How does a cell division process work in humans (Mitosis and Meiosis)?

         A cell is the building block of a living thing. Every living thing is made up of tiny little cells. Especially humans have a very large amount of different types of cells like skin cells, muscle cells, sperm cells, egg cells and other cells. All these cells have the same basic parts like cell membrane, cytoplasm, endoplasmic reticulum, mitochondria, nuclear membrane, nucleus and more. Here the nucleus is considered as the main part of the cell. Inside the nucleus, there is a thread-like structure called chromatin which is made up of DNA (Deoxyribo Nucleic Acid) wounded around proteins called histones. These chromatins are essential for our life to sustain. The DNA in the nucleus decides the way our life should evolve. The DNA stores all the information about our body and the operations that need to be performed by each cell.

Credit(Animal cell: Mostly same as Human cell):By OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30131195

                                             

       Every type of cell in our body divides and duplicates itself several times in its life. This process of division is called cell division. In order to grow and heal our wounds and for producing offspring, cell division must occur in our body. Cell division is the process of producing a new cell from an existing cell. To accomplish this task a cell must undergo Mitosis or Meiosis.

Division of Cell

   Both Mitosis and Meiosis are the process of cell division. But mitosis only takes place in somatic (non-sex) cells like skin cells, blood cells, nerve cells etc., while meiosis takes place only in gamete (sex) cells like spermatocytes and oocytes. Let us discuss the process of both mitosis and meiosis in detail.

MITOSIS:

   Mitosis is the most common type of cell division that occurs in our day-to-day life. To get considerable growth and to heal our wounds when we get hurt, mitosis must occur. Our cells divide daily, by mitosis, to take place of the inactive and dead cells. The main goal of mitosis is to produce an exact single copy of the existing cell. The process of mitosis is a small part of the life cycle of a cell. A cell that has to be replicated (duplicated) by mitosis has 46 chromatins inside its nucleus. Among these 46 chromatins, any two chromatins will be identical to each other having different characteristics. Thus 46 chromatins can be made as 23 pairs of chromatins. These identical chromatins are homologous which means in one pair of chromatin, one will be from the father and one will be from the mother. Such types of cells are called diploid cells (2N) where the 2N means, a single pair of chromatin made from two persons. Both the chromatins from the father and mother have the same number of genes and carry the same genes in the same location but can have different alleles. Alleles are the property of the genes that will be exhibited in the corresponding organ. For example, if the chromatin from the father has a recessive (not powerful) gene for blue-eye and the chromatin from the mother has a dominant (powerful) gene for red-eye, we would get red-eye, as we have our chromatin of the mother as dominant. So genes are mentioning the organs, while alleles are representing the properties of the organs.

Diploid Cell

      DNA molecules usually exist in the chromatin form, inside the nucleus. But when the cell is to be divided, the chromatin condenses into a chromosome and then goes into division. A chromosome is nothing but a fully condensed form of chromatin. So for example, if 46 chromatins condense then there will be 46 chromosomes. The below diagram shows the construction of the chromosome,

Credit (DNA):By OpenStax - https://cnx.org/contents/FPtK1zmh@8.25:fEI3C8Ot@10/Preface, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=30131204

Interphase Cycle

    The G0 phase makes a particular cell, to get not divided and arrests it. That particular cell remains undifferentiated for a long time but in need, it kicks into action and gets divided. In the G1 cycle, all the contents in the cell are duplicated like mitochondria, Golgi apparatus etc., except the chromatin which is present inside the nucleus. In the S phase, the chromatin present inside the nucleus is duplicated by the process called DNA replication. A brief explanation of the DNA replication is provided in another post; please refer to it,

https://sciencetopic03.blogspot.com/2021/11/how-dna-replication-works-during-cell.html

Through the S phase, a new chromatin is made to form, which is exactly the same as the existing chromatin. After the DNA replication, the number of chromatins in the nucleus will be 92 chromatins which is double the 46 chromatins that are already present in the nucleus. Next in the G2 phase, necessary enzymes are produced and the replicated DNA is double-checked for any errors. If any mismatching occurs in the new DNA, it is corrected in the G2 phase.

