Cell Division 1

 

The mechanism of cell division both mitosis and meiosis

And Cell Cycle regulation

 

CELL DIVISION

Cells of all organisms undergo cell division at one of the other stages of their development.  In many unicellular forms, cell division is an important mode of multiplication.  But in multi-cellular organisms, cell division is absolutely required for growth.  Reproductive elements like gametes are the other important products of cell division.

 

Types Cell divisions:  Organisms exhibit two types of cell divisions.  This is based on the pattern of distribution of parental chromosomes to the daughter cells.  They are Mitosis and Meiosis; however in prokaryotic organisms like bacteria and blue green algae, where there is no organized nucleus, the cell division is equational and it is called Amitosis, for the mitotic apparatus and such complicated chromosomal movements are absent.  Whatever may be the types, all cell divisions involve two important events viz, Nuclear division called Karyokinesis and cytoplasmic division called Cytokinesis.

 

Amitosis

Amitosis is also referred to as Binary fission or direct division.  This type of division is employed by lower prokaryotes, not only for multiplication of cells but also for reproduction.  In this context, a bacterial cell has been taken as an example of describe the process.

At the time of bacterial cell division, the circular DNA molecule that is attached to the mesosome membrane, starts replicating at the initiating point which is located very near to the mesosome.

 

 

 The chemical components of mesosomes are responsible for initiating replication, which will be soon completed.  Then the daughter molecules, still attached to the membrane, open out and segregate.  A little later, almost in the middle region of the cell, the plasma membrane produces an inward invagination all-round the cell and it progresses till the inwardly growing membranes fuse with one another in the center.  This results in the division of cytoplasm into two compartments.  Soon, the newly formed plasmalemma secretes the cell wall materials into the space found between them.  Then the middle wall splits across in the middle and two daughter cells separate.

 

It is important to not that this type of cell division does not involve any complicated structures but the cell divided by just DNA duplication and cytoplasmic division by cleavage.

 

MITOSIS

Multicellular organisms start their development as a unicellular zygote.  Under favourable conditions unicellular organisms multiply and produce a huge population. A fertilized egg may develop into a giant plant.  Some haploid organisms produce spores or gametes as a means of reproduction.  All the above processes are achieved by mitotic divisions.  This process involves equal distribution of genetic material.

 

 

 

The process involves two important steps.  The first is the division of nucleus (karyokinesis) and the second which normally follows karyokinesis is called cytokinesis.  Further-more depending upon the presence or absence of astral elements, it has been classified into astral type and anastral type.  However, the whole process of cell cycle progresses sequentially through different stages, which merge with one another smoothly.  Basing on the complex biochemical and physical changes, different stages have been recognized.  The stages are interphase, prophase, metaphase, anaphase, telophase and finally cytokinesis.

 

The time required for these different stages vary from cell type to cell type and organisms to organisms.  Nevertheless, the interphase is the longest stage and the most variable.  For example: in the root meristems of Vicia faba and Pisum sativum, the total time required for the whole cell cycle is about 24 hours, out of which interphase occupies about 21 hours and a half.  On the other hand prophaseto telophase requires just about two hours and a half.  On the contrary, in the corneal epithelial cells of rats, the whole process requires just 60 minutes whereas the interphase takes up 14-24 hours.

 

Interphase:  In yester years, interphase was considered as the resting stage, but in actuality it is one of the most active stages for it is the stage at which the cell prepares itself for the entire proceedings.  Because of its long duration and varied biochemical activities this stage has been further sub-divided into phases like G1, S and G2 of which G1 phase is the most variable in duration.  Intense biochemical activities take places in this stage and all precursors for DNA synthesis, histone synthesis, assembly of proteins etc., are either mobilized or synthesized.  Sometimes, the cell cycle gets arrested at this stage and undergoes a resting period called Go stage which ranges from few hours to years.  At this stage all activities of the cell come to stand still; such a stage is called G0 phase.  However, once the G1 stage is on, single stranded chromosomes move on to the next stage called ‘S’ phase.  At this stage chromosomes undergo duplication.  During this process, the long DNA double helix unwinds and semi-conservative replication is initiated at several points simultaneously.  Then the replication work progresses in both the directions till they meet the neighbouring replicons.  During replication, one of the daughter DNA molecules retains the parental histone core proteins and the other gets associated with the newly synthesized histone units to form a new chromatin thread.  Thus two chromatin strands are formed in about seven hours of time, while G1 stage takes about 5 hours.  Once the chromatin threads are duplicated, G2 phase is initiated.  At this stage intense biochemical activities required for chromosomal contraction and development of mitotic apparatus take place.  This state lasts for about 3 hours.  All these activities ultimately result in the increase of nuclear size and now the cell is set to enter into a dramatic condensation and movement phase.  It is important to note that through out this stage various types of RNAs are synthesized.  Even as the chromatin DNA is undergoing replication, RNA synthesis continues.

