Cell division (Meiosis) :
Describe the process of meiosis? Discuss the role of meiosis in sexual reproduction. What is meiosis? Describe the major features of each meiosis phase. Also discuss, why is meiosis needed for the production of gametes. Which phase of meiosis are the same as the corresponding mitotic phase and which are difference?
In this article we will discuss about meiosis:(1) Introduction of meiosis, (2) Factors which initiate meiosis, (3) Process of meiosis (4) First meiosis Division, (5) Second meiosis Division, (6) Significance of meiosis.
(2)Factors which initiate meiosis
(3)Process of meiosis
(4)First meiosis Division:
(5) Second meiosis Division:
*Meiosis-ll (Homeotypic Division Or equational division)
(6) Significance of meiosis
J.B. Farmer and J.E. Moore (1905) coined the term meiosis (Gr., meioun-to diminish) to describe a special kind of cell division occurring in sexually reproducing organisms. In comparison to mitosis, meiosis is rather an elaborate and complicated process of cell division. The germ cells from the gonads (ovary in males and testes in females) undergo meiosis and are called meiocytes.In sexual reproduction, the male gamete (sperm) and the female gamete (ovum) fuse to form the zygote which develops into an individual by repeated mitotic cell division. The gametes are haploid (n) whereas the organism and the meiocytes are diploid (2n). Thus, meiosis provides an opportunity by which the chromosome number is reduced to half i.e.. from diploid to the haploid condition. After fertilization, the diploid number of chromosomes is restored in the zygote Meiosis ensures genetic continuity and constant chromosome number in a species.
(2)Factors which initiate meiosis:
Meiosis is initiated by certain biochemical changes that occur in a cell. The ratio of RNA to DNA is significant for meiosis. A low value may lead to meiosis. Certain hormones are also known to influence t meiotic behavior in dividing cells.
Meiosis consists of two successive cell divisions. The first meiotic division i.e, meiosis I is called the heterotypic or reductional division during which the chromosome number is reduced to half. The second meiotic division i.e, meiosis II is very much similar to mitosis. It is called homeotypic or equational division as the chromosomes are equally distributed among the daughter cells. In meiosis, the chromosomes replicate only once but the nucleus and the cytoplasm divide twice in succession. As a result, a single diploid cell produces four haploid daughter cells.
(3) Process of meiosis:
(4)First meiotic division:
A cell in the pre-meiotic interphase is characterized by the following features.
- Increase in cell size.
- Increase in the number of cellular organelles.
- Synthesis of RNA, DNA and protein.
- Each chromosomes consists of two chromatids.
Meiosis I is a long continuous process. It comprises the following four phases which are repeated in meiosis II.
Prophase I is highly complicated and the longest phase in meiosis. It is further divided into five sub phases such as: (i) leptotene, (ii) zygotene, (iii) pachytene, (iv) diplotene and (v) diakinesis.
Leptotene (Leptonema) or Thin-strand Stage:
Leptotene is marked by the following events.
- The chromosomes look like slender threads, hence the name leptonema.
- Though each chromosome consists of two sister chromatids, it appears as a single strand.
- The chromosomes bear a large number of bead like structures called chromomeres. Chromosomes àre granular in appearance.
- Sometimes the centromeres of the chromosomes are oriented towards the centriole with their arms stretched out forming a flower bouquet like structure.
- Nucleus increases in size.
- Nucleolus grows in size.
- Seine must of DNA, RNA and protein synthesis takes place.
- Centrioles, asters and spindle fit constitute mitotic apparatus which starts appearing.
- The chromosomes further condense due to ceiling and folding.
Zygotene (Zygonema) Synaptotene or Mating Strand :
- Condensation of chromosomes continue with further coiling and looping. The chromosomes become short and thick.
- The centrioles move away from each other and spindle formation proceeds.
- Nucleolus becomes more prominent.
- The most important event of zygotene in the pairing af homologous chromosomes The maternal and the paternal chromosomes come closer to each other by mutual force of attraction. They lie side by side along their entire length in a ladder like fashion The pairing occurs part by part between homologous regions. Sometimes it begins at the centromere and proceeds towards the end or it was start at the end and nove towards the centromeres This pairing of homologous chromosomes is variously named such as synapsis, syndesis or syniusis. The synaptic pair of chromosomes are called bivalent of dyad.
Pachytene (Pachynema) or Thick-strand stage:
- After synaptie is over, the cells enter into pachytene.
- With continued thickening and shortening of chromosomes, the bivalents appear as thick strands.
- Among all the phases of prophase-l,pachytene is of longest duration.
- Each homologous chromosome from each pair of bivalents split longitudinally except at the centromeres The bivalents thus, assume a four strand appearance and constitute a tetrad.
- Intense ceiling occurs between the non-sister chromatids as a result of which they are twisted around each other.
- The points at which the non sister chromatids make contact and cross over each other are called chiasmata.
- Vigorous coiling puts severe strain on chromatids and the force caused due to such coiling is called synaptic force.
diplotene (Diplonema) or Double-strand stage:
- Diploene is marked by growth and metabolic activities
- Chromatoid bodies formed by the of intracellular substance appear in the cytoplasm.
- Nuclear membrane starts disappearing.