    

DNA Replication

PROPHASE

    After the G2 phase, the cell enters into mitosis. In mitosis also there are various stages. The first stage that takes place in mitosis is the prophase, in which the nucleus membrane disappears and all the chromatin condenses to a pair of chromatids (1N2C, a single chromosome), which forms in the order of 46 pairs of chromatids or 46 chromosomes. A chromatid is said to be one, when a single chromatin condenses to a single chromosome and it is replicated. At the point of replication that single chromosome will be called a pair of chromatids due to the replication process. And those two chromatids will be together called a chromosome. So in each chromosome, a single chromatid is an exact copy (formed during the DNA replication) of another chromatid in the pair. But before replication, each single chromatid will be called as a chromosome. Now, if a single condensed chromatin also called a chromosome, does not replicate and attaches with another chromosome also called a homologous chromosome, then each strand will be separately called a chromosome and together will be called a pair of chromosomes.  So during the mitotic process, 23 pairs of chromatin(2N2C) which is present before the DNA replication becomes 46 chromosomes or 46 pairs of chromatids after replication(2N4C).

Difference between chromosome and chromatid

     Now let’s see what the alphanumeric numbers 1N2C, 2N2C, and 2N4C mean. For example, let’s take a single diploid chromosome which is said to be 2N2C also known as diploid in number. This diploid chromosome consists of two chromosomes attached together. Here one chromosome is from the mother and one from the father, which is also called a homologous chromosome. It is said as 2N because the chromosome is made from two persons through whom there are two chromosomes and those two act as separate chromatids. If there is one chromosome, which will be from one person, and has its own chromatid, it is said as the haploid chromosome (1N1C). If the diploid chromosome is replicated once (i.e., the father is replicated once and the mother is replicated once) it will be said as 2N 4C, because there is a copy of the same chromosome, which means there are totally four chromatids (4C). If it is replicated for another time it will be said as 2N 6C. The C value represents the number of chromatids whereas the N value gives the number of types of chromosomes. 

Difference between chromosome and chromatid

 

    The next part that takes place is the metaphase, in which the 46 chromosomes or 46 pairs of chromatids (2N4C) arrange themselves in a straight line at the centre of the cell. Then two centrioles originate at right angles to the arrangement of the chromosomes and spread their web, which sticks to the centromere (centre part) of the chromosome.

 

METAPHASE: Centrioles appear and spread their web and attaches to the centromere.

       At next during Anaphase, the spindle pulls one part of 46 chromatids to one side and another part of 46 chromatids to another side. Now as it is separated, the chromatid will be now called as a chromosome. Next during Telophase, the tightly condensed chromosome starts to unwind to form chromatin and two nuclei start to form. Now each cell will get 46chromosomes or 23 pairs of chromosomes (2N2C).

 

ANAPHASE: Each one of the chromatids is pulled at each side.

TELOPHASE: Each of the separated sides will be surrounded by a separate nuclear membrane.

     The last part in the life cycle of a cell is the cytokinesis, in which the two nuclei that are formed, start to separate completely and two cells are produced.  Thus the goal of mitotic cell division is achieved, in which a single diploid cell consisting of 46 chromatins or 23 pairs of chromatins (2N2C) replicates into two diploid cells consisting of 46 chromatins or 23 pairs of chromatins (2N2C). This is how we grow and heal our wounds during our life cycle.

CYTOKINESIS: Each of the separated cells becomes a new cell thereby each cell has 23 pairs of chromosomes.

CYTOKINESIS: Now the chromosomes get uncondensed and become chromatin. Both cells will be identical to each other.

