A short Review on Mitosis : A process of Somatic Cell Division

mi·to·sis/mīˈtōsəs/nounBIOLOGY : a type of cell division that results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus, typical of ordinary tissue growth. (GoogleDictionary)

Mitosis is a part of the cell cycle in which the chromosome is duplicated and distributed between identical daughter cells. Walther Flemming coined the term mitotic process and described the chromosome behavior during cell division (W Flemming, 1965). Study of mitosis was greatly accelerated with the development of the microscope. Images of mitosis in cells of different living organisms were presented by scholars and every description involves the chromosome, nobody has an idea about spindle at that time.

Weismann was the person who proposed that cell genetic information was stored in the chromosome (A Weismann, 1885).  Imaging was done of both the living cells and stained cells. Spindle fibers were seen in the stained cells but not in the living cells. This was the reason that they have a doubt about the presence of spindle since the fixatives used can lead to the formation of aster-like structure in egg white (W.B Hardy, 1899). It was only when the phase optics was developed by Zernike in the early 1930s, spindle fibers were more readily visible in the flagellates living in the hindgut of the wood-feeding roach, Cryptocercus (Cleveland et al., 1934). With more advancement to microscopy, the study of mitosis reached to a new horizon with the invention of electron microscopy. The previous method implies harsh fixations that can make the fiber visible under a light microscope but when seen at higher resolution the fibers appear more like a bundle of fine fibrils (Rozsa et al., 1950). When glutaraldehyde was used as a fixative better preserved tubular fibers were found in all spindles studied (Sabatini et al., 1963)  and those were attached to specializations on each chromatid of a metaphase chromosome. Those identified specializations appeared as paired structures at the centromere of chromosome (H Fuge, 1973).

Serial cross-sectioning and electron microscopy elucidate the structure of the cold-stable bundle of MTs that associates with each kinetochore in a mammalian cells (Rieder  et al., 1981), and the structure of both kinetochore-associated MTs (McDonald et al., 1992) and other spindles MTs that contribute to mammalian spindle structure  (Mastronarde et al., 1993).

Chromosome segregation is diverse but small spindles build a distinct interpolar spindle, a common feature as found in most fungi and algae. Some of the microtubules are not in this bundle and they run from one pole to each kinetochore making a physical connection that is important for normal chromosome motion. The chromosome aggregate at metaphase plate but the connection between sister chromatids are not strong enough allowing the metaphase chromosome to separate for a short period of time, showing a “breathing” as they move back and forth about the spindle equator (He et al., 2000). This breathing is so extreme in some diatoms that the sister kinetochores are pulled to opposite poles even before the start of anaphase (Pickett-Heaps et al., 1980). In the cells of mammals, fruit fly and nematodes there a number of microtubules attached to kinetochore but only a few of them run from kinetochore to the pole (Rieder et al., 1981). Many of the microtubules start and end in the body of spindle and some of them blend with the kinetochore attached microtubules that make the kinetochore fiber (Mastronarde et al., 1993).

In higher plants and animals the nuclear envelope disperses but in many unicellular organisms the envelope remains intact throughout mitosis.  In chlamydomonas, the nuclear envelope is largely intact but there is a window near the spindle poles that allows the centrosome in the cytoplasm to extend microtubules into the nucleoplasm thus affecting chromosome position. Same is observed in the syncytial blastoderm stage of the Drosophila embryos. In some cases spindle forms in the cytoplasm and the nuclear envelope remains intact throughout the process (Ritter et al., 1978). These all suggest that mitosis requires a bipolar array of microtubules and they interact with the chromosomes linking kinetochore to the poles.

Quantitative descriptions of chromosome movements in living cells show some diversity in the anaphase, there were two phases of chromosome movement that involves a change in spindle length and kinetochore separation (Hughes et al., 1948, H.A Ris, 1943). In one of the phase, the chromosomal fibers are shortened and in the other phase, there is the elongation of the spindle with consequent chromosomal movement. Previous studies suggest that there is a variation in the extent to which an organism relies on Anaphase A or B. Tradescantia use only Anaphase A and Primary spermatocytes of the Aphid, Tamalia use only Anaphase B (H.A Ris, 1943). Some cells used both but at a different time period of the development. Secondary spermatocytes and embryonic cells of the Aphid, Tamalia; Hemiptera and Homoptera use both Anaphase A and B. (H.A Ris, 1943).

Mitosis was observed before the physical structure of the protein and nucleic acids were studied, so the hypothesis proposed at that time was not efficient, as for example, the spindle of marine eggs and embryo resemble that of the lines of magnetic force. So, models for mitosis were proposed based on this observation but none of this model led to experiments that clarified the mitotic mechanism (E.B Wilson, 1925). The first ideas that were efficient to explain the mitotic mechanism were ones that assigned contractile properties to the apparent connections between chromosomes and spindle poles (I Cornman, 1994).

Due to the complex and small structure of mitotic spindles technological advances in microscopy, genetics, biochemistry and molecular biology was very much necessary to identify important functional components of this complex process.


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Binod G C

I'm Binod G C (MSc), a PhD candidate in cell and molecular biology who works as a biology educator and enjoys scientific blogging. My proclivity for blogging is intended to make notes and study materials more accessible to students.

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