BIOLOGY Volume 3 - A Guide to General Biology - 2004

23. THE CONTINUITY OF LIFE

23.3. Mitosis

Events occurring in The Nucleus during mitosis are usually observed in fixed and stained preparations. Such preparations allow one to see the phases through which Chromosomes pass during Cell Division, but they do not reveal their chronological sequence. It should be remembered, however, that mitosis is a continuous process with no sharp boundaries between phases. Phase-contrast Microscopy and time-lapse cinematography make it possible to observe nuclear division taking place in a living cell. When the film is played back at high speed, mitosis is perceived as a continuous process. For convenience, it is divided into four phases. The changes taking place in an animal cell throughout these phases are shown in Fig. 23.6. Photomicrographs of mitosis in animal and plant Cells are shown in Figs. 23.7 and 23.8.

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Fig. 23.6. Mitosis in an animal cell.

Interphase

The duration of interphase varies depending on the specific Functions of the cell. Immediately prior to nuclear division, the DNA of each chromosome replicates. As a result, each chromosome is now represented as a pair of chromatids joined by a centromere. At this stage, The Cell contains 4 copies of each DNA molecule (4n), two in each chromosome of the homologous pair. During interphase, the chromosomal material exists as a loose mass of intertwined threads called Chromatin. The centrioles have replicated.

Prophase

Usually the longest phase of division. Chromosomes shorten and thicken as a result of coiling and tighter packing of their components. When stained, they are clearly visible. Each chromosome consists of two chromatids joined by an unstained centromere. In animal cells, centrioles move apart to opposite poles of the cell. Short microtubules can be seen radiating from the centrioles, forming what is known as the aster. Nucleoli disappear as their DNA is incorporated into certain chromosomes. At the end of prophase, the nuclear envelope disintegrates, forming numerous small vesicles. The spindle begins to form.

Metaphase

Chromosomes align along the equatorial plane of the cell. They are attached to the spindle fibers (microtubules) by their centromeres.

Anaphase

This stage proceeds very rapidly. Centromeres split in two, and the spindle fibers pull the daughter centromeres toward opposite poles. The centromeres drag the separated chromatids behind them.

Telophase

Chromatids reach the poles of the cell, uncoil, and elongate; they revert to chromatin and become indistinct. The spindle fibers break down, and the centrioles replicate. Around the chromosomes at each pole, a new nuclear envelope reforms and nucleoli reappear. Telophase is immediately followed by cytokinesis (cell division).

Fig. 23.7. Stages of mitosis and cell division in an animal cell.

Fig. 23.8. Cytology/practical/54.html">Longitudinal section of a ROOT tip, showing stages of mitosis typical of a plant cell. Try to identify these stages based on the information presented in Fig. 23.7.

23.3.1. Centrioles and Spindle Formation

Centrioles are Organelles located in the Cytoplasm near the nuclear envelope; they are found in animal cells and lower plant cells. They are paired organelles positioned at right angles to one another.

Each centriole is approximately 500 nm long and 200 nm in diameter, and consists of nine groups of microtubules, with three microtubules per group. Adjacent microtubule triplets are believed to be interconnected by fibrils (Fig. 23.9). Microtubules are long, hollow tubes 25 nm in diameter, composed of tubulin protein subunits (section 5.10.7).

Fig. 23.9. A. Electron micrograph of a Cross section of a centriole from a chick embryo Pancreas cell. B. Diagrammatic representation of a centriole cross section.

Centrioles are invariably located within an amorphous, unstructured region of material that initiates microtubule growth. This region of the cell is known as the centrosome. It is the centrosome that organizes the spindle, since the "spindle fibers" are actually microtubules. This explains how plants and Fungi, which lack centrioles, are still able to form a spindle. The function of centrioles in nuclear division remains unclear; they may be involved in orienting the spindle, thereby helping to determine the plane of cell division. Some spindle fibers extend from one pole to the other, while others run from the poles to the centromeres. The shortening of these spindle fibers through the removal of tubulin subunits accounts for the movement of chromosomes and chromatids during cell division. They are effectively "reeled in" by the centrosomes.

