What is Meiosis?
A cell can be rightly called as the base structure of each living species. There are different kinds of cells which make a living body and these cells do have their own functions.
Chromosomes are those tightly warped DNA which are present in the nucleus of every cell in a living body. The total numbers of chromosomes present in human body are 23 pairs of chromosomes.
An Eukaryotic process wherein In this process a cell is divided into two stages which eventually results into reduction of half chromosomes in comparison to the original amount.
In Meiosis a cell is divided into twice the amount in order to create four daughter cells which only have half of the amount of Chromosomes than the original cell. This process is responsible for producing sex cells in humans which are also known as Gametes.
These sex cells are also known as Sperm in males and Eggs in females. It is an important function for sexual reproduction which ensures that the offspring gets the similar amount of chromosome as the parent.
This process is found in both animals and human beings.
Meiosis v/s Mitosis:
A cell is divided and is reproduced in two ways. Each cell is divided into two in the Eukaryotic process which are meiosis and mitosis cells. Just before the process of meiosis the DNA is simulated this is also known as mitosis.
Mitosis was discovered by Walter Flaming whereas Meiosis was discovered by Oscar Hertwig. Mitosis takes part in healing and repair whereas Meiosis takes part in the formation of Gametes and maintenance of chromosome number.
In this process a cell division takes place where a cell is divided into two daughter cells which are identical in nature.
Mitosis results into two identical daughter cells whereas Meiosis results in four
Mitosis is a process of asexual reproduction in which the cells divides into producing a replica with the same number of chromosomes in each diploid cell.
Meiosis is a type of cellular reproduction in which the numbers of chromosomes are reduced by half by the separation of Homologous chromosome which produce two Haploid cells.
The major difference between Mitosis and Meiosis is the creation of cells. Mitosis is responsible for resulting into two identical daughter cells. On the other hand, Meiosis is responsible for creating four sex cells.
Difference between Mitosis and Meiosis are as follows:
- Mitosis is a type of asexual reproduction whereas meioses are a sexual type of reproduction.
- Meiosis is genetically different whereas Mitosis is genetically similar.
- In mitosis the number of division is one whereas in Meiosis the number of division is two.
- In mitosis number of daughter cells produces two diploid cells whereas in Meiosis the number of daughter cells produced is four haploid cells.
- In mitosis the number of chromosome remains same whereas in meiosis they are reduced by half.
- Mitosis takes place in somatic cells whereas meiosis takes place in germ cells.
Stages of Meiosis:
In meiosis cell division occurs twice. One stating cell can produce four gametes: eggs or sperm. The cell division in this phase goes through four stages: Prophase, Anaphase, Metaphase and Telophase.
Phases of Meiosis:
DNA is replicated as Mitosis before Meiosis. Meiosis consist two cell divisions: Meiosis I and Meiosis II. These phases further goes through four stages each. Let us briefly discuss on each phase of Meiosis.
Let us through some light on the phases of Meiosis I here briefly:
This is the first step of Meiosis I. It is similar to Prophase in Mitosis in which the chromosome compact and move towards the centre of the cell.
In this the nuclear envelope is festered which originates microtubules from the centrioles on both side of the cell to attach kinetochores in the centomeres of each chromosomes.
Each set of DNA creates genetic variation with the help of exchanging bits with each other and combining again after doing so. This very process is known as recombination or crossing over.
This process occurs between two chromosomes and is possible only between small areas of those chromosomes.
The chromosomes in Mitosis pair with their homologous partner which is different from Meiosis. It can be seen in red and blue chromosome.
At the end the nuclear membrane divides into several parts in the Prophase I.
In this the homologous pair of chromosomes line up on the Metaphase plate near the centre of the cell. The step is referred as reductional division.
The homologous chromosomes separate the two alleles from each gene which have different alleges. During this process there is no order found on the metaphase plate identifying whether the chromosomes present there is maternal or paternal and on which side.
Thus this process results into the law of segregation through which it can be found out the each allele present during the process is to be padded on to the offspring.
Law of segregation can be defined as the phenomena of segregation or separation of each trait of two alleles at the time of gametes and zygotes formation after which alleles combine with other alleles randomly. This phenomenon ensures to pass on either allele to the offspring with two copies of each gene by the parent. It leads to the chances of ending each allele into the Zygote.
This phase is more similar to the Anaphase of Mitosis. In this the chromosomes are pulled towards the centrioles at each side of the cell.
These cells are separated by the meiotic spindle that is responsible for pulling one chromosome to the one side of a cell and the other chromosome to the other parallel side of the cell.
Thee centromes holding the sister chromatides together doesn’t dissolve in Anaphase I of Meiosis. It means the homologous chromosome are separated but the sister chromates aren’t.
