Meiosis : Cell Division

Meiosis : Cell Division 

Meiosis is a method of cell division in which daughter cells are produced with half the original number of chromosomes. 

Thus the diploid or double set of chromosomes (2n)gets reduced to haploid or single set (1n). 

Hence meiosis is also called reduction division.

The chromosomes of the cell undergoing meiotic division duplicate only once while the nucleus and cytoplasm divide twice. 

As a result, at the end of the process four haploid cells are formed from a single diploid cell. 

Meiosis takes place once in the life cycle of all sexually reproducing organisms in order to prevent the doubling of chromosome numbers in every generation. 

Meiosis may take place either before the gametes are formed (gametic meiosis) as in most animals and some lower plants, or after the gametes have fused (zygotic meiosis) as in many lower plants, or before the formation of spores (sporic meiosis) as in higher plants. 

The process of meiosis is very similar in both the sexes as well as in plants and animals. 

The two successive divisions of meiosis are termed Meiotic Division I and Meiotic Division II.

For every paternal chromosome in the diploid nucleus there is a maternal chromosome which is similar to it in form, structure and function. 

Maternal and paternal chromosomes showing such similarities are called homologous chromosomes. 

During the early stage of the first meiotic division the homologous chromosomes pair and interchange genetic material between them. 

The paired chromosomes separate and move to opposite poles and then the cytoplasmic division takes place. 

Meiotic division I is a slow process and may take days and weeks to complete. 

The daughter cells produced show a haploid number of chromosomes, but each chromosome is actually double. 

The second meiotic division that follows is similar to mitotic division. 

Now there is neither pairing of chromosomes nor gene exchange. Each double chromosome divides and the haploid chromosome number is maintained in the daughter cells.

Both the meiotic divisions occur in four successive stages described in the case of mitosis, namely prophase. metaphase. anaphase and telophase. 

There is also an interval termed interphase between the two divisions. 

The prolonged prophase of meiotic division I can be further divided into six substages. namely, preleptotene. leptotene, pachytene, diplotene and diakinesis. 

Thus the various stages of meiosis are as follows:

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Meiotic Division I :

The melotically dividing cells undergo a preparatory stage called premeiotic interphase. 

The cells grow in size during this phase and the DNA undergoes replication to provide for the doubling of chromosomes.

(1.) Prophase I : 

This can be said to be the most characteristic phase of the entire meiosis. Many changes take place within the nucleus, and some also in the cytoplasm, during the long and complicated prophase I. 

The chromosomes appear and get rearranged and modified. 

These changes are necessary for the halving of chromosome number that will follow. 

The prophase I can be divided into six substages as given earlier. The substages are based on the differences of chromosomal behavior. 

By the end of prophase I the nuclear membrane and the nucleolus disappear and the centrioles, if present, move to opposite poles. The substages of prophase I are the following.

(a) Preleptotene (or Preleptonema): 

During this first stage of prophase the nucleus increases in size very greatly. 

This is mainly due to hydration or absorption of water. 

The chromosomes now start appearing. However, they are extremely thin and are difficult to make out.

(b) Leptotene

 (or Leptonema):

 Chromosomes become visible as extremely thin, long, slender, thread-like structures.

  The chromosomes appear single. but are in fact made up of two chromatids which are very closely placed. 

The two chromatids share a kinetochore. Bead-like swellings are present along the length of leptotene chromosomes. 

All the chromosomes in leptotene stage often move to one side and remain attached to the nuclear membrane at one point. 

This is called bouquet formation because of the resemblance of the chromosomes to a flower bouquet. 

The nucleolus and nuclear membrane are still present at this stage and the cell transcribes a large amount of RNAs.

(c) Zygotene

 (or Zygonema): 

During this phase the homologous chromosomes are attracted towards one another and come to lie side by side, close to each other along their entire length. 

This process is called pairing or synapsis.

 The pairing may start from one end of the chromosome and proceed to the other end like the closing of a zip or it may begin in the middle of the chromosome and proceed to either end.

 These two types of pairing are termed proterminal and procentric respectively. 

The way in which pairing takes place is constant for a given pair of chromosomes. The paired chromosomes are termed bivalents. 

The number of bivalents at this stage corresponds to the haploid number of chromosomes.

(d) Pachytene 

(or Pachynema): 

The stage is begun with the completion of pairing between homologous chromosomes. 

Chromosomes NOW become shorter and thicker due to contraction. 

The two chromatids of each pairing member now become very clear. So the bivalent now consists of four chromatids.

 The term tetrad is used to describe this condition. 

 Each of the two chromosomes of the tetrad has its own kinetochore and so a tetrad will show two kinetochores placed side by side.

One remarkable feature of pachytene stage is the occurence of crossing over. 

Soon after the formation of tetrad. chromatids belonging to separate chromosomes of a synaptic pair exchange bits of chromatids. 

A break appears at the same level in the two chromatids. 

The fragments are then exchanged at these points and this is followed by the fusion of the fragment with the chromatid to which it has been donated.

This process is called crossing over. It enables the intermixing of maternal and paternal genes. 

This is an important factor in producing recombinations and variations in organisms.

(e) Diplotene 

(or Diplonema): 

After the crossing over is completed, the paired homologous chromosomes start repelling each other, and thus begin to separate. 

However, the separation is not complete because they are held together at the points of crossing over. 

These joints are termed chiasmata (singular: chiasma). The chiasmata number of appearing over a chromosome is proportionate to its length. 

The diplotene stage is a prolonged one and may last for a few days to over an year.

(f) Diakinesis: 

In the diakinesis, repulsion of homologous chromosomes continues but the chiasmata are now shifted from the middle to the terminal ends of chromosomes. 

The phenomenon is termed terminalization.

