Module 4. Stages and Significance of Mitosis and Meiosis

Chromosomes play a vital role in cell division. They are in the nucleus of a cell; each chromosome contains the DNA which comprises the genes. Cell division is important in retaining or reducing the number of chromosomes. Having a correct number of chromosomes is critically important to all living organisms.

\n Mitosis and Meiosis

Mitosis and meiosis undergo series of stages to divide and produce daughter cells. Every stage in cell division has its own characteristics for the cell to reproduce. How is mitosis differing from meiosis? What are the characteristics of each stage?

1. Mitosis (apparent division) – is nuclear division; the process by which the nucleus divides to produce two new nuclei. Mitosis results in two daughter cells that are genetically identical to each other and to the parental cell from which they came.

Cytokinesis – is the division of the cytoplasm. Both mitosis and cytokinesis last for around one to two hours

Prophase – is the preparatory stage. During Prophase, centrioles move toward opposite sides of the nucleus.

• The initially indistinct chromosomes begin to condense into visible threads.
• Chromosomes first become visible during early Prophase as long, thin and intertwined filaments but by late Prophase, chromosomes are more compacted and can be clearly discerned as much shorter and rod-like structures.
• As the chromosomes become more distinct, the nucleoli also become more distinct. By the end of Prophase, the nucleoli become less distinct, often disappearing altogether.

Metaphase – is when chromosomes become arranged so that their centromeres become aligned in one place, halfway between the two spindle poles. The long axes of the chromosomes are 90 degrees to the spindle axis. The plane of alignment is called the metaphase plate.

Anaphase – is initiated by the separation of sister chromatids at their junction point at the centromere. The daughter chromosomes then move toward the poles.

Telophase b- is when daughter chromosomes complete their migration to the poles. The two sets of progeny chromosomes are assembled into two groups at opposite ends of the cell. The chromosomes uncoil and assume their extended form during Interphase. A nuclear membrane then forms around each chromosome group and the spindle microtubules disappear. Soon, the nucleolus reforms.

2. Meiosis – reduces the amount of genetic information. While mitosis in diploid cells produces daughter cells with a full diploid complement, meiosis produces haploid gametes or spores with only one set of chromosomes. During sexual reproduction, gametes combine in fertilization to reconstitute the diploid complement found in parental cells. The process involves two successive divisions of a diploid nucleus.

   First Meiotic Division

• The first meiotic division results in reducing the number of chromosomes (reduction division). In most cases, the division is accompanied by cytokinesis.

Prophase I – has been subdivided into five substages: leptonema, zygonema, pachynema, diplonema, and diakinesis.

• Leptonema – Replicated chromosomes have coiled and are already visible. The number of chromosomes present is the same as the number in the diploid cell.
• Zygonema – Homologous chromosomes begin to pair and twist around each other in a highly specific manner. The pairing is called synapsis. And because the pair consists of four chromatids, it is referred to as bivalent tetrad.
• Pachynema – Chromosomes become much shorter and thicker. A form of physical exchange between homologues takes place at specific regions. The process of physical exchange of a chromosome region is called crossing-over. Through the mechanism of crossing-over, the parts of the homologous chromosomes are recombined (genetic recombination).
• Diplonema – The two pairs of sister chromatids begin to separate from each other. It is at this point where crossing-over is shown to have taken place. The area of contact between two on-sister chromatids, called chiasma, become evident.
• Diakinesis – The four chromatids of each tetrad are even more condensed and the chiasma often terminalize or move down the chromatids to the ends. This delays the separation of homologous chromosomes.

In addition, the nucleoli disappear, and the nuclear membrane begins to break down.

Metaphase I – The spindle apparatus is completely formed, and the microtubules are attached to the centromere regions of the homologues. The synapsed tetrads are found aligned at the Metaphase plate (the equatorial plane of the cell) instead of only replicated chromosomes.

Anaphase I – Chromosomes in each tetrad separate and migrate toward the opposite poles. The sister chromatids (dyads) remain attached at their respective centromere regions.

Telophase I – The dyads complete their migration to the poles. New nuclear membranes may form. In most species, cytokinesis follows, producing two daughter cells. Each has a nucleus containing only one set of chromosomes (haploid level) in a replicated form.

2. Second Meiotic Division

• The events in the second meiotic division are quite similar to mitotic division. The difference lies, however, in the number of chromosomes that each daughter cell receives. While the original chromosome number is maintained in mitosis, the number is reduced to half in meiosis.

