About mitosis and meiosis

Cells differentiate and replicate using two processes: meiosis and mitosis. Mitosis produces two equivalent daughter cells, while meiosis produces four sex cells (Brachet, 2014). The mechanisms of Mitosis and Meiosis are compared and contrasted in this article.

Mitosis

A mitosis is a form of eukaryotic cell division that produces two identical daughter cells that have the same genetic makeup as the parent cell. Mitosis is a continuous process that is classified into five phases: prophase, prometaphase, metaphase, anaphase, and telophase (Brachet, 2014).

In prophase, the nuclear membrane breaks down and results in the appearance of spindles and condensation of chromosomes. In prometaphase, the spindle fibers attach to chromosomes, and chromosome condensation continues. The metaphase stage is characterized by the alignment of chromosomes, while anaphase involves the division of centromeres, and the movement of sister chromatids to opposite poles of the cell. The telophase stage involves the reformation of the nuclear membrane, decondensation of chromosomes, and the disappearance of the spindle fibers. Finally, the cytoplasm divides through a cytokinesis process, and the parent cell becomes two daughter cells with similar or identical genetic formation (Brachet, 2014).

Meiosis

Meiosis refers to a form of eukaryotic cell division that results in gametes from diploid cells. The meiosis process occurs in the form of a replication of one DNA followed by two successive divisions, nuclear (Meiosis I) and cellular (Meiosis II). Meiosis, like in mitosis, gets preceded by a DNA replication process that changes each chromosome to two sister chromatids (Brachet, 2014).

Meiosis I

Meiosis I division involves the separation of the pairs of homologous chromosomes and results in the reduction of the cell from diploid to haploid. It has five stages, which include prophase I, prometaphase I, metaphase I, anaphase I, and Telophase I.

In prophase I stage, the homologous chromosomes pair up and exchange their DNA to produce recombinant chromosomes. Prometaphase I stage involves the formation of spindle apparatus and the attachment of spindle fibers by kinetochores (Brachet, 2014).

In metaphase I stage, the homologous chromosome pairs get arranged as a double row along the cell's metaphase plate. The paired chromosomes' arrangement in the context of the spindle apparatus' poles is random along the metaphase pole. That, therefore, forms the source of genetic variation through the process of random assortment. The maternal and paternal chromosomes in a homologous pair remain similar but not identical, and the possible number of arrangements is 2n, where n represents the number of chromosomes is a haploid set. The different number of chromosomes in human beings is 23, meaning a possible number of combinations is 223. Anaphase I stage is characterized by the separation of the homologous chromosomes in each bivalent to the cell's opposite poles. In telophase I stage, the chromosomes reach the cell's opposite ends, and the membrane reforms. Finally, cellular division occurs through a cytokinesis process to form two new cells. What follows is meiosis II, which is a reduction division (Brachet, 2014).

Meiosis II

This involves the separation of each chromosome into two separate chromatids and has four stages, which include prophase II, metaphase II, anaphase II, as well as anaphase II and telophase II. Prophase II involves the condensation of chromosomes, dissolution of the nuclear membrane, and the formation of spindle fibers. In metaphase II, spindle fibers get attached to chromosomes, and the chromosomes get aligned at the center of the cell. Anaphase II involves the division of centromeres, the movement of the sister chromatids to the cell's opposite ends, and shortening of the spindle fibers. In telophase II, chromosomes reach the cell's opposite ends, and the nuclear membrane forms before cell division occurs in a cytokinesis process (Brachet, 2014).

Therefore, meiosis generates variation or genetic diversity during meiosis I through the exchange of genetic material between the homologous chromosomes. Meiosis also creates variation through the random alignment of the paternal and maternal chromosomes in meiosis I, as well as the random alignment of the sister chromosomes in meiosis II.

Despite the differences between mitosis and meiosis, the two forms of cell division also have a broad range of similarities.

One of such similarities is that they both have diploid parent cell and involve five stages (Gustaffson, 2010). Additionally, both their metaphase stages involve the alignment of the individual chromosomes, while both their anaphase stages involve the separation of sister chromatids to opposite poles. Besides, they both end with cytokinesis (Gustaffson, 2010).

Mendel's Law of Independent Assortment

According to Mendel's law of independent assortment, when two or more characteristics or allele pairs get inherited, the individual hereditary factors separate independently during gamete formation, resulting in the independent transmission of traits to the offspring (Gustaffson, 2010).

For example, let us assume crossing two different pure-breeding pea plants. One has yellow and round seeds (YYRR), while the other has green and wrinkled seeds (yyrr). Since every parent plant is homozygous, Mendel's law of independent assortment says that the gametes resulting from the green, wrinkled plant are all ry, while those resulting from the yellow, round plant are all RY. That gives an end offspring that are all RrYy. Meaning, the allele that specifies the yellow seed color is dominant to that which specifies the greed seed color. Also, the allele that specifies round seed shape is dominant to that which specifies the wrinkled seed shape (Gustaffson, 2010). Mendel's law of independent assortment, therefore, reflects the variation resulting from meiosis.

References

Brachet, J. (2014). Meiosis and Mitosis. Elsevier Science.

Gustafsson, Å. (2010). A General Theory for The Interrelation of Meiosis and Mitosis. Hereditas, 25(1), 31-32. http://dx.doi.org/10.1111/j.1601-5223.1939.tb02681.x