This very process in this stage is different to the process of Meiosis II and Mitosis I.
In Telophase I the chromosomes are entirely pulled apart and a new nuclear envelope is formed. With the help of Citokinesis the plasma membrane is separated due to which two new cells are produced.
In this stage the chromosomes fully separates each other to the parallel side f the cell. After this the chromosomes are fully prepared to align together.
Two nuclei are created by the membrane formed around the chromosomes. With the help if the Cytokinesis process these cells are aligned in order to create two daughter cells which contains right amount of chromosomes in a nucleus.
Outcome of Meiosis I:
By the end of this process 2 new cells which have haploid in their DNA along with its two copies are formed. The genes here have 2 alleles in it but these genes share sister chromatid copies due to which these cells are known as Haploid cells.
After the end of the cell division process in Meiosis I these cells takes a rest of short duration before entering the second stage of Meiosis II.
Meiosis II is a phase which is much more like Meiosis I. In this process the amount of chromosomes on the cells are not changed. This eventually leads to creating daughter cells.
Meiosis II also has four phases just like Meiosis I. They are: Prophase II, Metaphase II, Anaphase II and Telophase II.
Let us throw some light on each phase briefly here:
This phase is much similar like Prophase I in Meiosis. In Prophase II the chromosome compact and move towards the centre of the cell.
In this the nuclear envelope is festered which originates microtubules from the centrioles on both side of the cell to attach kinetochores in the centomeres of each chromosomes just like Prophase I.
The chromosomes present in the two daughter cells created from the Meiosis I phase compresses together again which eventually creates an X-shaped structure. This structure can be seen under the microscope.
These daughter cells further dissolve the membrane present surrounding the nucleus which eventually results into releasing chromosomes present in the cells.
After this the centrioles in the cells duplicate and through which the meiotic spindle is formed again to go to the metaphase II stage.
After this the chromosomes starts heading in order to line up on the metaphase plate.
Just after the Prophase II stage the next stage is Metaphase II stage. In this stage, the chromosomes of the cells are aligned along with their centomeres. These chromosomes are aligned on the metaphase plate.
The centrioles present in the cells are placed on the parallel sides of each daughter cell. After this the sister chromatids attach to each Meiotic Spindle fibres present in the cells.
The centomeres here on these cells are attached with their protein cohesion. Each of these cells have a sister chromatid on each side when they are aligned on the metaphase plate for further process.
The third stage of Meiosis II is Anaphase II. In this stage the sister Chromatid which were aligned on the Metaphase plate are separated.
Due to the process that has taken place in the meiotic spindle the siser chromodites are again pulled to the parallel sides off the cells.
These chromatids after being separated are known as sister chromosomes. These sister chromosomes are further aligned to centrioles.
With this there is a final separation of the DNA this is why this process or division is also known as Equational Division.
The chromosomes now are individual chromosomes at this state due to the separation of the chromatids.
In this phase during the division of the cell each cell has equal number of chromosome as it was in at the time of the division. These cells do not have any copy.
The last stage of Meiosis II is Telophase II. Just like the Telophase I in Meiosis in this stage too the cells are further divided into two.
The chromosomes present at this stage are moved towards the parallel side of the cells present. After which the chromosomes are prepared fully to align together.
Two new nuclei are formed due to the formation of a membrane surrounding the chromosomes available in the cells.
After the completion of the Cytokinesis process in the meiosis phase creation of four granddaughter cells are done. These cells contain half the amount of chromosomes or haploid that was present originally.
The chromosomes present in these cells are present in the opposite end of each cell. New nuclear envelopes surrounding the chromosomes are formed here after the process of Cytokinesis division.
The cells formed at last are sperm and eggs. In males all the cells available are sperm and in females only one cell is egg while the other three are polar bodies.
Outcome of Meiosis II:
By the end of this phase 4 cells containing haploid are formed. These cells only have one copy of the genome.
The cells formed at this stage are fully functional and can be developed as Gametes. These cells can be developed as eggs and sperm. Eggs are developed in females and sperm in males.
Importance of Meiosis:
Meiosis is definitely an important phase which is important for reproduction process in every species. When it comes to generating sperm and egg cells in human body chromosomal segregation is essential.
And this separation of sister chromatids is done in the phases of Meiosis I and Meiosis II by extension and healthy embryos generation.
Missing or Extra chromosomes in gametes can be formed can be produced if the segregation of chromosomes fails completely.
If this phenomenon of missing chromosomes in gametes occurs then the embryo formed through it has no chance of survival. Though it doesn’t mean that other chromosomal abnormalities; apart from extra or missing chromosomes gametes; are not favorable for the embryo to survive.
Meiosis is highly responsible highly responsible for generating genetic diversity. This very process of generating genetic diversity is the essential factor for survival of species.
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