Mitosis and Meiosis

Mitosis and Meiosis

Introduction

Mitosis and Meiosis are both biological processes that involve cell division; each process is preceded by a process of DNA replication (University of Leicester n.d.). Each of the above cell divisions has stages in which the cell undergoes and they are different in each case. Mitosis and Meiosis are of significance to organisms as it will be seen in this article.

The Significance of Mitosis to Organisms

Mitosis is a type of eukaryotic cell division that gives rise to two daughter cells that have the same genetic component as the parent cell (University of Leicester n.d.). This process takes an hour while dividing animal cells. Mitosis plays a major role in single-celled organisms as it creates the basis for asexual reproduction to occur (University of Leicester, n.d.). There are organisms that are known as diploid multicellular organisms whereby sexual reproduction takes place (Armstrong 2007). This reproduction involves the coming together of two haploid gametes and the result is a diploid zygote. Mitotic divisions between the zygote and daughter cells then take place and these processes are responsible for the ‘growth and development of the organism’ (University of Leicester, n.d., n.p.). Old cells die and they are replaced by new ones however, there must be continuous reproduction of new ones. There are wounds that occur in organisms and they also require healing.  Mitosis is, therefore, responsible for cell replacement, tumor formation, and wound healing in adult organisms (University of Leicester, n.d.).

The Process of Mitosis

As seen earlier, Mitosis is a form of cell division whereby each of the resulting daughter cells has a similar genetic component (Cregan 2007). During the process which is known as the S phase, ‘the chromosomes gets divided and the daughter cells a copy of every chromosome’ (University of Leicester n,d, n,p,).  The sister chromatids are further separated when the replicated chromosomes get placed in a mitotic apparatus. ‘Karyokinesis is the separation of the genetic material in a mitotic nuclear division and it is followed by cytokinesis which is the division of the cell cytoplasm in a cellular division’ (University of Leicester n.d., n.p.). Mitosis is divided into five phases and the first stage is the prophase.

Prophase

Prophase takes the largest part of mitosis, occupying half of the whole process. The very first separation that takes place is the breaking down of the nuclear membrane to form tiny vesicles and the nucleolus disintegrates (Armstrong 2007). The above is then followed by the duplication of the centrosome into two daughter centrosomes which in turn place themselves on the opposite edges of the cell. Mitotic spindle is constituted by spindle fibres which are formed from the production of microtubules by the daughter centrosomes (University of Leicester, nd). The replicated chromosome consists of two identical chromatids that are held together by a centromere (Cregan 2007).

The Significance of Meiosis to Organisms

Meiosis which is a procedure involving cell division leads to the production of haploid sex cells (gametes) from haploid cells (University of Leicester n.d.) This process happens in two phases that is Meiosis I (nuclear division) and Meiosis II (cellular division). The process of Meiosis is important to organisms in that it leads to genetic diversity (University of Leicester n.d.). There is an exchange of genetic materials that takes place between the homologous chromosomes when Meiosis I occurs (Armstrong 2007). Another form of genetic diversity is the alignment of maternal and paternal chromosomes which also occurs during Meiosis I. Random alignment of the sister chromatids takes place in Meiosis II and it is an important aspect of the organisms. As mentioned earlier, Meiosis occurs in two major phases which are then subdivided into five stages each. The stages in Meiosis I are; prophase I, metaphase I, anaphase I, telophase I, and cytokinesis. The stages in Meiosis II are; prophase II, metaphase II, anaphase II, telophase II, and cytokinesis (University of Leicester n.d.).

The separation of the pairs of homologous chromosomes takes place in Meiosis I. We take a look at the happenings of prophase I in males and the first occurrence is the condensation of the chromosomes (University of Leicester n.d.). Condensation is then followed by the crossing over of the homologous chromosomes pair which leads to recombinant chromosomes. Prophase I is further divided into five phases including leptotene which has been described above as the condensation of chromosomes, zygotene; which involves the association of homologous chromosomes (synapsis) which forms pairs of chromosomes (bivalents) and they consist of four chromatids (tetrads) (University of Leicester n.d.). Pachytene is the next stage whereby homologous chromosomes cross over each other to form chiasmata (Morgan 2007). The chromosomes begin to separate but there is an attachment between them created by chiasmata in the stage known as diplotene. The final stage of prophase I is known as Diakinesis whereby the chromosomes continue to detach from each other and the chiasmata migrate to the edges of the chromosomes (Morgan 2007)

Metaphase I

The second stage of meiosis I is metaphase I. After the homologous chromosomes undergo various processes in prophase I, they enter metaphase I where they are aligned as a double row along the metaphase plate. The alignment of the homologous chromosomes is random in consideration of the spindle apparatus. The genetic variation takes place during this process through random assortment. The paternal and the maternal chromosomes within the homologous pair are similar but not identical and this notion explains the latter genetic variation. There are certain numbers of alignments involved and in this case, it is 2ⁿ, where n represents the number of the chromosomes found in a haploid set (University of Leicester n.d.). There are 23 different chromosomes in a human being and so the number of probable combinations could be 2²³ (University of Leicester n.d.). Other Meiosis I follow suit to complete the whole process and Meiosis II sets in.

Differences between Mitosis and Meiosis

Although the two processes involve cell division, Mitosis and Meiosis are different in the way they take place, their significance to organisms to the final results of each of the processes. One of the major differences is that Mitosis happens in the somatic cells while Meiosis occurs in the germ cells. Mitosis takes place in both sexually and asexually reproducing organisms, unlike Meiosis which takes place in sexually reproducing organisms (Your Genome 2016). Mitosis has only one major stage whereas Meiosis takes place in two stages which are Meiosis I and Meiosis II. The cells involved in the process of Mitosis only divide once while in Meiosis there are two cell divisions taking place. There are stages of interphase that occur during cell division. In Mitosis, the interphase takes place prior to each cell division while the interphase only takes place after Meiosis I, with none occurring in Meiosis II (Morgan2007).

DNA replication precedes each of the two processes. In Mitosis it happens during interphase I whereas in Meiosis it does so in interphase I and none during interphase II. The mode of replication is also different in Mitosis whereby it happens once for the cell division and it also happens once in Meiosis but for the two cell divisions (Dashek & Harrison 2006). Mitosis is meant for cell replacement, tumor formation, and wound healing while Meiosis is responsible for genetic diversity. In Mitosis prophase takes a few hours to be completed while the same stage in Meiosis takes days to get completed. The process of synapsis does not occur in mitosis but the same takes place during the prophase stage of meiosis.

References

Armstrong V (2007), Mitosis and Meiosis: The Science of Cell Division. User-Friendly Resource

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Cregan, E (2007), All About Mitosis and Meiosis, Teacher Created Materials.

Dashek, WV & Harrison, M (2006), Plant Cell Biology, London: Science Publishers.

Hill Mc G. nd. Chapter 11: Sexual reproduction and Meiosis. Comparison of meiosis and mitosis. [Online]

Morgan, DO (2007), The Cell Cycle: Principles of Control, Sidney: New Science Press

University of Leicester. (n.d.). The cell cycle, mitosis and meiosis. [Online].

Your Genome (2016). Mitosis versus meiosis. [Online].