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An In-Depth Look at Mitosis and Meiosis

March 03, 2012 | Comments: 0 | Views: 229

Mitosis and Meiosis are both cellular processes in nature which result in the division of cells and formation of new life. Mitosis involves the division of a cell into two identical cells, each containing the exact genetic makeup (genome) in their chromosomes as the original cell, but go through differentiation during a eukaryote organism's development in order to accommodate for the different cells that the body requires. Mitosis takes place in both single-celled organisms, being the method in which they are able to reproduce, and also in eukaryote somatic (body) cells, allowing them to proliferate and repair damaged tissue. Meiosis on the other hand only occurs in the sex cells of organisms and is the method by which sex cells produce gametes for sexual reproduction.

These sex cells are called germ cells, which when divided are termed gametes. Gametes are sperm in males and ova in females. Sperm and ova are haploid (1n), meaning that they contain one set of 23 chromosomes. This is because they divided from a germ cell containing 46 chromosomes. All cells in the human body have 46 chromosomes, except for gametes. When the sperm and ova meet during sexual reproduction, they form a diploid (2n) cell containing two sets of 23 chromosomes, 46 in total (one set having came from the mother and the other from the father). This diploid cell is called a zygote, which divides through mitosis into multiple copies of somatic cells and germ cells that will undergo meiosis for sexual reproduction.


So what are the steps for mitosis and meiosis? Let us begin with mitosis. Take note that mitosis in its simplest definition means that a diploid cell will divide into two diploid cells and a haploid cell will divide into two haploid cells.


Mitosis begins with interphase, the stage where the cell grows and stores energy in order to proceed with its division. The cell divides its chromatin, chromosomes in the form of an unwound mass of genetic information composed of DNA coiled around histone proteins, producing two sister chromatids from each chromatin. The centrosome, the organelle responsible for mitotic spindle formation, also divides in this stage.


The cell moves into prophase, where the chromatids (still in an unwound from) condense into chromosomes. Chromosomes are either made up of two pairs of sister chromatids or a single strand of conjoined sister chromatids. At this time, the "x" structure of two pairs of sister chromatids is counted as a whole chromosome. The region of attachment between the sister chromatids is known as the centromere and is made possible due to the kinetochore protein. Centromeres are also useful how many chromosomes there are by counting the number of centromeres. The centrosomes migrate to the opposite poles of the cell and release spindle fibers that attach to the kinetechores located in the centromeres. The disassembling of the nuclear envelope is sometimes referred to a whole other stage called prometaphase.


Next, in metaphase, the spindle fibers align the chromosomes in the middle of the cell.


Once aligned, the chromosomes are pulled apart by the spindle fibers and towards the centrosomes, making each single strand of sister chromatid pairs a chromosome.


When the chromosomes reach the centrosomes, the spindle fibers break down, releasing the chromosomes. Nuclei reform on each opposite side and around the chromosomes. The chromosomes uncoil and revert back into chromatin. The center of the cell then contracts and the cell splits in two, a process known as cytokinesis. The end result is two cells with two nuclei harboring the same set and number of chromosomes as the original cell.


Now that we have covered mitosis, we can move onto meiosis. Mitosis and meiosis have some similar phases but they differ in two major distinct ways. One is that meiosis involves a total of two divisions (meiosis 1 and meiosis 2) as opposed to one division in mitosis. The second is that meiosis produces cells that have half the amount of chromosomes as the parent cell, this being the production of gametes. Take note that meiosis does not occur during sexual reproduction, but is the process within an organism that prepares it for sexual reproduction by shuffling the genes it inherited from its mom and dad in its gametes.

First Division: Meiosis 1

Prophase 1

Meiosis undergoes the same interphase process and proceeds to prophase 1. Just like in mitosis, the chromatin become chromosomes consisting of two sister chromatid pairs linked to each other at the centromere. The nuclear envelope also breaks down and the centrosomes move towards opposite sides of the cell, where they begin forming the spindle fibers. The difference is that homologous chromosomes (chromosomes from the mom and dad that have related genetic information) pair up next to each other and cross-over. Crossing-over involves homologous chromosomes to break off and exchange parts of their chromatids that code for similar traits, leading to genetic variation in species.

Metaphase 1

The homologous chromosomes are lined up in pairs at the middle of the cell. Also, they are aligned in a random order called independent assortment to add more variation.

Anaphase 1

The spindle fibers pull in the entire homologous chromosomes away from each other and towards the centrosomes.

Telophase 1

Telophase 1 is very similar as telophase in mitosis. Spindle fibers disintegrate, the nucleus develops around the chromosomes, and the cell divides in two by cytokinesis. The difference here is that each cell contains half the starting number of chromosomes, 23 out of 46, and these cells are now considered haploid. The second division, meiosis 2, then takes place in both of these newly formed haploid cells.

Second Division: Meiosis 2

There is no further chromosome replication in meiosis 2, nor crossing-over. The two haploid cells go through all the steps in meiosis 1 (they are referred to as prophase 2, metaphase 2, anaphase 2, etc) but this time both of the cells' 23 double-stranded chromosomes pairs are pulled away from each other, resulting in 23 separate single-stranded chromosomes in four haploid gamete cells (four 1n cells). Males end up with four sperm cells but females generate only one mature egg out of the three because and egg requires a lot of cytoplasm to feed the zygote, which cannot be divided equally in all four eggs. This leads only one egg to get the concentrated large amount required to sustain the zygote. The three remaining eggs with minimal amount of cytoplasm, polar bodies, do not mature and break down within the body. That concludes our look at mitosis and meiosis.

Salvador Barajas

Source: EzineArticles
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