CELL DIVISION – MITOSIS AND MEIOSIS

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Cell division is simply defined as the general way in which a microbial cell or a living organism (the parent cell in this case) reproduces itself in order to generate new offspring. It is the process in which parent or mother cells share their genetic materials (or DNA) with their progeny; and in some cases the offspring look like their parent cells while in other instances the progeny are not identical with their parents. Cell division is an important process in microbial cells because DNA replication only occurs in cells that are actively dividing.    

MITOSIS

Mitosis is the type of nuclear or cell division in which the mother cell divides into two identical daughter cells.  It occurs mainly in asexual type of reproduction. Mitosis is generally a sequence of events that occurs during the reproduction or cell division of eukaryotic cells that produces two genetically similar or identical nuclei that resemble their parent cells. Mitosis is characterized by several stages including interphase, anaphase, prophase, metaphase and telophase; all of which play critical role in the asexual division of eukaryotic cells. But the four phases of mitosis are prophase, metaphase, anaphase and telophase.

Interphase is the stage in which the eukaryotic cell is not experiencing any form of cell division. A eukaryotic cell is always at a stage of interphase before it enters mitosis for cell division to occur. Interphase is a stage of a eukaryotic cell between divisions; and the partitioning of the mother cell does not occur at this phase. DNA replication occurs at this stage of interphase. Interphase perhaps is usually the first stage that precedes cell division in a living cell; and after this stage of interphase, the first stage of mitosis – which is known as prophase begins.

Prophase is the first stage of mitosis; and it is the stage in which the chromosomes of the parent cell condense in order to separate into two sister chromatids (Figure 1). Under the microscope, the two sister chromatids are seen as individual cells; and each moves towards the equator of the cell. A cell first replicates its entire DNA every time it divides, and the chromosomes also divide during this time to form sister chromatids which eventually separates and becomes individual cells of their own. This stage of mitosis is generally known as prophase. Mitotic spindle is formed at this stage of prophase. The center of the chromosome is known as centromere; and it differentiates the two sister chromatids of the chromosome. Centromere is defined as the point of adhesion or attachment of two chromatids in a chromosome. It is formed during cell division in cells.

Figure 1.  Illustration of prophase. Prophase is the first phase of mitosis in which the chromatin becomes visible. During this stage of mitosis, there is usually a condensation of the complex of DNA and proteins (i.e. the chromatin) contained in the nucleus of the cell.

Metaphase: Metaphase is the stage of mitosis in which the chromosomes are arranged in the center of the mitotic spindle fiber (Figure 2). It is usually the second stage of mitosis. At this stage, the nuclear envelope of the nucleus breaks down and disappears. And a metaphase plate in which the chromosomes attaches to is formed at this stage. During metaphase, the chromosomes become visible and can pick up stains. The chromosomes reach their maximum degree of coiling and condensation; and they can be counted at this stage. Metaphase is the third phase of mitosis. During metaphase, the chromosomes of the cell align themselves in the middle of the cell, and this leads to the formation of a type of cellular structure that resembles a tug of war. In metaphase, the chromosomes of the cell are at their most highly coiled and condensed state; and the chromosomes are aligned at the metaphase plate.   

Figure 2. Illustration of metaphase.

Anaphase: Anaphase is the third stage of mitosis. It is the stage of mitosis in which each of the sister chromatids move in opposite direction towards the ends of the spindle pole as the centromere of each chromosome splits into two (Figure 3). It is the stage in mitosis or meiosis following metaphase in which the daughter chromosomes move away from each other to opposite ends of the cell. During anaphase the spindle apparatus pulls the two sister chromatids of each chromosome apart and drags them toward opposite poles of the cell as shown in Figure 3.

Figure 3. Illustration of anaphase.

Telophase: Telophase is the fourth stage of mitosis; and it is the stage of cell division after anaphase. At this stage, nuclear membrane or envelop assembles around each sister chromatids to form two new nuclei, with each having a new nucleolus (Figure 4). The chromatids become less visible at this stage since the nucleolus reappears and the sister chromatids become enveloped to from two new nuclei. And the separated chromatids are now known as chromosomes.

Figure 4. Illustration of telophase.

The cell can now replicate its DNA again during interphase and the mitosis cycle continues (Figure 5). During telophase, the sister chromatids arrive at the poles; and they start to uncoil and lose their morphology. Telophase is the process that separates the duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. Cell division becomes complete once this stage of telophase is reached. Telophase eventually passes into the next interphase as the nuclear envelopes and nucleoli of the cell reforms and the chromatin becomes diffused. Cytokinesis (cytoplasmic division) as shown in Figure 5 follows and occurs concurrently with telophase. Cytokinesis is simply defined as the division of the cytoplasm. And it includes those other activities that occur during the cell division (with the exception of nuclear division) of a parent eukaryotic cell into its progeny or daughter cells. 

Figure 5. Illustration of cell division showing interphase and cytokinesis.

MEIOSIS

Meiosis unlike mitosis is the type of cell division that occurs during the formation of gamete cells (e.g. spermatozoa and ova). It only occurs in reproductive cells; and meiosis results in the formation of a diploid cell that gives rise to four haploid cells, with each having a single set of chromosomes. Diploid cells are cells that have two sets or copies of chromosomes. Each haploid cell (i.e. one copy of the chromosome) formed during meiosis can afterward fuse with the gamete of the opposite sex during sexual reproduction to form new diploid cell. Though meiosis and mitosis are similar in their mode of division, meiosis occurs only in reproductive cells during sexual reproduction while mitosis occurs during asexual reproduction. Meiosis involves two stages: meiosis I (the first meiotic division) and meiosis II (the second meiotic division). Each of the divisions in meiosis exhibits the prophase, metaphase, anaphase and telophase stages that are characteristics of cell division by mitosis. At the end of meiosis I and meiosis II, the initial diploid cell will be changed into four haploid cells.

