Cell Reproduction

Cell Reproduction

The reproduction of cells is described as cell division, a process that occurs in a cyclic manner, hence the term cell cycle. In a typical person’s cell cycle, the stage at which the cell divides is termed the mitotic phase and is divided into mitosis and cytokinesis (Simon, Dickey, Reece, and Campbell, 2019). Mitosis involves the duplication and division of the nucleus and all its contents into two identical daughter nuclei. In cytokinesis, the cytoplasm and the organelles divide into two. Finally, mitosis and cytokinesis produce two daughter cells that are genetically identical to each other.

Mitosis follows a process of four main stages: prophase, metaphase, anaphase, and telophase. Prophase is preceded by the interphase stage, where new molecules and organelles are produced and, in the nucleus, the chromosomes are duplicated but remain tightly packed in chromatin fibers. In prophase, the chromatin fibers begin to coil such that the chromosomes become thick. Also, the formation of the mitotic spindle is initiated. Late in this stage, the nuclear envelope breaks while the mitotic spindle attaches to the chromosomes’ centromeres, aligning the chromosomes at the center of the cell. In metaphase, the centromeres of the chromosomes align themselves along the two poles of the completely formed mitotic spindle. Anaphase follows when each chromosome splits into two chromatids. These chromatids move towards the contrasting poles of the cell as the mitotic spindle shortens. Telophase follows when the two chromosome sets meet at the opposite poles of the cell. Finally, the mitotic spindle disappears entirely, the chromosomes unwind, and a new nuclear envelope is made, ending the process of mitosis. Cytokinesis then proceeds to form two diploid daughter cells, which are genetically identical.

Sexual reproduction in humans relies on the process of meiosis and fertilization. Therefore, the reproductive development of an unborn baby is majorly dependent on meiosis. Meiosis is the type of cell division whereby four haploid cells are produced. It occurs in two phases: meiosis one, where the homologous chromosomes separate, and meiosis two, where the sister chromatids segregate. In meiosis one, prophase one is preceded by interphase, just like in mitosis. Here, homologous chromosomes pair up due to specific protein complexes leading to the formation of four chromatids (Simon, Dickey, Reece, and Campbell, 2019). Also, the crossing over of the chromatids occurs in this phase. Subsequently, the crossing over leads to the exchange of genetic content between homologous chromosomes. The spindle then begins to form, and the pairs of homologous chromosomes move towards the midpoint of the cell. Next, the pairs are arranged at the center of the cell in the metaphase one stage. The homologous are arranged in such a way that the spindle attached to one chromosome comes from one pole, and the spindle attached to the other chromosome is from the opposite pole. This alignment helps the homologous chromosomes move towards opposite poles of the cell. Anaphase one then follows, and here the homologous pairs split. The chromosomes then move to the poles of the cell in telophase one.

Following is the transition to meiosis two, beginning with prophase two, whereby spindles form, and the chromosome becomes aligned at the center of the cell. Metaphase two comes about as the chromosomes are arranged like in mitosis, and the spindle tracks attached to the sister chromatids of each chromosome come from opposite poles. Following is anaphase two. This is where the sister chromatids then segregate (Kindfield, 1994), and each pair of the chromatids moves towards the pole of the cell. Lastly, telophase two takes place. The primary activities involved at this point are the formation of nuclei and cytokinesis that occur simultaneously to form four haploid daughter cells, completing the process of meiosis.

Advantages and Disadvantages of Mitosis and Meiosis

One of the advantages of mitosis is that cells are able to make identical copies of themselves, thus replacing injured, damaged, or dead cells. Another one is that it requires minimal energy and resources, unlike in meiosis, where a lot of energy and resources are needed for reproduction (Crow, 1994). In addition, mates are not required since self-reproduction is possible; on the other hand, for meiosis, species need to find mates, which is a disadvantage in a scenario of a sparse population, for instance, or in the case where there is limited mobility. The rapid growth of cells is observed in mitosis since little energy and resources are required and also due to the fact that cells can self-reproduce.

Further, meiosis has its advantages. Firstly, it involves great genetic recombination, leading to a population with a genetic makeup better than the parents. This implies that the offspring would have a better chance of survival as it quickly adapts to environmental changes, unlike mitosis, where the daughter cells identical to the parent cells have few genetic variations. Secondly, meiosis makes it possible to eliminate unfavorable mutations due to the fact that gene shuffling occurs, and the resulting genetic recombination allows the harmful mutations to be quickly eliminated from the gene pool (Simon, Dickey, Reece and Campbell, 2019).

When normal cells like skin cells and liver cells incur an injury, it is upon mitosis to occur and replace these injured cells. Skin cells are diploid cells since they are somatic, meaning that they enclose pairs of chromosomes. When the body is experiencing difficulties in repairing skin cells resulting from a cut, this implies that these diploid cells are not dividing enough to make new cells to repair the cut. For a pregnant woman, cells that are not effectively undergoing division might lead to complications with the pregnancy since she might lose a lot of blood or get infections through the wound.

Meiosis is important in sexual reproduction because, during gamete formation, the chromosome number in cells is divided into two so as to make the gametes haploid. When two haploid gametes fuse, the resulting zygote is diploid; thus, the normal chromosome number in the cells is restored (Kleckner, 1996). However, when accidents occur during meiosis, the chromosome number in the daughter cells becomes altered. This leads to the formation of cells with an abnormal number of chromosomes. A common example of an accident is nondisjunction, which happens when homologous chromosomes fail to separate during meiosis one or when the sister chromatids fail to segregate in meiosis two. Therefore, when the child’s reproduction development malfunctions, like in the case of nondisjunction, cells with missing or extra chromosomes are produced. Abnormal chromosome numbers lead to medical disorders such as Down Syndrome.

In conclusion, cell reproduction involves distinctive stages of mitosis and meiosis types of cell division. These two types of cell division are, in turn, involved in the sexual reproduction of organisms. Mitosis is essential for growth and maintenance, while meiosis is important in accomplishing sexual reproduction.

References

Crow, J. (1994). Advantages of sexual reproduction. Developmental Genetics, 15(3), pp.205-213.

Kindfield, A. (1994). Understanding a basic biological process: Expert and novice models of meiosis. Science Education, 78(3), pp.255-283.

Kleckner, N. (1996). Meiosis: How could it work? Proceedings of the National Academy of Sciences, 93(16), pp.8167-8174.

McIntosh and Koonce, M. (1989). Mitosis. Science, 246(4930), pp.622-628.

Simon, E., Dickey, J., Reece, J., and Campbell, N. (2019). Campbell essential biology.