Nuclear division or Mitosis

MITOSIS OR NUCLEAR DIVISION 
Nuclear division precedes cell division. The frequency with which cell division takes place depends on many factors and naturally is greatest in young cells in a condition of active growth, although even old, diHerentiated tissue-cells may divide occasionally. In many plants a daily rhythm can be traced, divisions in root tips bfing most active at about midday and in stem apices about midnight.
The process of nuclear division is called mitosis (Gr. mitos, a thread) with reference to the fibrils of cytoplasm which play an important part in it. An older name, karyokinesis, is now rarely used.
There is another form of nuclear division called amitosis, in which no fibrils appear and the nucleus divides directly into two, by constriction, without change of structure. Amitosis in plants is only found in certain specialized tissues. It is still uncertain whether it is ever accompanied by cell-division, and the balance of evidence is against it being a phenomenon of cellular reproduction.
Mitosis takes place in a regular series of four phases, of which the first is the prophase. This first stage involves the resolution of the chromatin reticulum in the metabolic nucleus into a number of thin, separate threads, the chromonemata, which do not stain very deeply. Each of these chromonemata is double. The two together represent one chromosome, and each individual thread or half-chromosome is also called a chromatid, for convenience in tracing it through the subsequent changes.
Each chromatid shows a double series of spiral coilings or gyres. Firstly, the two members of a pair are loosely twisted round each other (relational coiling), these coils being relic coils, left over from the previous mitosis. As prophase advances the relic coils disappear and a new series of minor gyres develops, each chromatid coiling independently.
The two chromonemata of each pair are linked together at one point, where there is an unstained spot or granule, known as the kinetochore or centromere, usually situated not far from one end. Its function will appear later. The portions of the chromosome on each side of the kinetochore are called the arms.

As prophase draws to a close three things happen.Firstly, the chromonemata begin to uncoil, so that the gyres are fewer and larger. Secondly, a densely stainable matrix begins to accrete around each chromatid, forming a double, thickened chromosome. Thirdly, each chromonema splits length-ways, so that each chromatid now contains, within the matrix, two chromonemata which are relationally twisted together as well as coiled. The entire chromosome at this stage therefore consists of two thickened chromatids, conjoined at the kinetochore, and in each chromatid there are two chromonemata. The chromosomes shorten and become thicker and denser and the coils of the chromonemata become closer. In this condition they enter the Metaphase.

