Meiosis takes place in the reproductive cells that produce gametes. Meiosis is a modified mitosis in which chromosomes divide once and the nucleus divides twice, after which the number of chromosomes are reduced to half. Thus, the four cells resulting from a meiotic division have a haploid number of chromosomes.
It means that the number of chromosomes becomes half in each sex cell. This is because when the male and female gametes fuse during fertilization, the diploid (double) number of chromosome pairs are re-established. Meiosis is a Reductional division.
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Stages of Meiosis:
(As per syllabus, it is not necessary to study the stages of meiosis. However, it is useful to know). Meiosis includes two nuclear divisions:
i. First meiotic division (reduction division), and
ii. Second meiotic division (mitotic division/ equational division)
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Thus, in meiotic cell division, all the stages, i.e. prophase, metaphase, anaphase and telophase are repeated twice.
(A) First Meiotic Division:
In this division, homologous chromosomes come together (associate) and subsequently segregate into daughter cells. Thus, the number of chromosomes are reduced from diploid (double) to the haploid (single) state. That is why it is known as reduction division.
The following events take place during this division.
(I) Chromosomes Pair and Separate:
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a. Chromosomes become shorter and thicker.
l. The chromatin network undergoes condensation and shortening and changes into chromosomes.
2. The homologous chromosomes (one received from father and one received from mother) attract each other and come to lie in pairs. The pairing of homologous chromosomes is known as synapsis and the pair is known as bivalent.
(II) Chiasmata Formation:
1. Chromosome continues to shorten and thicken. Each chromosome splits lengthwise into two chromatids so that each homologous pair now has four chromatids and is termed as tetrad.
2. The non-sister chromatids of a tetrad break open and rejoin each other. This is known as crossing over or chiasmata (singular: chiasma) formation.
3. Exchange of some genes or portions of chromatids takes place between two non-sister chromatids of homologous chromosomes during this stage.
i. Due to crossing over of homologous chromosomes, chromosomes separate out. Nucleolus and nuclear membrane disappear.
ii. The members of homologous chromosomes completely separate from each other and move towards the opposite poles. Nuclear membrane reappears leading to the formation of two daughter nuclei.
(B) Second Meiotic Division:
It is similar to mitosis. During this the two chromatids of each chromosome separate and move to opposite poles. Nuclear membrane reappears and four cells are formed. Finally each cell formed has haploid (n), half the number of chromosomes of the original cell (diploid, 2n).
Significance of Meiosis:
i. The number of chromosomes are reduced to half in the daughter cells.
ii. It results in the formation of haploid sex cells (sperms and ova), which after fertilization restore the diploid number of chromosomes in the zygote.
iii. During crossing over which occurs in meiosis, part of chromatids are exchanged between homologous chromosomes which bring about variations in the offsprings.
iv. The four chromatids of a homologous pair of chromosomes are passed on to four different daughter cells. This also causes gametic variation.
v. It avoids the multiplication of chromosomes, and thus maintains the stability of species.