Growth may occur at different levels in a living system. In order to see that growth is a universal characteristic of living systems, let us consider its occurrence at various levels of organization. Cellular and intracellular levels of growth. At the cellular and intracellular synthesis, the two cells attain the size of the original cell.
The process may be repeated in each of these two cells, then in their progeny, with the result that an overall exponential growth results.
The forces which initiate the process of cell division are not quite clear, but division of a given cell is usually preceded by the synthesis of sufficient protoplasm that, when the cell reaches its maximum size, division occurs to produce two daughter cells from the original one.
There is remarkable uniformity among organisms with regard to the .mechanics of cell division. With the exception of certain forms, division may be said to consist of two processes, namely, nuclear division, or mitosis, and cytoplasmic division, or cytokinesis.
Division of the Nucleus-Mitosis
The first indication that a cell is about to undergo division is a visible change in the chromatin “network”‘ of the nucleus. Special staining and microscopic techniques reveal that this material is not really a network at all but that it consists of elongated threads which are distinct from each other.
As the nucleus undergoes further change, these threads gradually condense and thicken. Because this is the first and most obvious of the nuclear changes associated with division of the nucleus, early cytologists settled upon the name mitosis to describe the entire process of nuclear division. From the beginning of the mitotic process to the formation of two daughter nuclei, four progressive and interconnected stages of phases are recognized.
Prophase: This stage begins with the condensation of the chromatin threads, which are called chromosomes as they become distinctly visible. The number of chromosomes which finally make their appearance at late prophase is generally constant for a species exhibit sixteen chromosomes, and those of the parasitic roundworm Ascaris lumbricoides possess only four.
The chromosome number per human cell is forty-six. In the cells of some organisms, the chromosomes number into the hundreds. Apparently, there is no correlation between chromosome number and degree of complexity of an organism.
Great variation in chromosomal morphology occurs among species, there being differences both ill size and shape. Even within a species, the chromosomes can often be distinguished from one another, and can be named or numbered on the basis.
In the cells of a morphological partner, that is, the chromosomes exist in pairs, although paired chromosomes show no tendency to associate closely in the nucleus.
Hence, it is frequently said that an organism exhibits a certain number of chromosome pairs; onion has eight pairs, Ascaris has two pairs; and so on.
The members of these pairs are the descendants of those contributed by the individual’s two parents. If the chromosomes are closely examined as they become visible at prophase, they will be seen to consist of two parallel halves called chromatics, which are connected by a centromere.
Hence, this stage of mitosis reveals the presence of twice as many chromatics as there are chromosomes. At some time during prophase, the nucleoli and nuclear membrane disappear.
The full significance of this is not completely understood, but one immediate result is that the nucleolus is no longer separated from the cytoplasm. Consequently, beginning with late prophase, a cell does not really possess a nucleus during its division.
Metaphase: Near the end of prophase, the chromosomes become oriented in such a way that a lateral view of the cell shows that the chromatics of a given chromosome are in position to move toward opposite poles of the cell; at the point when the centromeres of all the chromosomes are so oriented as to be in an equatorial plane, metaphase is said to begin.
In a polar view of the cell, a ring or plate of chromosomes is characteristically seen, and this is usually the most advantageous view for counting or studying the chromosomes. Meanwhile, a spindle, so called because of its shape, appears in conjunction with the orientation of chromosomes, and some of the fibres which compose it attach to the centromeres of the chromosomes, while others simply run from pole to pole. Perhaps the most significant event which occurs during metaphase is the division of chromosomal centromeres.
This results in the possession of a centromere by each chromatic. Half the original number of chromatics are thus prepared for movement toward one pole and half toward the other, each set constituting the chromosomal complement of a daughter nucleus.
Anaphase: With the division of centromeres, which occurs simultaneously m all the chromosomes of a given nucleus in most cases, anaphase begins. There is a shortening of those spindle fibres that attach to the centromeres of daughter chromosomes, as though a pulling force were being exerted.
Actually, the forces responsible for chromosomal movement in this situation are not clearly understood, but the attachment of spindle fibres to the centromeres is suggestive of some active role by the fibres in chromosomal migration. At late anaphase, chromosomes which are moving toward opposite poles of the cell are widely separated.
Telophase begins when chromosomal migration is complete, and it is somewhat the reverse of prophase. The chromosomes gradually lose their apparent individual identity and they collectively form the chromatin “network”, or mass, typical of a nucleus that is not involved in division.
Nucleoli and the nuclear membrane reappear, and telophase end when the two daughter nuclei are identical to the original no dividing nucleus except in size. A nucleus which is not undergoing mitosis is said to be in interphone. In actively dividing cells, this is a period of synthesis and growth on the part of the nucleus, which enables it to enter again into mitosis at a later time.
During interphone the chromosomes lose their definite stain ability, but it has been shown that their individual identity is retained. In other words, chromosomes are not dissolved and reformed at telophase and prophase respectively; they simply assume different morphological forms.
It should be recognized that the stages of mitosis are portions of a continuous division cycle and that there is no definite point between each; the phases are recognized by cytologists simply as convenient divisions for reference purposes.
By observing the process closely with proper optical equipment, it can be seen that the nucleus moves smoothly from one phase to another. Intimately associated with the process of mitosis in animal cell is the behaviour of the centrosome and centrioles.
During prophase the centrosome divides and each half undergoes migrating in such a way that the two centrioles lie opposite to each other. At metaphase, a centriole is thus situated at either side of the nucleus, and each serves as a center from which the spindle fibres and astral rays radiate.
Division of the Cytoplasm- Cytokines Is
Is considerably less dramatic than that of mitosis. Usually, cytoplasm organelles or inclusions such as mitochondria or plastids are distributed more or less equally in the process. In animal cells, cytokinesis is accomplished by constriction, the process generally being initiated at about the time of mitotic anaphase.
By the end of telophase, when daughter nuclei are full}’ formed, constriction is complete. A variation is constriction is seen in the furrowing process characteristic of certain embryos and other cellular aggregates in which groups of cells divide simultaneously and remain in close contact with each other following cytokinesis. As an accompaniment of their cellulose walls, whose rigidity renders constriction impossible, plant cells exhibit a mode of cytoplasm division different from that described above for animal cells.
Cytokinesis begins in the plant cell by the formation of a cell plate in the middle of the cell at about the time mitotis anaphase ends. The cell plate continues to grow in circumference until it meets the outer walls of the cell. In the meantime, the nucleus has progressed from telophase to interphone, and division of the cell is completed by differentiation of a new cell wall on both surfaces of the cell plate. In certain plants, cytokinesis is somewhat animal like.
Each division product may undergo subsequent divisions, but at any rate, daughter cells will have secreted new cell walls when the original wall breaks open and releases them. Except for certain minor variations, cell division is a remarkably uniform phenomenon among organisms.
This is highly significant, since it lends further support to the view that living forms are fundamentally srmilar. Whatever the physical and chemical factors involved, mitosis is an effective means for ensuring a qualitatively and quantitatively equal distribution of certain key nuclear substances to newly synthesized cytoplasm.
As we have already observed, the chromosomes are bearers of hereditary- determiners, and it is significant that each cell of an organism normally possesses exactly the same complement of these as any other cell. Specialization, and repair tissues through cell replacement As a fundamental biological process, therefore, cell division ranks exceedingly high.