The nuclear membrane now dIsappears, so that the chromosomes are no longer separated from the cytoplasm, and the nucleolus also disappears. The chromosomes arrange themselves across the middle plane of the cell, forming the equatorial plate. If the chromosomes are short they may lie horizontally on the plate, but if they are long it may be only the portions containing the kinetochore which actually lie on the plate, the arms pointing in different directions .
The kinetochores then divide, so that the chromatids separate and become independent chromosomes. Simultaneously two sets of fine kinoplasmic pear, one on each side of the nucleus, converging towards the two les of the cell. These are the spindle fibres, and together they make up a double-ended cone, the achromatic spindle, so called because of its slight affinity for stains. Studies by micro-dissection have shown that the spindle is not a mere appearance in the cytoplasm, but a definite body, which can be extracted entire from the cell, with the enclosed chromosomes. One end of each spindle fibre is attached to the kinetochore of a chromosome. There is indeed some evidence that the kine to chores may be concerned in their formation by the secretion of nuclear material. At any rate if a chromo­some loses its kinetochore it has no spindle attachment and can take no part in the subsequent movements.
The next stage, or anaphase opens with the chromosomes moving away from the equatorial plate in two groups, one of the two chromatids of each original chromosome going into each group. The distribution of chromosomes into the two groups is thus exactly equal, in a qualitative as well as a quantitative sense, and the division is said to be equational. The two groups moye convergently towards the two poles of the spindle and form a very characteristic figure, of two radiating clusters, known as a diaster. In this movement the kinetochores lead the way, the arms trailing behind, so that the chromosome looks as if folded in two. As the chromosomes approach the poles the matrix gradually disappears and the double chromonemata again come into view. Their coils relax and they lengthen out, though they retain the relational twist round each other until the prophase of the next mitosis.
The cause of the movement is still uncertain and has aroused a good deal of controversy, but it is not unlikely that there is, as \yas first supposed, a real tractive force exercised by the spindle fibres, due to the contraction of the long protein molecules of which they are composed.
Mitosis now enters the telophase, during which the two daughter nuclei are organized. The chromonemata arrange themselves into a reticulum, and in the process they appear to become linked together. If there is any considerable interval between divisions, a finely granular reticulum is formed, in which the individual chromonemata become in­distinguishable, but if divisions follow one another rapidly the reticulum may not be completed and the next prophase follows almost directly.
Nuclear membranes now appear round the daughter nuclei, and the nucleolus is reformed in each. The relation of the latter body to the chromo­somes is peculiar and interesting. Among the chromosomes in a diploid nucleus there is always one pair which show a marked constriction near one end, on which no matrix accumulates. The small terminal portion, beyond the constriction, is thus isolated from the main body of the chromosome and is termed a satellite, while the chromosomes themselves are known as the sat-chromosomes. This constriction is associated with the formatiop. of the nucleolus and is called the nucleolar organizer. The nucleolus makes its appearance attached to the base of this constriction, which is usually heterochromatic, and as the nucleolus enlarges the satellites may adhere to its surface. The nucleolar material is apparently secreted by the sat-chromosomes during telophase, while during prophase it is dissipated. It is either adsorbed on to the chromosomal surface as a pellicle, or, according to another view, it is wholly or in part dispersed into the cytoplasm. It contains ribo-nucleic acid, which is found in cytoplasm, especially in the mitochondria.
There is at least one pair of sat-chromosomes in a diploid nucleus and each of them forms a nucleolus. If they happen to lie close together at telophase the two nucleoli may coalesce into one, otherwise they remain apart. The presence of more than two nucleoli in a nucleus is usually a sign that more than the diploid complement of chromosomes is present, that is to say that the nucleus is polyploid.
The matrix of the metaphase chromosomes is known to consist of thymo­nucleic acid, the molecules of which align themselves in rows, parallel to the protamine molecules of the chromonemata. Underlying the other changes during mitosis there is thus a cycle of the charging and discharging of nucleic acid upon the chromonemata. Certain parts of a chromonema may, however, retain their charge throughout the metabolic phase; these portions remaining deeply stained in telophase and even in the metabolic nucleus, where they may be seen as knots or chromocentres in the reticulum. This retained material is called heterochromatin, and the rest is dis­tinguished as euchromatin. A chromonema \yhich carries heterochromatin is called heteropycnotic. Charges of heterochromatin occur most commonly near the kinetochore and near the nucleolar organizer in sat-chromosomes. The heterochromatin appears to be responsible for the formation of the thymo-nucleic acid of the euchromatin in the rest of the matrix and possibly also for the ribo-nucleic acid of the nucleolus, since the latter always appears in contact with a heterochromatic region.
The chromonema itself is not uniform throughout its length, but shows a beaded structure, in \yhich the beads or chromomeres stain more deeply than the intervening portions. These chromomeres are the centres of attach­ment of the matrix substance. They are generally regarded as the seats of the genes or units of heredity.
The chromosomes which are formed in mitosis have always a constant and characteristic number in every true species of plant and animal, and they are also constant in form. Each chromosome in a set has a definite length, thickness and shape, so that it can be recognized at any nuclear division and may be given a name or number. Its history can thus be traced through the development of the individual and from generation to generation. There is no escape from the conclusion that the chromosomes are persistent entities, despite the fact that they are not usually recognizable as individuals in the metabolic reticulum, though their persistence in that state may be inferred from the number and distribution of the chromocentres.
It is important to realize that in every diploid nucleus there are two sets of chromosomes, one derived from the male parent and the other from the female. It follows that, in all normal cases, the diploid number of chromo­somes must be even. There is thus a pair of chromosomes of each type, called homologous chromosomes. Unpaired chromosomes are of exceptional occurrence. In hybrids, in cases where the chromosome com­plements of the two parents are not identical, some chromosomes may be unpaired, and if there are many of these the cell cannot survive, that is to say, the union of the parents is infertile. In some species there are also unpaired sex chromosomes, concerned with the determination of sex. As the single set of chromosomes, derived from one parent, contains the complete set of that parent's genes it is called the genome.
The time relationships of the phases in mitosis are fairly constant, though the total time occupied and also the interval between successive mitoses depend not only on the type of tissue but on external factors such as temperature. Mitosis can be watched in living cells in some cases, such as hair cells or the stigma cells in grasses. In the stigmas of Anhe­natherum at 19° C. the total time for the process was found to vary be­tween 80 and 110 minutes.
Those phases which last longest will obviously be those most often seen in stained preparations.

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