 The considered the last stage of prophase or the beginning of metaphase. 

 It is characterised by the disappearance of nuclear membrane and nucleolus. 

 The chromosomes also become shorter and stouter due increased contraction.

Prometaphase: 

This can be chromosomes undergo further condensation. The spindle apparatus appears and the microtubules get attached to the kinetochores of chromosomes.

(2.) Metaphase I: 

The chromosomes now get arranged at the equator of the spindle. 

They are still in the form of tetrads. The chromosomal microtubules of the spindle are attached to the kinetochores of chromosomes. 

The kinetochores of homologous chromosomes are now directed towards the opposite poles. 

These kinetochores lie on either side at an equal distance from the equator. With the pulling of the bivalent homologues in opposite directions they tend to move apart from each other, but are still held by their terminal points of chiasmata. 

The chromosomes are widely separated in between the chiasmata leaving a wide annular, i.e. ring-like. space between them.

(3.) Anaphase I: 

The chromosomal microtubules contract further. As a result each homologous chromosome with its chromatids moves towards the opposite poles of the cell. 

In this process all the remaining chiastama get detached.

This movement of homologous chromosomes towards the opposite poles is responsible for the reduction of chromosomal number. 

In this process, paternal and maternal chromosomes which had come together during the process of fertilization segregate. However, crossing over that has taken place during pachytene would have led to a mixing up of paternal and maternal genes to some extent. 

Moreover, all the paternal chromosomes may not move to the same pole, and the same is true of the maternal chromosomes. Each pole will receive a random mixture of paternal and maternal chromosomes.

(4.) Telophase I: 

During this phase a nuclear membrane is formed again around each group of chromosomes at the opposite poles. 

The chromosomes which are in the form of bivalents get uncoiled. The nucleolus does not reappear.

The nuclear division is then followed by cytoplasmic division as in the case of mitosis. 

This results in two haploid cells, but the chromosomes are really double. 

The cells then pass through an interphase before the beginning of meiotic division II. However, in some cases, meiosis II may start even before the cytokinesis of division I is over.

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Meiotic Division II

2. Homotypic division or Second Meiosis 

A. Karyokinesis. It is a simple mitotic division.

 The nuclear division includes the follow- ing four successive stages:

(i) Prophase II

(ii) Metaphase II

(iii) Anaphase II

(iv) Telophase II

(i) Prophase II:

This stage is of very short duration. The chromosomes do not undergo any appreciable change. 

They are already split into two sister chromatids during pachytene of first prophase. 

The two chromatids of each chromosome remain loose except at the centromere. 

At late prophase, the chromosomes are much thicker and shorter than at early stage. 

The centromeres of the chromosomes remain still undivided. Disappearance of the nuclear membrane and simultaneous forma- tion of the spindle terminate the prophase II. 

In some cases the prophase II is absent and the chromosomes after anaphase I directly form metaphase II plate. 

(ii) Metaphase II:

The doubled chromosomes in each daughter cell move in middle region of the cell and are finally arranged on the equator of the newly formed spindle.

The centromere of each chromosome now divides in the longitudinal plane to form two daughter centromeres; each daughter centromere thus becomes the centomere of the chromatid.

Finally, the centromeres of chromatids become connected with attachment fibres from their respective spindle poles.

(iii) Anaphase II:

The anaphase II starts with the repulsion of centromeres of sister chromatids.

Now the daughter chromatids of chromosomes start moving apart towards the opposite poles of spindle. 

At the end of this process one chromatid of each chromosome goes to one pole and the other to opposite pole.

(iv) Telophase II:

When the daughter chromosomes have reached to the spindle poles, the chromonemata anastomose to form the nuclear reticulum. 

The nucleolus begins to organise on one chromosome and the nuclear mem- branes appear around the chromosomal groups.

B. Cytokinesis:

Intermediate walls develop between two daughter nuclei of each cell after the completion of telophase II. 

Division of the cytoplasm is accom- plished either by furrowing or by cell plate formation. 

Finally, four cells, each with single haploid nucleus, are formed in meiosis.

From the above analysis of meiosis, it is concluded that the chromosomes in the leptotene stage are found in singlet state. 

At zygotene stage homologous singlet chromosomes pair and then the reproduction (duplication) of each chromosome occurs during pachytene.

After duplication the homologous chromosomes show a tendency to repel each other during diplotene and diakinesis stages. 

 The duplication of chromosomes is followed by two divisions. 

 First division is heterotypic or reductional which separates the homologous chromosomes into two equal sets. 

 Thus the original number of chromosomes is reduced by half. 

 The second division is a simple mitosis which takes place in the two nuclei resulted from heterotypic division and separates the daughter chromatids of the chromosomes.

  The two successive divisions of diploid nucleus result into four nuclei having haploid set of chromosomes.

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Significance of Meiosis:

Meosis is a complex process and is of great significance in the life cycle of plants and animals which reproduce sexually.

Significance of meiosis is as follows:

(1) If the mitosis is the only method of cell division, after every act of fertilization the chromosome number of individual will become doubled in subsequent generations. 

The increase in number of chromosomes is harmful and creates imblalance between cytoplasm and the nucleus which may ultimately cause several variations and malformations in the organisms. 

Meiosis helps in keeping the number of chromosomes constant in the species.

(2) During the Diplotene stage of meiosis I, crossing over provides an opportunity for the exchange of genes between homologous chromosomes. 

Thus chromosomes with changed genetic constitution are formed which may cause mutations in the species. Variations are root causes of evolution.

(3) Failure of meiosis leads to the formation of diploid gametes which after fertilization form polyploid forms.

(4) Meiosis which results in the formation of haploid gametes also provides a way for the segregation and independent assortment of genes.

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