Adding colchicine to actively dividing cells inhibits spindle formation, leaving pairs of chromatids stranded at their metaphase positions. This technique allows for chromosome counting and microscopic examination of their Structure. Modified centrioles are also found in Cilia and flagella, where they are referred to as basal bodies.

23.3.2. Cell Division

The division of the cytoplasm is known as cytokinesis. It typically follows telophase and leads into the G_I phase of interphase. In preparation for division, cellular organelles along with the chromosomes are evenly distributed toward the two poles of the telophase cell. In animal cells, The Plasma Membrane begins to invaginate during telophase at the level of the former spindle equator. This process is thought to be driven by microfilaments located in this region. As a result of this invagination, a continuous furrow forms, encircling the cell along the equator. Eventually, the cell membranes pinch together at the furrow, completely separating the two cells.

In plant cells, spindle fibers begin to disappear during telophase, persisting only in the region of the equatorial plate. Here, they shift toward the periphery of the cell, increase in number, and form a barrel-shaped structure called the phragmoplast. Microtubules, Ribosomes, Mitochondria, Endoplasmic reticulum, and the Golgi apparatus also migrate to this region; the latter produces numerous small, fluid-filled vesicles. These vesicles first appear in the center of the cell and then, guided by microtubules, move and fuse with one another to form the cell plate situated in the equatorial plane (see Fig. 5.30). The Contents of the vesicles contribute to The formation of the new middle lamella and the walls of the daughter cells, while their membranes give rise to the new outer cell membranes. As the cell plate expands, it ultimately fuses with the parent Cell wall, completely partitioning the two daughter cells. The newly formed cell walls are termed primary cell walls; subsequently, they may undergo additional thickening through the deposition of Cellulose and other substances, such as Lignin and suberin, forming a secondary cell wall. In certain areas, the vesicles of the cell plate do not fuse, leaving cytoplasmic connections between adjacent daughter cells. These membrane-lined cytoplasmic channels form structures known as plasmodesmata.

23.3.3. Comparison of Mitosis in Animal and Plant Cells

The most critical event occurring during mitosis is the equal distribution of replicated chromosomes between two daughter cells. Mitosis proceeds in much the same way in both animal and plant cells, though several differences do exist (Table 23.1).

Table 23.1. Differences between mitosis in PLANT AND ANIMAL cells

23.3.4. Summary

As a result of mitosis, the Nucleus of the parent cell divides into two daughter nuclei, each containing the same number of chromosomes as the parent nucleus. This is followed by the division of The Cell as a whole. For this to happen, chromosomes are first replicated during interphase. The paired structures resulting from this Replication are called chromatids, and during mitosis, they segregate into separate cells.

23.3.5. Significance of Mitosis

1. Genetic stability. Mitosis yields two nuclei, each containing the identical number of chromosomes as the parent nucleus. Because chromosomes are derived from parental chromosomes through precise METABOLISM/36.html">DNA replication, their genes carry the exact same Genetic information. Daughter cells are identical to the parent cell, meaning mitosis cannot introduce any alterations into the genetic information. Consequently, cell populations (clones) originating from the same parent cells remain genetically stable.

2. Growth. Mitotic divisions increase the Total Cell Count within an Organism, which forms the basis for the growth of Multicellular Organisms (Chap. 22).

3. Cell replacement. The replacement of Cells and Tissues is also driven by mitosis. Cells are constantly dying and being replenished by new ones—Skin cells provide a clear example of this process.

4. Regeneration. Some animals are capable of regenerating lost body parts, such as limbs in crustaceans or arms in starfish. The cells required for this process are produced through mitosis.

5. Asexual reproduction. Mitosis serves as the basis for asexual reproduction—The production of new individuals of a species from a single parent. Asexual reproduction is widespread among many species, and the various modes of this reproduction are covered in greater detail in Chap. 21.