MEIOSIS

The special type of cell division in which the numbers of chromosomes in ‘tighter cells are reduced to one half, as compared to the p. rent cells is called meiosis. It takes place only in diploid cells. Meiosis takes place in animals at the time of gamete formation. But it takes place during spore formation in plants. Two consecutive divisions take place after single replication of DNA during meiosis. So each diploid cell produces four haploid cells as a result of meiosis. The two divisions are meiosis I and meiosis II.

(a)   The first meiotic division is the reduction division.

(b)  The second meiotic division is just like the mitosis.

Both divisions can further be divided into sub stages like prophase I. metaphase I, anaphase I. telophase I and same names are also used for meiosis

Meiosis I Prophase I

This is very long phase. The chromosomes behme as homologous pairs during meiosis. So it differs from the prophase of mitosis. Chromosomes are not arranged in homologous pairs. Each diploid cell has two chromosomes of each type. One member of this pair comes from each parent by the fusion of male and female gametes. The chromosomes arc replicated during interphase. So each chromosome has two chromatids. The interphase of meiosis lack Gi stage. The similar but not necessarily identical chromosomes are called as homologous chromosomes.

Prophase I further consists of the foil ONN i rigs stages:

(a)Leptotene (ribbon like): The chromosomes become shorten and thick. So they become visible. The size of the nucleus increases and homologous chromosomes start getting closer to each other.

(b)Zygotene: (pairing): The pairing of homologous chromosomes. starts during zygotene.. It is called synapsis. It is the first essential phenomenon of meiosis. This pairing is highly specific. Exact point to point pairing takes place. But this pairing has no

definite staffing point. Each paired (but not fused) complex structure is called as bivalent or tetrad.

Two ‘sister’ chromatids

fanning one

homologous chromosome

(c)      Pachytene (Packing): The pairing of homologous chromosomes is completed. Chromosomes become more and more thick. Each bivalent has four chromatids. These chromatids wrap around each other. Non-sister chromatids of homologous chromosomes exchange their segments by chiasmata formation during crossing over. The exchange of segment of the non-sister chromatids of homologous chromosomes is called crossing over. Thus reshuffling of genetic material takes place. Crossing over produces 11lnk recombination. Pachytene may last for days, weeks or even years. But leptotene and zygotene can last only for few hours.

(d)  Diplotene: The paired chromosomes repel each other. So they beg iii to separate from each other. The homologous chromosomes remain united bv chiasmata. So separation is not complete. Each bivalent has at least one chiasmata. Othemise the chromatids can separate from each other.

(e)  Diakinesis: ^.1he condensation of chromosomes reaches to its maximum point during this phase. At the same time separation of the homologous chromosomes is completed. But still they are united at one point at ends (not by chiasmata). The nucleoli disappear.

Fig: Metaphase I of meiosis

Metaphase I

The nuclear membrane disorganize (disappear) at the beginning of this phase. Spindle fibers originate and the kinetochore fibers. They attach to the kinetochore of homologous chromosome from each

pole. They arrange the bivalents at the equator. The sister chromatids of individual chromosome in bivalent behave as a unit.

Anaphase I         •

The kinetochore fibers contract and the spindle or pole fibers elongate. It pulls the individual chromosome (each having two chromatids) towards their respective poles. The sister chromatids are not separated. But the sister chromatids are separated in anaphase of mitosis. Each pole receives half of the total number of chromosomes. So this is actually a reduction phase.

Telophase I

Nuclear membrane reorganizes around each set at two poles. The nucleoli reappear. Thus two nuclei are formed. Each nucleus has half number of chromosomes. The division of the cytoplasm divides the cell into two. It terminates the lirst meiotic division. The chromosomes decondense during this stage.

Meiosis II

There is small interphase in two daughter cells after the telophase I. But replication of chromosomes does not take place during this interphase. The meiosis II is divided MI sub stages. The sub stages of the meiosis are just like the stage of m.teisis.

I. Prophase 11: The chromosomes comli Ise and mitotic apparatus is appeared.

2. Metaphase 11: The chromosomes are arranged at the equators.
3. Anaphase II: The sister chromatids of the chromosomes moves towards the poles.
4. Telophase I: The chromosomes decondense. Cvtokinesis takes place. Thus each cell di \ ides and thrills four haploid daughter cells.

Importance of Meiosis

1. Variations: Two significant processes take place during meiosis. These are:

(a)Crossing over: The parental chromosomes exchange segments with each during crossing over other. It results in large number of recombinations.

(b)Random assortment of chromosomes: The separation of homologous chromosomes is random during anaphase It gives a large variety of gametes.

Both these phenomena cause variations and modifications in the

genome. These variations are the bases of evolution. These variations also make every individual specific. particular and unique in his characteristics. Even the progeny of very same parent, i.e. brothers and sisters are not identical to each other.

2. Constant number of chromosomes in each generation: Meiosis takes place during sexual cell (gamete) formation and spore formation in plants. Thus it reduces the.. number of chromosomes to one half in each gamete or spore. The original number of chromosome is estored after fertilization. So it maintains chromosome number constant generation after generation. The chromosome number will become double after every generation without meio -is.

Difference between mitosis and meiosis

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