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Meiosis

Meiosis is a special type of cell division in which number of chromosomes in daughter cells is reduced to half, as compared to parent cell.

It is the process leading to formation of the haploid gametes from diploid cells.

It is the process of nuclear division which occurs during the final stage of gamete formation.

It takes place in diploid cells only, in animals at the time of gamete formation (gametogenesis) while in plants at the time of spore formation (sporogenesis). Each diploid cell after meiosis produces 4 haploid cells, because it involves two consecutive divisions after single replication of DNA.

= Introduction =

The essential feature of sexuality is the bringing together of the genomes of cells from diﬀerent individuals of a given species. After mixing their genetic information by breaking and rejoining DNA molecules (homologous recombination), at least one complete and functional genome is established again.

·        Prokaryotic organisms achieve this by a variety of mechanisms whose common feature is that only parts of the two input genomes are retained in a single recombinant genome.

·        Eukaryotes have developed meiosis, a process allowing for full-length pairing of two parental genomes, their recombination without loss of genes, and then segregation of the recombined genetic material into four cells, each with complete haploid sets of chromosomes. Eukaryotes, having many chromosomes, achieve genome variation also by random assortment of the parental chromosome sets during meiosis.

= The Eukaryotic Life Cycle and Meiosis =

The body of a multicellular animal (soma) consists mainly of diploid cells that do not contribute genetic information to the progeny. Only cells of the germline undergo meiosis, and form haploid gametes in the sexual organs. In contrast, no germline is set apart during the development of the soma of plants and fungi. They diﬀerentiate sexual organs from somatic cells. Gametes may be the direct products of meiosis, or diﬀerentiate after further divisions of the primary products of meiosis.

Despite pronounced diﬀerences, most eukaryotic species can be accommodated in a common basic life cycle with a diploid and a haploid phase (Figure 1). Most eukaryotic species, in particular multicellular organisms, predominantly use the diploid phase for formation of the soma. A tendency for haploid soma is found in some multicellular eukaryotes (e.g. mycelial fungi) and in some unicellular eukaryotes (e.g. yeasts and algae). Other species have two diﬀerent types of soma, one in the haploid, the other in the diploid phase of their life cycle (e.g. mosses). In a life cycle involving mainly the haploid phase, the zygote formed by gamete fusion is the only diploid cell. It undergoes meiosis without intervening mitotic divisions. In contrast, gametes are the only haploid cells in species where the life cycle is predominantly in the diploid phase. In higher plants the haploid cells resulting from meiosis form a small tissue within the ﬂowers of the diploid plant, producing gametes and other specialized cells.

= The Principal Events in Meiosis =

The crucial features speciﬁc to meiosis assure that each of the four resulting cells obtains a haploid set of chromo- somes. These features involve pairing of homologous chromosomes, crossover (chiasma) formation, and the reductional meiosis I division. To achieve this, a number of meiosis-speciﬁc processes and structures have developed (Figure 2). The following overview of meiotic events may not fully apply to all eukaryotes (see ‘Variations of meiosis’).

= Premeiotic DNA replication and sister chromatid cohesion =

Diploid cells ready to undergo meiosis (meiocytes) are speciﬁcally diﬀerentiated.

This diﬀerentiation is completed at the latest in the G1 phase immediately before premeiotic DNA replication.

As in the S phase of mitotic cells, cohesion of sister chromatids is established during premeiotic DNA replication. This is achieved by protein complexes (cohesins) attaching to a number of sites along the sister chromatids.

In meiosis, sister chromatid cohesion contributes to proper pairing and recombination of the chromosomes during prophase. In addition, sister chromatid cohesion is essential for correct completion of meiosis I and II.

= Divisions of Meiosis =

There are two divisions of meiosis

1.      Meiosis I (Reduction Division)

2.      Meiosis II (Mitotic Division)

= Meiosis I =

This is sometimes referred to as the reduction division because it is during the first meiotic division that the chromosome number is reduced from 46 to 23.

Meiosis I consist of four stages:

1.      Prophase I

2.      Metaphase I

3.      Anaphase I

4.      Telophase I

Prophase I
This is very prolonged phase.

It differs from the prophase of mitosis, because in this chromosomes behave as homologous pairs.

Each diploid cell has two chromosomes of each type one member from each parent because of fusion of gametes.

Each chromosome has two chromatids, because chromosomes have been replicated during interphase.

The interphase of meiosis lacks G2 stage.

These similar but not necessarily identical chromosomes are called as homologous chromosomes.

Sub stages of Prophase I
Prophase I further consists of following sub stages:

The prophase stage of meiosis I is relatively long and can be subdivided into five stages.

a)     Leptotene

b)     Zygotene

c)      Pachytene

d)     Diplotene

e)     Diakinesis

a)     Leptotene
i.           The chromosomes become visible, shortened and thicker. ii. The size of nucleus increases and homologous chromosomes start getting closer to each other. iii. Leptotene can last for only few hours.

b)     Zygotene (pairing)
i.           First essential phenomenon of meiosis i.e., pairing of homologous chromosomes is called Synapsis starts. ii. This pairing is highly specific and exactly pointed but with no definite starting point. iii. Each paired but not fused, complex structure is called as bivalent or tetrad. iv. Zygotene can last for only few hours.

c)      Pachytene (Chiasmata formation)
i.           The pairing of homologous chromosomes is completed. ii. Chromosomes become more and more thick. iii. Each bivalent has four chromatids, which wrap around each other. iv. Non sister chromatids of homologous chromosomes exchange their segments due to chiasma formation, during the process called Crossing over. In this                   way reshuffling of hereditary material takes place which results in genetic recombination. v.           Pachytene may last for days, weeks or even years.

d)     Diplotene
i.           The paired chromosomes repel each other and begin to separate. ii. Separation however is not complete, because homologous chromosomes remain united by their point of interchange (chiasmata). iii. Each bivalent has at least one such point, the chromatids otherwise are separated.

e)     Diakinesis
i.           During this phase the condensation of chromosomes reaches to its maximum. ii. At the same time, separation of homologous chromosomes (started during diplotene) is completed, but still they are united  at one point, more often at ends.

