Proper understanding of heredity depends on some knowledge of the mechanism of meiosis. Meiosis is a very complicated process of nuclear division and by this process the chromosome number is reduced to half.
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In higher animal, meiosis takes place shortly before the germ cells are formed. In meiosis there are always two successive divisions. During the first division the chromosome number is reduced to half, while the second division is mitotic in nature. The process of meiosis is briefly as follows:
Stages
(i) First Prophase
(a) Leptotene:
In this stage the chromosome in the form of slender thread in diploid number appears in the nucleus. Each chromosome bears a definite number of granules known as chromosomes, and as such the chromosomes look like chains of beads of various sizes.
(b) Zygotene:
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The identical chromosomes come close to each other and they undergo pairing throughout the whole length. The phenomenon is called synopsis. The paired chromosomes arc said to be in bivalent condition. The pairing, however, does not mean fusion.
(c) Pachytene:
The bivalent chromosomes coil around one another and become short and thick at this stage the chromosomes are present in haploid number.
(d) Diplotene:
The chromosomes split longitudinally and as a result each chromosome produces two chromatids. The chromosome threads begin to separate. The separation is, ho’ not always complete, as they remain attached to one or several points.
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At this stage paternal and one maternal chromatid exchange parts by breakage at corresponding plan of the two chromatids. This process is known as crossing over. At certain stage the point of breakage and rejoining come to lie at right angle forming a crosslike structure, which called a chiasma. The chromosomes further shorten and thicken through coiling.
(e) Diakinesis:
The chromosomes further coil and become very short. They remain sparsely distributed throughout the nucleus.
(ii) First Metaphase:
With the disappearance of the nuclear membranes and the nucleolus, the nuclear spindle develops and the bivalent chromosomes move towards the equatorial plane. Each pair of chromosomes has one centromere on each side by which it is attached to the spindle fibers.
(iii) First Anaphase:
The two pairs now get separated and they move to opposite poles.
(iv) First Telophase:
Two daughter nuclei, each with a pair of chromatids, are formed at the poles. Thus the two daughter nuclei contain haploid number of chromosomes. Two separate cells may or may not be formed.
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Each nucleus then may rest for a while (interphase) or it may not, after which it passes on to the second division; which is mitotic in nature.
(v) Second Metaphase:
The paired chromatids are widely separated and are attached with each other only at the centromeres.
(vi) Second Anaphase:
The behaviour of the chromosomes are same as in case of the first anaphase.
(vii) Second Telophase:
Four daughter nuclei are formed, each with haploid number of chromosomes.
Divisions of Meiosis:
During the first division of meiosis the centromere fails to divide causing reduction in the number of chromosomes, while during the second division, the centromeres divide and not the chromatids which simply move apart.