Meiosis and reproductive cycle

MEIOSIS AND REPRODUCTIVE CYCLE

The term meiosis was termed by J.B Farmer in 1905.Meiosis produces as total of four haploid cells form each original diploid cell.These haploid cells either become or give rise to gametes,which through union support sexual reproduction and a new generation od diploid organisms.So,the meiosis is required to run the reproductive cycle of eukaryotes such as microorganisms.Chlamydomonus,Neurospora;bryophutes plants and animals.The reproductive cycle includes a long dominant diploid and multicellular generation and a short,multicellular haploid generations called gametophyte generation.The tiny gametophyte is nurtured in specialized tissues of saprophyte.Male and female haploid cells called spores,are produced by meiosis in the diploid organism.In both animals and plants,male and female gametes unite during fertilization to produce a zygote in which the diploid chromosomes number is restored.In animals and simpler plants,the zygote matures to a new diploid organism.In the seed-producing plants,development is arrested at an early multicellular stage as a seed,which may remain stable for long time before germination permits a continuation of growth.Thus,reproductive cycle includes alternation of two generations :haploid and diploid and involves meiosis.

Kind of Meiosis

Meiosis occurs in the germ cells of sexually reproducing organisms.In both plants and animals germ cells are localized in the gonads.The time at which meiosis takes place varies among different organisms,and on this basis the process can be classified into : terminal,intermediate or initial.

1. Terminal meiosis. It is also called gametic meisis and its found in animals and a few lower plants.In terminal meiosis,the meiotic division occurs immediately before the formation of gametes or gametogenesis.
2. Intermediary or sporic meiosis. It is the characteristic of flowering plants.This meiosis takes place at some intermediate time between fertilization and the formation of gametes.It is also involved in the production of microspores and megaspores or in microsporogenesis and negasporogenesis respectively.
3. Initial or zygotic meiosis. It occurs in some algae,fungi,and diatoms.Meiotic division occurs immediately after fertilizations.In this case,only the egg is diploid.

Melocytes.The cell in which meiosis takes place are known as meiocytes.The meicytes of gonads are called gonocytes that may be spermatocytes in male and oocytes in female.The meiocytes of the plants sporangium are called sporocytes (i.e.,microsporocytes and megasporocytes).

Process of Meiosis

Meiosis superficially resembles two mitotic divisions without an intervening period of DNA replication.The first,meiotic division includes a long prophase in which the homologous chromosomes become closely associated to each other and interchange of hereditary material takes place between them.The first meiotic division is also known as the heterotypic division.In the second meiotic division the haploid cell divides mitotically and results into four haploid cells.The second meiotic division is also known as the homotypic division.Both the meiotic divisions occur continuously and each includes the usual stages of the meiosis.viz.,prophase,metaphase,anaphase and telophase.The prophase of first meiotic divisions is very significant phase because the most cytogenetical events such as synapsis, crossing over,etc.,occur during this phase.The prophase is the longest meiotic phase,therefore,for the sake of convenience it is divided into six substages,viz.,proleptonema,leptonema,zygonema,pachynema,diplonema and diakinesis.The successive meiotic substages can be represented as follows:

Heterotypic Division or First Meiotic Division

Meiosis starts after an interphase which is not very different form that of an intermitotic interphase.During the premeiotic interphase DNA duplication has occurred at the S phase..In the G₂ phase of interphase apparently there is decisive change that direcys the cell toward meiosis,instead of toward mitosis.The first meiotic division the nucleas of the meiocyte starts to swell up by absorbing the water from the cytoplasm and the nuclear volume increases about three folds.After these changes the cell passes to the first stage of first stage of first meotic division that is called prophase.

Prophase I

The first prophase is the longest stage of the meiotic division.It includes following substages:

1. Proleptotene or Prolepto-nemA. (Gr.,pro=before;leptas=thin;nema=thread).The proleptotene stage closely resembles with the early mitotic prophase.In this stage the chromosomes are extremely thin,long,uncoiled,longitudinally single and slender thread like structures.
2. Leptotene or Leptonema. In the leptotene stage the chromosomes become more uncoiled and assume a long thread-like shape.The chromosomes at this stage take up a specific orientation inside the nucleus ; the ends of the chromosomes converge toward one side of the nucleus that sidr where the centosomes lies.
3. Zygotene or Zygonema. (Gr.,zygon=adjoining).In the zygotene stage,the pairing of homologous takes place.The homologous chromosomes which come from the mother (by ova)and father (by sperm)are attracted towards each other and their pairing takes place.The pairing of the homologous chromosome is called synapsis .Three types of synapsis have been recognized:
4. Proterminal synapsis .In proterminal type of synapsis the pairing in homologous chromosomes starts from the end and continues towards their centromeres.
5. Procentric synapsis. In procentric synapsis the homologous chromosome starts pairinf from their centromeres and the pairing progresses towards the end of the homologues.

• Localized pairing or Random synapsis. The random type of synapsis occurs at various points of the homologous chromosomes.The pairing of the homologous is very exact and specific that is alignment of chromosomes is exactly gene-for-gene).The paired homologous chromosomes are joined by a roughly 0.2-um thick,protein-containing framework called a synaptonemal complex.SC helps to stabilize the pairing of homologous chromosomes and to facilitate the cytogenetical activity,called recombination or crossing over.SC is not found in those organisms in while crossing over does not occur (e.g.,the male fruitfly,Drosophilla melanogaster .

