In this article we will discuss about the role of meiotic divisions in reduction of chromosomes.
The somatic (body) cells in multicellular animals have, as a rule, a diploid or double number of chromosomes; that is, apart from some exceptions due to the presence of sex chromosomes, every kind of chromosome in the set is represented by a pair of chromosomes of which one is derived from the male parent and one from the female parent.
The total number is therefore an even one. The two chromosomes of each pair are called homologous chromosomes. The haploid number of chromosomes is found in the gametes of animals and is half the number in the somatic cells.
The process of meiosis consists of two divisions of the nucleus, as a result of which the diploid number of chromosomes present immediately before is reduced to the haploid number.
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The reduction of the number of chromosomes from the diploid to the haploid state can be considered as a means of preventing a continuous increase in the chromosome numbers which become doubled at each fertilization through the addition of the chromosomes of the egg and the spermatozoon, with a subsequent perpetuation of the increased number in mitotic divisions.
This reduction can also be considered to be the last stage in the follow-up of the preceding sexual process which had caused a doubling of chromosome numbers in the zygote, after which the chromosome numbers again revert to the original haploid state. In many unicellular organisms the reduction of chromosome numbers immediately follows the fusion of the gametes, the very first division after the fusion being a meiotic division.
The cells not involved in the sexual process are then all haploid. In multicellular animals, however, the reduction of chromosome numbers is delayed and the somatic or body cells are diploid. Reduction occurs only during the next reproductive cycle.
The mechanism by which a reduction of chromosome numbers is achieved involves a sequence of two divisions during which, however, the chromosomes divide only once. In ordinary mitosis, each chromosome splits during the prophase to form two chromatids; these separate in anaphase to become the daughter chromosomes. In this way each of the two daughter cells receives as many chromosomes as there were in the original cell. The chromosome numbers remain the same from cell generation to cell generation.
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In meiosis the chromosomes also split into two chromatids during the prophase, but instead of one division, producing two cells, two divisions occur in more or less quick succession, so that the available chromatids are distributed among four cells, each of which then receives only half the chromosomes originally present. The distribution of the chromosomes among the four cells is not haphazard but is the result of a peculiar feature of the meiotic prophase which must now be considered.
When the chromosomes first become visible as thin chromatin threads in the early meiotic prophase (the leptonema stage), the number of chromosomes is a diploid one- there are as many threadlike chromosomes as could be seen during mitosis of somatic cells.
It can often be noticed that the chromosomes at this stage take up a specific orientation inside the nucleus; the ends of the chromosomes converge toward one side of the nucleus, the side where the centrosome lies (the bouquet stage).
Next, the chromosomes become sorted out in pairs- the two homologous chromosomes of each pair apply themselves to each other at first only at certain points, especially at the end where they are nearest to the centrosome, but later the two homologous chromosomes become joined along their whole length (the zygonema stage).
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The joining together of the homologous chromosomes is known as the process of synapsis and is the most important part of the meiotic prophase. The pair of chromosomes become twisted spirally around each other and cannot be distinguished separately; they appear to form a single thick thread (pachynema stage).
The number of these thick threads corresponds to the number of pairs of chromosomes in the somatic set and is half the number of thin threads appearing in the preceding stage. Sometime during the pachynema stage, each of the two conjoined chromosomes splits lengthwise to form two chromatids.
Actually, the doubling of the DNA molecule strands, which is necessary for the subsequent duplication of chromosomes, occurs earlier, before the beginning of meiotic prophase, as can be shown by measuring the amount of DNA in the nucleus. Through the earlier part of the meiotic prophase, however, the DNA molecules in each chromosome remain closely joined together, and each chromosome behaves as a single body.
In the pachynema stage this is changed; the two chromatids of each chromosome, containing half of the DNA present in the chromosome at the start, become partially independent of each other, although they still continue to be linked together by their common centromere. Nevertheless, it is important to know that a pachynema chromatin thread actually consists of four chromatids closely joined together in one complex unit called a bivalent, because it contains a pair of chromosomes.