- Nucleolus begins to disappear.
- The force of attraction between the homologous chromosomes is replaced by a force of repulsion and the chromosomes suddenly start moving away from each other.
- The non-sister chromatids unwind and start separating but at the chiasmata the attachment is so strong that the chromatids undergo. breakage.
- The broken peices of chromatids rejoin with broken segments of non-sister chromatids. This physical and reciprocal exchange of corresponding chromosomal segments between the homologous chromosomes is called crossing over Crossing over results in the recombination of genes and genetic variability.
- The number of chiasmata formed depends on the length of the chromosome; shorter chromosomes contain fewer chiasmata while longer i chromosomes may have more than 12 chiasmata.
- Due to increased coiling and condensation the terminal chiasma slips off the end of the chromosome while the interstitial chiasma slide along the chromosome and becomes the terminal chiasma. This is called terminalisation.
Diakinesis or Moving Apart Stage:
- Certain events in this substage are similar to those of late prophase of mitosis.
- The nucleolus completely disappears.
- The mitotic apparatus is fully formed.
- The chromosomes continue to thicken, shorten and become more compact.
- The nuclear membrane breaks up and disappears into the cytoplasm.
- The bivalent chromosomes get separated and are evenly distributed in the cytoplasm. They move towards the periphery of the cell.
- The metaphasic chromosomes are still in bivalent condition.
- Each number of the bivalent migrates towards the equator. They are arranged in such a way that their centromeres come to lie on the opposites sides of the equatorial plane with their arms directed towards the equator.
- The spindle fibres extend between the centrioles and the centromere region of the bivalents.
- Each member of the bivalent now move towards the opposite poles of the cell. As a result, the tetrads with 4 chromatids get separated into dyads with 2 chromatids.
- In the process, the centromere moves ahead carrying the arms of the chromosome behind them.
- The number of chromosome at each pole is half of the number of chromosome in the parent cell.
- Thus, the chromosome number (2n) is reduced to half (n) at this sub-phase.
- The centromeres do not divide in anaphase-l.
- Most of the events of telophase I correspond to those of telophase of mitis.
- The appearance of haploid set of chromosome at each pole of the dividing cell marks the beginning of telophase 1.
- The chromosomes become thinner and longer by unfolding and uncoiling:
- The nucleolus ordinarily does not appear.
- A nuclear membrane is reformed around each set of chromosome.
- The spindle fless begin to disappear .
- DNA synthesis does not occur in telophase l .
- A construction appears on the coll membrane at the level of equatorial plane which progressively deepens and divides the parent cell into two daughter cells.
- The Cytokinesis in plant els occurs by cell plate formation.
Sometimes telophase-l is not followed by cytokinesis. The second nuclear division starts immediately. This results in two nuclei ist a common cytoplasm.
(5) Second meiotic division:
- The period between the first and the second meiotic division is insignificant as neither DNA synthesis occurs nor the chromosomes undergo replication Hence. it is appropriately termed as interkinesis. The meiocyte with two nuclei may directly enter into prophase-l from telophase-l.
Meiosis II (Homeotypic division or Equational division):
- The second meiotic division is identical to mitosis. The haploid (n) number of chromosomes are equally distributed between the two daughter cells. It consists of the following phases.
- Compared to the prophase 1. prophase short in duration.
- The disappearance of nuclear membrane marks the beginning of prophase II.
- Centrioles move towards the opposite poles.
- The spindle fibres start forming between the poles.
- The axis of the nuclear spindle in prophase II is at right angles to the axis of the spindle of prophase I.
- The two chromatids of a chromosome remain attached by a common centromere but their arms are widely separated.
- After the disappearance of nuclear membrane, the chromosomes are free and move towards the equator.
- The chromosomes arranged in the al plane in such a to that the center at the middle the equator with their arms directed towards the poles.
- The centromeres divide and the chemise away.
- The spindle fibres are fully formed and extend between the aster and the centromeres.
- The chromatids after being separated are now called chromosomes.
- The chromosomes along with their commer start moving towards their respective poles.
- Each pole of the dividing cell receives a haploids set of chromosomes.
- As in mitis, the telophase-ll in second meiotic division is involved with the reconstitution of nucleus.
- A nuclear membran appears around each haploid set of chromosome at each poles.
- The nucleolus is reorganized.
- The chromosomes uncoil and elongate forming a network of chromation reticulum.
- Cytokinesis or division of the cytoplasm follows telophase-ll and soon two haploids daughter cells are formed.Two successive cell divisions results in four daughter cells. In testes, meiosis (spermatogenesis) results in the production of four male gametes or sperms but in every oogenesis produces me um and three polar bodies.
(6)Significance of meiosis:
- Meiosis is a prerequisite for sexual reproduction. The gametes are formed by gametogenesis which is essentially a process of mitosis. The haploids gametes (n)fuse to form diploid zygote (2n) Thus, meiosis ensures constancy in chromosome number in a species.
- Crossing-over takes place during meiosis which brings about genetic recombination and variation.
- Variation is useful in evolution. Individuals with a wide range of variable characters are adaptable in a changing environment.
- Meiosis provides an opportunity for random assortment of genes.
- New linkage groups arise due to meiosis.