Essay on the Process of Meiosis

As much as sexual reproduction involves the union of gamete nuclei and an accompanying association of the chromosomes which come from each parent, it becomes important to understand certain phenomena that are involved in these processes. Let us suppose that a certain species is characterized by individuals all of whose cells, including gametes, possess ten chromosomes.

This means that it gametes of male and female individuals unite, there are twenty chromosomes in the zygote nucleus, and it is not difficult to visualize that unless some mechanism were to reduce the number at one point or another, the chromosomal complement of these organisms would not remain numerically constant.

Fortunately for our understanding of biology, such a mechanism is operative in all organisms that reproduce sexually, except for a few unusual types. It involves a special type of nuclear division called meiosis, accomplished by two successive divisions with the production of four daughter nuclei, each of whose chromosome number is exactly one-half that of the original cell nucleus.

Hence, if meiosis occurs in gamete formation, as it does in animals and in a few plants, it results in eggs and sperm whose union merely restores the “double” chromosome number of the species to the zogote, and subsequent mitotic divisions ensure that all cells of the individual possess this characteristic number.

In order to understand the events which take place in the meiotic process, let us review certain details of ordinary , or mitotic, nuclear division. It will be recalled that prophase is marked by the appearance of distinct chromosomes, each of which is composed of two chromatids connected by a centromere.

The chromosomes line up independently of one another along an equatorial plate at metaphase. At anaphase, the centromeres divide, chromatids become separated, and with the subsequent events of telophase and daughter cell formation, each new nucleus comes to possess a representative chromatid of each original chromosome.

Thus, if the number of chromosomes appearing at prophase is ten, each new daughter nucleus receives ten chromatids which duplicate themselves before the onset of new prophases in actively dividing cells. Because of this mechanism, chromosome numbers remain constant, and in a quantitative sense, mitotic division is purely educational.

As is so often the case with difficult problems in biology, it must be admitted that the forces responsible for initiation of meiotic rather than mitotic division in a given cell are not entirely clear. At any rate, the nucleus of such a cell enters prophase as though it was going to divide mitotic ally, but the chromosomes behave quite differently than do those in a mitotic nucleus.

The descendants of the parental chromosomes, brought together in the zygote that produced the individual and exactly’ duplicated by many mitoses, now exhibit a strong attraction for each other, and actually unite in a process called synopsis. In this union, homologous chromosomes become intimately attached to each other, and because the four chromatics constitute a unit, they are sometimes referred to collectively as a tetrad.

During synapsis, opposing chromatics of homologous chromosomes frequently become coiled and twisted about each other, and they may even exchange portions, and event which has considerable genetic significance.

Eventually, there is a meiotic metaphase, and the tetrads line up on a spindle. Characteristically, they separate in the plane of their original union, and the two original chromatics of a chromosome move toward one pole in meiotic anaphase, while those of the homologous chromosome move toward the other.

With regard to the number of original chromatics, this first meiotic division accomplishes precisely what a mitotic division does, that is, half of the prophase chromatics is delivered to each daughter nucleus. There is a considerable difference, however, in the distribution of this chromatics. In mitosis, one chromatic from each original chromosome becomes situated in a daughter nucleus.

This is not the case in the first meiotic division; whole chromosomes go into one daughter nucleus, and a second division separates the chromatics.

To state the matter differently, a mitotic daughter nucleus receives one chromatic of each chromosome, while a meiotic daughter nucleus receives one chromosome of each chromosome pair characteristic of the species.

Following the separation of chromosomes during the first meiotic division, it would be expected that each nucleus should pass through telophase into interphase. Although nuclear membranes are characteristically formed, the chromosomes tend to retain their individual appearance, and interphase is thus greatly reduced.

There is, of course, a great deal of individual variation among organisms and their cells in this respect. In many cells, the chromosomes remain in a prophase-like condition, with their individual identity and form being retained. Regardless of telophase- interphase details following the first meiotic division, each daughter nucleus enters second meiotic metaphase. This time, the chromosomes line up on the spindle in such a way that centromeres divide and sister chromatids separate as in mitosis.

This time, the chromosomes line up on the spindle in such a way those centromeres divide and sister chromatics separate as in mitosis. Since both daughter nuclei-of the original cell undergo this second division, the result is four nuclei, each of which receives a chromatic representing each chromosome pair of the original cell. Thus each nucleus resulting from meiotic division has exactly one- half the number of chromatics as the original cell had chromosomes.

Perhaps the full significance of meiosis will not be apparent immediately since all its implications can hardly be appreciated at once. At least two things should be apparent at this point, however. One of these is that the process of meiosis is educational in terms of chromosome number, resulting in nuclei which possess only one of each original chromosome pair. This is a result of the chromosomes having divided only once while the original nucleus divided twice.

It might be pointed out here that a cell whose nucleus exhibits homologous chromosomes is said to be diplioid, and one whose nucleus possesses only one member of each chromosome pair is said to be haploid. In the formation of gametes, the reduction of the chromosome number from 2n to n eliminates the difficulty which would otherwise exist in maintaining a constant chromosome number for a species.

A second significant feature of meiosis is that it provides opportunity for arandom mixing of chromosomes in gametes. Whereas each haploid “set” ordinarily must include a representative chromosome of each homologous pair, the distribution of original parental chromosomes seems to be entirety fortuitous. This particular aspect of meiosis and its importance will be developed more fully in a later topic.

Whenever meiosis occurs in the formation of gametes, as described above, it is said to be gametic. This is characteristics of animals, where eggs and sperm are normally the only haploid cells in otherwise diploid bodies. It is interesting to note that, in typical gamete production in the male animal, the cell which undergoes meiosis produces four sperm cells, as we would predict, but in the production of eggs, the first meiotic division results in only one functional cell, while the other receives a very small amount of cytoplasm in the divisional process.

Although the polar body eventually degenerates and thus plays no further part in the reproductive process, it may undergo the second meiotic division. The functional cell undergoes its final meiotic division with the production of a second polar body, which also degenerates.

The net functional egg instead of the four that would be expected theoretically. This egg, however, has the advantage of possessing all the cytoplasm of the original cell, a feature which makes for considerable biological advantage, since the egg is thus provided with a quantity of food materials which are utilized in embiyonic grown following fertilization of the egg.

Most plants exhibit sporic rather than gametic meiosis with reduction of chromosome numbers occurring in the production of four haploid spores from a diploid spore mother cell. Pollen grains of flowering plants, for example, are such haploid spores.

In the life cycle of plants whose meiotic process is sporic, there is alternation of a haploid, gamete- producing generation which grows from a spore with a diploid, spore-producing one which arises from a zygote. Although gametic and sporic meiosis are generally characteristic of animals and plants, respectively, certain algae and fungi display a third type, zygotic meiosis, where the zygote is the only diploid stage in the life cycle. Meiosis occurs in such plants during first divisions of the zygote.