (procentric) or at the chromosome ends (proterminal)  or at any other portion (intermediate). 
  • Two chromosomes in a pair are equal in length and in position of the centromere, making the pairing highly specific and exact.
  • These paired chromosomes are called bivalent or tetrad chrmosomes. 

Pachytene 
  • The pachytene stage is also known as pachynema and is derived from Greek which means "thick threads".This is the stage where chromosomal crossing over occurs.
  • Nonsister chromatids of homologous chromosomes exchange segments over homologous regions. 
  • Sex chromosomes are not identical and they exchange information over a small region of homology.
  • Chiasmata is formed where exchange happens. 
  • There is exchange of information between the non-sister chromatids and this results in a recombination of information. 
  • Every chromosome has a complete set of information and there are no gaps formed as the result of the process.

Diplotene 
  • The diplotene stage is also known as diplonema, which is derived from Greek word meaning "two threads".
  • During this stage there is degradation of synaptonemal complex and the homologous chromosomes separate a little from one another. 
  • The chromosomes in this stage uncoil a little, this allows transcription of DNA. 
  • The bivalent homologous chromosomes remain tightly bound at the region of the chiasmata,where crossing over occurred. 
  • The chiasmata regions remains on the chromosomes until they are separated in the anaphase. 
In the oogenesis of humans the developing oocytes in the fetal stage stop at this stage of diplotene before birth. This state is referred to as the dictyotene stage and it remains in this suspended stage until puberty.

Diakinesis 

  • During the stage of diakinesis the chromosomes condense further.
  • The word diakinesis is derived form Greek word which means "moving through". 
  • This stage is the first part of meiosis where the four arms of the tetrads are visible. 
  • The sites where crossing over has occurred entangle  together, overlapping effectively and making the chiasmata visible clearly. 
  • This stage resembles the prometaphase of mitosis, where the nucleoli disappear and the nuclear membrane disintergrates into vesicle and also there is formation of the meiotic spindle. 

Prophase 1 Meiosis

Metaphase I 

  • The homologous pairs of chromatids move together along the metapahse plate. 
  • The kinetochore microtubules from the centrioles attach to their kinetochores respectively. 
  • The homologous chromosomes align along the equatorial plane, this alignment happens due to the continuous counterbalancing forces exerted on the bivalents by the microtubules emanating from the kinetochores of the homologous chromosomes.  

Anaphase I 

  • During this phase the kinetochore microtubules shorten, this severs the recombination nodules and pulls the homologous chromosomes apart. 
  • As each chromosome has only one functional unit of a pair of kinetochores, the whole chromosomes are pulled towards the opposite poles which results in the formation of two haploid sets. 
  • Each chromosomes contains a pair of sister chromatids. 
  • Disjunction occurs during this time, this is one of the process that leads to genetic diversity as the chromosomes end up in  either of the daughter cells. 
  • The nonkinetochore microtubules lengthen and pushes the centrioles farther apart. The cell is elongated and it prepares for division at the center. 

Telophase I

  • The first phase of meiotic division ends when the chromosomes arrive at the poles.
  • The daughter cells now have half the number of chromosomes, the chromosomes consists of a pair of chromatids. 
  • The microtubules of the spindle network disappear and nuclear membrane surrounds each haploid set.
  • The chromosomes uncoil and return back to the chromatin stage. 
  • The process of cytokinesis occurs where, the cell membrane in the animals cells is pinched off, and in plant cells there is formation of the cell wall in between the daughter cells.
  • This completes the creation of two daughter cells. The sister chromatids remain attached during the telophase I stage. 

Meiosis II 

It is the second part of the meiotic process and is also known as equational division. Meiosis II is similar to mitosis. The genetical results are fundamentally different from that of mitosis. The end result of mitosis II is the production of four haploid cells from two haploid cells produced in meiosis I, each cell consisting of 23 chromosomes in humans and the chromosomes consists of two sister chromatids.
In meiosis II there are four steps  
  • Prophase II
  • Metaphase II 
  • Anaphase II
  • Telophase II. 
Meiosis 2

Prophase II

  • During this stage there is disappearance of the nucleoli and the nuclear envelope, also there is shortening and thickening of the chromatids. 
  • The centrioles move to the polar region and the spindle fibers are arranged for the second meiotic division. 

Metaphase II 

  • During this stage the centromeres that contain two kinetochores attach to the spindle fibers at each pole from the centrioles. 
  • The equatorial plate formed here is rotated by 90 degrees, compared to meiosis I and is perpendicular to the previous metaphase plate. 

Anaphase II 

  • The metaphse II is followed by the anaphase II, in the anaphase II stage the centromeres are cleaved, this allows the microtubules attached to the kinetochores to pull the sister chromatids apart. 
  • The sister chromosomes move towards the opposing poles.   

Telophase II 

  • The meiosis II process ends at this stage, this stage is similar to the telophase I. 
  • In this phase there is uncoiling and lengthening of the chromosomes and disappearance of the spindle. There is also reformation of nuclear envelope. 
  • Cleavage or cell wall forms which eventually produces a total of four daughter cells, each cell having its own haploid set of chromosomes. 

Mitosis vs Meiosis

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Difference between Mitosis and Meiosis


                             Meiosis                            Mitosis
 End result   Normally there are four cells, each cell has half the
number of chromosomes as the parent cell. 
 Two cells, each cells has the same number of
chromosomes as that of the parent.
 Function   Sexual reproduction , production of gametes (sex cells).  Cellular reproduction, growth, repair wear and tear of cells,
 sexual reproduction. 
 Where it occurs?  Animals, fungi, plants, and some protists.  Occurs in all eukaryotic organisms.
 Stages   Steps in the process Prophase I, Metaphse I, Anaphase I,
 Telophase I, Prophase II, Metaphase II, Anaphase II, Telophase II.
 Prophase, Metaphase, Anaphase, Telophase.
 Genetical composition  Not similar to parents  Usually similar to parents. 
 Crossing over process  Occurs in Prophase I  Sometimes
 Pairing of homologous chromosomes  Yes   No
 Cytokinesis  Occurs in Telophase I and Telophase II  Occurs in Telophase
 Splitting of chromosomes  Does not occur in Anaphase I; occurs in
Anaphase II
 Occurs in Anaphase

Pictorial Comparison of Meiosis and Mitosis

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