MEIOSIS:

Diploid Cell

    The process of cell division in meiosis is much more similar to mitosis. Similar to mitosis the cell which has to be replicated through meiosis also has 23 pairs of chromatins (2N2C) inside its nucleus. But at the end of replication in the meiotic cell division, there will be in result of four haploid cells instead of two diploid cells like mitosis.  Haploid cells (n) are the types of cells that have the chromosome number half to the number of diploid cells, which means if the diploid cells have 23 pairs of chromatins then the haploid cell will have only 23 chromatins at the end, but both the types of cells originates the replication process by the cell which has 23 pairs of chromatins. Thus the main motive of the meiosis process is to produce four haploid cells from a single diploid cell. So in here also, the cell will enter into an interphase cycle in which cellular contents will be duplicated after which the cell will enter into meiosis. During duplication of the DNA, the 46 chromatins (2N2C) become 92 chromatins (2N4C) similar to the mitotic process. Here there are two stages in the meiosis process, namely Meiosis I and Meiosis II, to create a perfect four haploid cells. In Meiosis I a single diploid cell is divided into two and in meiosis II the divided cells again divide into two thereby resulting in four haploid cells.

 

DNA Replication

     Thus after duplication of the cellular contents, the cell enters into meiosis I which first takes through the prophase I stage, in which the nucleus disappears and the duplicated chromatins condense into chromosomes. Due to this, the 92 chromatins will be condensed into 92 chromatids or 46 chromosomes (2N4C) and get arranged in such a manner, that the homologous chromosomes come together.  

PROPHASE I: Chromosome form, and unlike arranging in a straight line like mitosis, two chromosomes come together and cross over.

     After crossing over occurs the next step happens is Metaphase I in which the centrioles appear at the poles and spread their web and attaches to the centromere of the chromosomes. Then during the next phase (Anaphase I), one chromosome is pulled to one side and the other one is pulled to the other side. 

METAPHASE I: Centrioles appear and spread their web and attaches to the centromere.

ANAPHASE I: Each of the chromosomes is pushed on each side, thereby each side gets 23 chromosomes, but has 46 chromatids (1N2C).

     After that, the spindle disappears and two nucleus starts to form through the process called Telophase I and Cytokinesis. This is the end of Meiosis I. During the end of Meiosis I, there will be two cells produced by a single cell, having 23 chromosomes or 23 pairs of chromatids (1N2C) on each.

 

TELOPHASE I: Each of the separated sides will be surrounded by a separate nuclear membrane.

CYTOKINESIS: Each of the separated sides becomes a new cell thereby each cell has 23 chromosomes, and 46 chromatids (1N2C).

CYTOKINESIS: Now the chromosomes get uncondensed and become chromatin. Each of the cells will be different to the other. 

    Now the cell enters into Meiosis II. In Meiosis II, the chromosomes present on each cell again lose their nucleus membrane through the process of Prophase II. Next during Metaphase II, the chromosomes align in the centre and the spindle fibres occur at the poles and spread their web to the centromere of the chromosomes.

PROPHASE II: Nuclear membrane of each cell disappears.

METAPHASE II: Again the chromosomes align and the spindles originate and attach to the centromere.

ANAPHASE II: Now in both cells, there will be 23 chromosomes with 23 chromatids (1N1C).

     Next during Anaphase II, the chromosomes at each nucleus, formed during meiosis I separate into two cells thereby forming 23 chromosomes (1N1C) on each side. These each 23 chromosomes become a new cell, through the process called Telophase II and Cytokinesis, which results in the formation of four new haploid cells. This is the end of Meiosis II. In males, each of the haploid cells develops into a sperm cell whereas in women, among these four cells only one becomes an egg cell, while the other becomes the polar body and does not get used up. Because in women all the cell that is got divided by the meiosis process does not get their full nutrients, during the division process and thus only one gets used up.

TELOPHASE II: Now in both cells, each side forms a nuclear membrane around the separated chromosome.

CYTOKINESIS: Now each cell gets 23 chromosomes with 23 chromatids resulting in a haploid cell (1N1C).

CYTOKINESIS: Now the chromosomes get uncondensed and become chromatin. Each of the four cells will be different to each other.

    This is how cell division occurs through the process of mitosis and meiosis. Please refer to this video regarding the operation of mitosis and meiosis.
https://youtu.be/zGVBAHAsjJM