4.Pachytene or Pachynema; In the pachynema stage the pair of chromosomes become twisted spirally around each other and cannot be distinguished separately.In the middle of the pachynema stage each homologous chromosome spilts lengthwise to form two chromatids.During pachynema stage an important genetic phenomenon called “crossing over” takes place.The crossing over involves reshuffling,redistributation and mutual exchange of hereditary material of two parents between two homologous chromosomes.According to recent views,one chromatid of each homologous chromosome of a bivalent may divide transversely by the help of an enzyme the endonucleasewhich is reported to increase in the nucleus during this stage by Stern and Hotta(1969).After the division of chromatids the interchange of chromatid segments takes place between the non-sister chromatids of the homologous chromosome.The broken chromatid segments are united with the chromatids due to the presence of enzyme,ligase.The process of interchange of chromatin material between one non-sister chromatid of each homologous chromosomes is called the crossing over which is accompanied by the chiasmata formation.Stern and Hotta have reported that during the pachytene and zygotene stage,synthesis of small amount of DNA takes place.This DNA amount is utilized in the repairing of broken DNA molecule of the chromatids during the chiasmata formation and crossing over. The nucleolus remains prominent up to this stage and it is found to be associated with the nucleolar organizer region of the chromosome.

5.Diplotene or Diplonema. In diplonema,unpairing or desynapsis of homologous chromosomes is started and chiasmata are first seen.At this phase the chromatids of each tetrad are usually clearly visible,but the synaptonemal complex appears to be dissolved,leaving participating chromatids of the paired homologous physically joined at one or more discrete points called chiasmata.

6.Diakinesis. In the diakinesis stage the bivalent chromosomes become more condensed and evenly distributed in the nucleus.The nucleolus detaches from the nucleolar organizer and ultimately disappears.The nuclear envelope breaks down.During diakinesis the chiasma moves from the centromere towards the end of the chromosomes and the intermediate chiasmata diminish.This type of movement of the chiasmata is called terminilization.

Prometaphase

In the prometaphase the nuclear envelope disintegrates and the microtubules get arranged in the form of spindle in between the two centrioles that occupy the position of two opposite poles of the cell.The chromosomes become greatly coiled in the spiral manner and get arranged on the equator of the spindle.

Metaphase I

Metaphase I consists of spindle fibre attachment to chromosomes and chromosomal alignments at the equator.During metaphase I,the microtubules of the spindle are attached with the centromeres of the homologous chromosomes of each terad.The centromere of each chromosome is directed towards the opposite poles.The repulsive forces between the homologous chromosomes increases greatly and the chromosomes become ready to separate.

Anaphase I

At anaphase I homologous are freed from each other and due to the shortening of chromosomal fibres or microtubules each homologous chromosome with its two chromatids and undivided centromere move towards the opposite poles of the cell.The actual reduction and disjunction occurs at this stage.Here it should be carefully noted that the homologous chromosomes that moves towards the opposite poles are the chromosomes are either paternal or maternal origin.

Telophase I

The arrival of a haploid set of chromosomes at each poles define the oneset of telophase I ,during which nuclei are reassembled.The endoplasmic reticulum form the nuclear envelope around the chromosomes and the chromosomes become uncoil.In case of Trillium telophase I and interphase I do not occur and the anaphase I is followed by prophase II directly.

Homotypic or Second Meiotic Division

The homotypic or second meiotic division is actually the mitotic division which divides which divides each haploid meiotic cell into two haploid cells.The second meiotic division includes following four stages.

Prophase II

In the prophase second,each centriole divides into two ends,thus pairs of centrioles are formed.The microtubules get arranged in the form of spindle at the right angle of the spindle of first meiosis.The chromosomes with two chromatids become short and thick.

Metaphase II

During the metaphase II,the chromosomes get arranged in the equator of the spindle.The centromeres divide into two and,thus each chromosome produces two monads or daughter chromosomes.

Anaphase II

The daughter chromosomes move towards the opposite poles due to the shortening of chromosomal microtubules and stretching of interzonal microtubules of the spindle.

Telophase II

The chromatids migrate to the opposite poles and now known as chromosomes.After the karyokinesis each haploid meiotic cell,the cytokinesis occurs,and thus four haploid cells are resulted.These cells have different types of chromosomes due to the crossing over in the prophase I.

Significance of Meiosis

The meiosis has the greatest significance for the biological world because of its following uses:

1.The meiosis maintains a definite and constant number of the chromosomes in the organisms.

2.By crossing over,the meiosis provides an opportunity for the exchange of genes abd thus,causes the genetical variations among the species.

Thus the meiosis is a peculiar taxonomic,genetical and evolutionary process.

References

Dhami, P S, and J K Dhami. A textbook of zoology Vol. II and Vol.III. Latest edition. New Delhi: Pradeep publication, n.d.

Kotpal, R L. Modern textbook of Zoology. Meerut, India: Rastogi Publication, n.d.

Rastogi, S C. Cell, and Molecular biology. New Delhi: New Age International (P) Limited, 2001.

Verma, P S, and V K Agrawal. cell biology,Genetics,Molecular Biology,Evolution, and Ecology. New Dehli, India: S. Chand and company Ltd., 2012.