Mieosis or Cell Reduction Division

MEIOSIS OR REDUCTION DIVISION 
The essential feature of the union of two gametes in the process of fertilization is the fusion of their nuclei. Each of these gametes possesses the monoploid or haploid (n) number of chromosomes, and hence the zygote will have double the number of chromosomes, that is to say, its chromosome number is diploid (2n). It follows therefore that, as the chromosome number of a species remains constant, a mechanism must be duced at some stage in the life-cycle by means of which a reduction or halving of the number of the chromosomes takes place. In animals this generally occurs at the formation of the gametes, but in most plants it takes place in the production of non-sexual spores.
The mechanism whereby this halving is effected is termed meiosis, from the Greek word meioo = to lessen. In every diploid nucleus half the chromosomes have been derived from one parent and half from the other parent, and hence we can refer to a maternal and paternal set. It has already been pointed out that in all nuclei the size and shape of the chromosomes differ among themselves, hence in a diploid nucleus the chromosomes all form pairs, and the members of each pair are termed homologous chromosomes. - each such pair one member is maternal and one is paternal.
The essential feature of meiosis as opposed to mitosis is that, in the meta­se of the division, instead of split halves of each chromosome passing to o poles of the spindle, the homologous pairs of chromosomes unite, then come on to the equatorial plane and separate, so that one whole chromo­e oi each pair goes to one pole and one to the other. Since it is a matter of pure chance how the chromosomes are arranged at the metaphase, it does necessarily follow that all the maternal chromosomes pass to the same pole. Usually there is mixing, so that the resulting nuclei contain some paternal  and some maternal chromowmes. In no normal case, however, do two homologous chromosomes go to the same daughter nucleus .
Meiosis used to be regarded as involving two separate nuclear divisions which closely followed one another. Since the first is not normally completed before the inception of the second, it is now considered preferable to regard the whole process as continuous but passed through in two stages, resulting in the production of four monoploid daughter nuclei.
The essential features of the process are the following: The first stage, formerly called the heterotypic division, begins with the separation from the reticulum of the diploid number of chromonemata. The two members of each homologous pair then come together side by side, the process being, kown as synapsis. This reduces the original diploid number to half that er of bivalents.
One member of each pair has been said above to be maternal in ongm the other paternal. Thus, although the members of a pair are homologous in form they differ in hereditary constitution, each corresponding to that of the parent from which it was derived.
The chromonemata on coming together, or sometimes at an earlier stage, split lengthways so that the bivalents comprise a tetrad or group of four units called chromatids. The four chromatids, lying side by side, may form unions between those of opposite origin at intervals along their length. These are called chiasmata.
The matrix now begins to form round the chromonemata, and the resulting bivalent chromosomes contract and thicken until they are much shorter and fatter than any seen in mitosis. At this stage they are scattered throughout the nucleus, an appearance known as diakinesis. We now enter metaphase and the bivalents arrange themselves on the equatorial plate; the spindle fibres appear and the nuclear membrane disappears. The separation that now occurs is not, as in mitosis, the separation of two halves of one chromosome. It is the disjunc­tion, i.e., separation, of two whole chromosomes, the paternal and mater­nal members of the homologous pair which, at synapsis, united to form a bivalent. These now move apart towards the poles of the spindle.
As disjunction occurs the chias­mata that were formed between the units in the tetrad are broken through, so that an interchange of segments takes place between the chromonemata in­volved. This exchange of paternal and maternal material is known as crossing over, and it im­plies a redistribution of hereditary material which is of the highest genetical importance.
It will be recollected that when the homologous pairs came together -he chromonemata split, so that each chromosome which now disjoins contains two chromonemata or, with the adherent matrix, two chromatids. As progresses the matrix on the chromosomes diminishes and the ouble internal thread becomes visible again.
When the chromosomes reach the spindle poles they retain their individuality until the second stage, or homotypic division, commences.
Two achromatic spindles are formed in the cell at right angles to the first, such a position that their equators coincide with the poles of the heterotypic spindle. In this way the anaphase of the heterotypic division passes directly into the metaphase of the homotypic division. The chromosomes now split longitudinally as in mitosis, the two halves pass to opposite poles, and the subsequent stages follow the same sequence as in ordinary nuclear division, the final result of the whole process being the reformation of four monoploid nuclei. This is usually followed by the formation of four cells, though all may not necessarily survive. These mono?loid cells mark the beginning of a new phase in the cytological cycle of the life history in the organism concerned, and they normally function as reproductive cells, beginning- also a new generation. 'Whether they act as non -sexual spores and germinate directly, or whether they function as gametes which only germinate after conjugation, depends on the type of alternation of generations which the organism displays, but in plants the former is universal outside the Thallophyta.

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