Metaphase I
Ø Nuclear membrane disorganizes at the beginning of this phase.

Ø Spindle fibers originate and kinetochore fibers attach the homologous chromosome from each pole and arrange bivalent at the equator.

Ø The sister chromatids of individual chromosomes in bivalent behave as a unit.

Anaphase I
Ø The kinetochore fibers contract and spindle pole fibers elongates, which pull the individual chromosome (each having two chromatids) towards their respective poles.

Ø It may be noted here that incontrast to anaphase of mitosis, sister chromatids are not separated.

Ø This is actually reductional phase because each pole receive half of total number of chromosomes.

Telophase I
Ø Nuclear membrane reorganizes around each set at two poles.

Ø The nucleoli reappear, thus two nuclei each with half number of chromosomes are formed.

Ø The cytoplasm divides thus terminating the Ist meiotic division.

Ø It is also to be noted that chromosomes may not condense during this.

= Meiosis II =

Ø After telophase I, two daughter cells experience small interphase, but in contrast to interphase of mitosis here is no replication of chromosomes.

Ø Prophase II, metaphase II, Anaphase II and Telophase II are just like the respective phases of mitosis.

Ø During which the chromosomes condense and mitotic apparatus is formed.

Ø The chromosomes arrange at the equator.

Ø Individual sister chromatids move apart.

Ø Ultimately four nuclei at the respective poles of two daughter cells (formed after meiosis I) are formed.

Ø Cytokinesis takes place and haploid cells with half number of chromosomes (chromatids are formed).

= Significance of Meiosis =

a. It maintains the same chromosome n umber in the sexually reproducing organisms. From a diploid cell, haploid gametes are produced which in turn fuse to form a diploid cell.

b. It restricts the multiplication of chromosome number and maintains the stability of the species.

c. Maternal and paternal genes get exchanged during crossing over. It results in variations among the offspring.

d. All the four chromatids of a homologous pair of chromosomes segregate and go over separately to four different daughter cells. This leads to variation in the daughter cells genetically.

= Nondisjunction =

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. There are three forms of nondisjunction: failure of a pair of homologous chromosomes to separate in meiosis I, failure of sister chromatids to separate during meiosis II, and failure of sister chromatids to separate during mitosis. Nondisjunction results in daughter cells with abnormal chromosome numbers (aneuploidy).

Meiotic Errors:
Abnormalities in which during meiosis, chromosomes fail to segregate or separate during anaphase, hence equal distribution of chromosomes among daughter nuclei does not take place.

Results and effects
Due to non- disjunction in gametes, there

1.      May be an extra chromosome than normal (n+1) or

2.      May be a decrease of a chromosome (n-1)

3.      If such gametes unite with a normal gamete (sperm), the resulting zygote will have a chromosomal abnormality.

4.      Thus an extra chromosome may be present in the new organism (2n+1) or an entire chromosome may be missing (2n-1).

5.      Result may be serious in the form of either physical, social or mental disorders.

1. Meiosis I 2. Meiosis II 3. Fertilization   4. Zygote The left image at the blue arrow is nondisjunction taking place during meiosis II. The right image at the green arrow is nondisjunction taking place during meiosis I. Non disjunction is when chromosomes fail to separate normally resulting in a gain or loss of chromosomes.

= I. Autosomal non-disjunction =

Cause
In this case, 21st chromosome fails to segregate, resulting gamete with 24 chromosome. If this gamete fertilizes normal gamete, the new individual will have 47 (2n+1) chromosomes.

Site
Non disjunction appears to occur in the ova and related to age of mother.

Occurrence
The chances of teenage mother having down’s syndrome child is one in many thousands, forty years old mother one in hundred chance and by forty-five the risk is three times greater.

Symptoms
The affected individuals have flat, broad face, squint eye with skin folds in the inner corner and protruding tongue, mental retardation and defective development of central nervous system.

= II. Sex Chromosomal non-disjunction =

Reason
These individuals have additional sex chromosomes e.g. 47 chromosomes (44 autosome + xxy)

Symptoms
These individuals are phenotypically male, but have frequently enlarged breast, tendency to tallness, obesity, small testes with no sperm at ejaculation and underdevelopment of secondary sex characters males with 48 chromosomes (44 autosomes + xxxy), with 49 chromosomes (44 autosomes + xxxxy), and male with 47 chromosomes (44 autosomes + xyy) are also observed.

Reason
These affected individuals have one missing x chromosome with only 45 chromosome (44 autosome + y)

Symptoms
Individuals with this condition often do not survive pregnancy and are aborted. Those who survive have appearance with short stature, webbed neck, without ovaries and complete absence of germ cells.