The first meiotic division comprises prophase I, metaphase I, anaphase I and telophase I while the second meiotic division (which is analogous to mitosis) consists of prophase II, metaphase II, anaphase II and telophase II. Meiosis I differs markedly from mitosis which exhibit similar characteristic with meiosis II. Meiosis is a reductive type of cell division because the chromosomes of the parent cell (which is diploid) is reduced by half or halved (i.e. as haploid cells); and each of the haploid cells receives a complete set of new chromosome. (Chromosome number is not halved during cell division by mitosis as is obtainable in meiosis). The major role of meiosis in the life cycle of microbial cells and other eukaryotic organisms is to ensure that the diploid cells of the parent cell are halved during sexual reproduction. Only meiosis I, which is different from mitosis, shall be highlighted in this unit. (Meiosis II is a mitotic division of each of the haploid cells produced in Meiosis I). The interval between the two stages of meiosis (i.e. meiosis I and meiosis II) is known as interkinesis or interphase II. After the first meiotic cell division (meiosis I), interkinesis occurs prior to the start of meiosis II.  

  • Prophase I: This stage is characterized by the development of the spindle fiber, and the breakdown of the nuclear membrane of the chromosomes. The chromosomes appear at this stage of meiotic cell division as sister chromatids; and they are visible under the light microscope. There is a complete disintegration of the nucleolus and nuclear envelop or membrane at this stage of division. Prophase I is similar in to prophase in mitotic cell division.  
  • Metaphase I: Metaphase I is characterized by the alignment of the chromosome pairs on either side of the metaphase plate. And this is different from metaphase in mitotic cell division in which all the chromosomes are aligned to the metaphase plate in no particular arrangement. Crossing over occurs at this stage of meiotic cell division; and it is characterized by the formation of a layer at the equator of the microtubule spindle by the homologous pairs of chromosomes or non-sister chromatids. During crossing over, the arms of the chromatids overlap and momentarily fuse together to form a single larger circular molecule or chromosome. Crossing over is a genetic recombination process in which the exchange of hereditary information occurs between two linear duplexes of chromosomes. Crossing over occurs in meiosis between non-sister chromatids in homologous chromosomes; and it is lacking in mitotic cell division. 
  • Anaphase I: In anaphase I, the microtubule spindle fibers contract, pulling the homologous pairs of chromosome away from each other and toward each pole of the cell. Basically, the two chromosomes in each homologous pair move to opposite poles of the spindle fiber. 
  • Telophase I: Telophase I is the meiotic cell division in which each of the two sets of chromosomes becomes enclosed by a nuclear membrane. The chromosome does not disappear at this stage, and there is no reformation of nuclear membrane as well. Cytokinesis occurs in telophase I. Cytokinesis is the changes that normally occur in the cytoplasm of a cell during cell division, and it excludes nuclear division. Examples of such changes include the synthesis of new materials for the cell wall of each progeny and distribution of organelles in the cell. Each daughter cell has a single set of chromosomes that is half the total number of chromosome in the parent cell.

Table 1. illustrates some of the basic differences between meiosis and mitosis. 

Table 1. DIFFERENCES BETWEEN MITOSIS AND MEIOSIS

S/NoMitosisMeiosis
1.It occurs in the division of somatic or body cells.It occurs in gamete cells.    
2.Mitosis does not introduce variation in the genetic makeup of an organism.Meiosis introduces variation in the genetic makeup of an organism.  
3.It starts with the zygote after fertilization and continues throughout the life of the organism.It occurs only after the age of maturity especially during the formation of gamete cells.  
4.No crossing over occurs during mitosis.Crossing over occurs during meiosis.  
5.Chiastmata is not formed during mitosis.Chiastmata is formed during meiosis.  
6.During mitosis, there is no association of homologous chromosomes.During meiosis, homologous chromosomes associate.  
7.In mitosis, two daughter cells are usually formed in one cycle.In meiosis, four daughter cells are formed in one cycle.  
8.One nuclear division and one cytoplasmic division occur per cycle during mitosis.Two cytoplasmic divisions and two nuclear divisions occur per cycle during meiosis.

FURTHER READING

Alberts B, Bray D, Lewis J, Raff M, Roberts K and Watson J.D (2002). The molecular Biology of the Cell. Fourth edition. New York, Garland, USA.

Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology: Exploring Life. Boston, Massachusetts: Pearson Prentice Hall.

Cooper G.M and Hausman R.E (2004). The cell: A Molecular Approach. Third edition. ASM Press.

Karp, Gerald (2009). Cell and Molecular Biology: Concepts and Experiments. John Wiley & Sons.

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of microorganisms. 12th edition. Pearson Benjamin Cummings Publishers. USA. Pp.795-796.

Nelson, David L.; Cox, Michael M. (2005). Lehninger Principles of Biochemistry (4th ed.). New York: W.H. Freeman.

Verma P.S and Agarwal V.K (2011). Cytology: Cell Biology and Molecular Biology. Fourth edition. S. Chand and Company Ltd, Ram Nagar, New Delhi, India.

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