Prophase II – The dyads contract.

Metaphase II – The centromeres are directed to the equatorial plate and then divide.

Anaphase II – The sister chromatids (monads) move away from each other and migrate to the opposite poles of the spindle fiber.

Telophase II – The monads are at the poles, forming two groups of chromosomes. A nuclear membrane forms around each set of chromosomes and cytokinesis follows. The chromosomes uncoil and extend.

Cytokinesis – The Telophase stage of mitosis is accompanied by cytokinesis. The two nuclei are compartmentalized into separate daughter cells and complete the mitotic cell division process. In animal cells, cytokinesis occurs by the formation of a constriction in the middle of the cell until two daughter cells are formed. The constriction is often called cleavage, or cell furrow. However, in most plant cells, this constriction is not evident. Instead, a new cell membrane and cell wall are assembled between the two nuclei to form a cell plate. Each side of the cell plate is coated with a cell wall that eventually forms the two progeny cells.

Disorders and Diseases

• Incorrect DNA copy (e.g., Cancer)
• Chromosomes are attached to string-like spindles and begin to move to the middle of the cell (e.g., Down Syndrome, Alzheimer’s, and Leukemia)

Other chromosome abnormalities:

• Arise from errors in meiosis, usually Meiosis I;

• Occur more often during egg formation (90% of the time) than during sperm formation;

• Become more frequent as a woman ages.

• Aneuploidy – is the gain or loss of whole chromosomes. It is the most common chromosome abnormality. It is caused by non-disjunction, the failure of chromosomes to correctly separate:

• homologues during Meiosis I or

• sister chromatids during Meiosis II

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The importance of Cell Cycle is very evident that the growth and sustainability of multicellular organisms depend on this process. Cells that are damaged and lost will be replenished when cells divide. Errors in mitosis lead to an incorrect copy of the DNA which may produce deadly functional consequences depending on the error. The positive correlation with the malfunction of these processes to the onset of major diseases such as cancer, stroke, atherosclerosis, inflammation, and some neurodegenerative disorders is increasingly proven in various studies.

\n Significance of Mitosis and Meiosis

Mitosis and Meiosis result in eukaryotic cell division. The primary difference between these divisions is the differing goals of each process. The goal of mitosis is to produce two daughter cells that are genetically identical to the parent cell. Mitosis happens when you grow. You want all your new cells to have the same DNA as the previous cells. The goal of meiosis is to produce sperm or eggs, also known as gametes. The resulting gametes are not genetically identical to the parent cell. Gametes are haploid cells, with only half the DNA present in the diploid parent cell. This is necessary so that when a sperm and an egg combine at fertilization, the resulting zygote has the correct amount of DNA – not twice as much as the parents. The zygote then begins to divide through mitosis.

Significance of Mitosis for Sexual Reproduction

Mitosis is important for sexual reproduction indirectly. It allows the sexually reproducing organism to grow and develop from a single cell into a sexually mature individual. This allows organisms to continue to reproduce through the generations.

Significance of Meiosis and Chromosomes

Number chromosomes are the cell’s way of neatly arranging long strands of DNA. Non-sex cells have two sets of chromosomes, one set from each parent. Meiosis makes sex cells with only one set of chromosomes. For example, human cells have 46 chromosomes, with the exception of sperm and eggs, which contain only 23 chromosomes each. When a sperm cell fertilizes an egg, the 23 chromosomes from each sex cell combine to make a zygote, a new cell with 46 chromosomes. The zygote is the first cell in a new individual.

Significance of Meiosis for Diversity

One of the benefits of sexual reproduction is the diversity it produces within a population. That variety is a direct product of meiosis. Every sex cell made from meiosis has a unique combination of chromosomes. This means that no two sperm or egg cells are genetically identical. Every fertilization event produces new combinations of traits. This is why siblings share DNA with parents and each other, but are not identical to one another.

Aberrations that Alter Chromosome Number

During cell division, chromosomes sometimes disappear. This occurs when there is some aberration in the centromere, and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell. Consequently, the lost chromosome never properly groups with others into a new nuclear envelope, and it is left in the cytoplasm, where it will not be transcribed. Also, chromosomes don’t always separate equally into daughter cells. This sometimes happens in mitosis, when sister chromatids fail to separate during Anaphase. One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during Anaphase I. This also results in daughter cells with difficult numbers of chromosomes.

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