While the chromatids are in close proximity, breakages occur along the length of the chromatids and subsequently parts of the chromatids join up in new combinations. Sections originally belonging to different partners of the pair of chromosomes now become joined in one and the same chromatid.
This is the phenomenon of crossing over, which enables an exchange of parts of the chromosomes to take place. The importance of crossing over stems from the fact that the hereditary material of the two parents, embodied in the two homologous chromosomes, is now reshuffled and redistributed to the four chromatids joined in synapsis.
At any point, breaks and crossing over involve only one of the two chromatids of each of the chromosomes; the other chromatid in each of the chromosomes remains intact. Barring some irregularities which occasionally happen, the two chromosomes exchange corresponding parts, that is, sections of the chromosomes containing the same genes. As a rule, therefore, each chromatid emerges with a full complement of genes.
After the pachynema stage, the chromatids united in each bivalent start separating from each other, as if the forces of attraction which had led to the joining of homologous chromosomes in synapsis have now been replaced by forces of repulsion. A distinct split appears between the two chromosomes in each bivalent (diplonema stage), and the two chromatids belonging to the same chromosome become distinguishable.
The complex, however, continues to be held together, because at every location where a crossing over occurred, the chromosomes are connected by a chiasma. The chiasma is a link between chromatids belonging to different chromosomes of a pair, resulting from the fact that a part of a chromatid of one chromosome has been joined by crossing over to a complementary part of a chromatid of the other chromosome.
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As the crossing over is a reciprocal process, the chiasmata appear as X-wise links between the chromosomes. Depending on how many times crossing over has occurred in any bivalent, there may be one or more chiasmata present.
Toward the end of the meiotic prophase, the chromatids joined in the bivalent contract and thicken in a very marked degree, leading to the diakinesis stage. In diakinesis the chromatids of every bivalent are typically short stout rods, still united by the chiasmata. With the attainment of this stage, the chromosomes are ready to enter into the meiotic divisions.
The two meiotic divisions which follow, in the case of spermatogenesis, are very similar to ordinary mitosis insofar as the mechanism of cell division is concerned. An achromatic figure is formed, and in the first meiotic division the bivalents are placed in the equatorial plate of the spindle.
At the start of anaphase, the two chromosomes forming a bivalent separate from each other and are drawn to the opposite poles and thus to the two daughter cells, which are known as the secondary spermatocytes. In each chromosome the two chromatids are still joined by the common centromere.
A short interphase follows which, however, may be very much reduced so that the chromosomes do not change substantially from their condition during the first, meiotic metaphase. Even if a resting nucleus is formed, there is no duplication of chromatids and no synthesis of genie material at this stage.
In the second meiotic division, each chromosome on the equatorial plate consists of only two chromatids linked together by the centromere. The centromere now breaks, and the chromatids pass to the opposite poles. As a result of the two divisions, four cells are formed from each primary spermatocyte. These are called spermatids.
Each spermatid receives one chromatid (now becoming an independent chromosome) from a bivalent. Because the bivalents are pairs of homologous chromosomes, it follows that the spermatids posses one chromosome for each pair of chromosomes present in somatic cells; that is, they have now a haploid set of chromosomes.
The two homologous chromosomes joined in the bivalent are normally derived from different parents- from the father through the spermatozoon and from the mother through the egg. Thus, the usual condition is that half the chromosomes in somatic cells are maternal and half are paternal.
In the bivalents, one of the component chromosomes is paternal and the other maternal. There is ample genetic evidence that during the first meiotic division the distribution of the paternal and maternal chromosomes is random, each chromosome having equal chances of being delivered into one or the other secondary spermatocyte.
It follows that a secondary spermatocyte and consequently also every spermatid may have any combination of paternal and maternal chromosomes, provided that it can have only one chromosome from each pair. This, together with crossing over, provides for an almost infinite variety of combinations of paternal and maternal genes in any gamete.