 

Prophase: 

Prophase sets in as cell cycle factors operating in regulating cell division events are ready to go.  The cell volume has reached a critical stage with all its components are build up, the nucleus meanwhile also enlarges as the  chromosomal DNA undergoes replication and the daughter chromatids fully form, the nuclear membrane starts dismemberment aided by lamin components as small membrane vecicles including pore complexes.  Appearance of mitotic apparatus and the dismemberment of Nuclear membrane coincide.

Metaphase: As the nuclear membrane disappears tractile fibers with continuous fibers appear in dome shaped form, which then called mitotic apparatus.  Tractile fibers join and attach with kinetochore microtubular elements.  By contactile mode the tractile fibers bring all the chromosomes (which divalent i.e contain two sister chromatids, but the centromere is still intact and single.  Interestingly, at CEN region kinetochore complex organizes on either side of the CEN region, thus all mitotic chromosomes contain two kinetochore complexes; they with their tractile fibers are oriented toward their respective poles.  It is at the end of the metaphase the CEN region appears to be split for the kenetochores bound by their respective tractile fibers are oriented towards their respective poles.

Anaphase: Once duplicated chromosomes with their chromatids placed in the mid region of the cell or what is called equatorial plate, start moving towards their respective opposite poles.

In astral type cells, the beginning of the prophase is indicated by the division of centrioles and the formation of radiating fibres all-round these daughter centrioles.  As the nuclear volume increases, one of the astral points start moving towards the other pole. Thus mitotic apparatus develops. Both in plant cells and animal cells tractile fibres which are associated with the kinetochore start depolymerizing at the poles.  Thus chromosomes move along with the tactile fibres.  Perhaps, simultaneously or at a little later stage, continuous fibres grow longer with the polymerization of tubulins from the polar ends.  Thus they stretch the mitotic spindle and greatly aid in the movement of chromosomes. This process requires ATP as the energy source.

 

Sliding Theory:  This theory envisages the presence of microfilaments associated with microtubules.  The presence of Actin and Myosin units at the polar regions has been detected by antibodies raized against actin and myosin.  Similar to that of muscular contraction, the microtubules of tactile fibres interact with acto-myosin proteins found at the poles and slide over each other.  Thus the contraction of tactile fibres towards the pole is brought about.  This process also requires ATP as the energy source.

 

Present concept:  Even though both the above said theories are attractive, each of them has its own drawbacks.  It is presumed that both the mechanisms may be operating simultaneously.  To begin with, the sliding mechanism starts pulling the tactile fibres at the poles, at the same time the tactile fibres undergo deploymerization and continuous fibres get elongated by polymerization of tubulins.  Nevertheless, the knowledge about the molecular mechanisms of the organization of mitotic apparatus and the exact role in chromosomal movement and cytokinesis is far from clear.

 

Telophase: In this stage single stranded chromatids that are pulled towards their respective poles start aggregating; simultaneously chromosomes start decondensation.   Thus the chromosomal strands become longer.  At this stage transcription activity of chromosomal DNA begins.  At the same time the nuclear membrane vesicles start appearing all-round the chromosomes and soon these membranous bits with pore complexes organize into nuclear envelope. The relaxed chromatin attaches to matrix proteins at regions called MARS.  As the chromosomal strands recoil and relax, the nucleolar DNA present in the region of secondary constriction loops out and nucleolar region begins to get organized. The nacked rRNA genes in clusters start transcribing precursor rRNAs.  It is at the same time very many pre-rRNA processing snoRNAs and their associated proteins assemble in the nucleolus.  A little later the incoming ribosomal structural proteins assemble on r.RNA and nucleolus gets organized.  It is important to note that chromosomes at this stage remain single stranded.

 

Cytokinesis

Generally karyokinesis leads to cytokinesis, but in certain organisms like plasmodia, siphonales algae and others, cytokinesis does not follow karyokinesis and repeated nuclear divisions lead to multinucleate cells or coenocytic cells.  Similarly, during early development of liquid endosperm in coconut fruits, nuclei divide repeatedly before cytokinesis sets in.  The mechanism of cytokinesis in animal cells and plant cells vary; in the former case it is achieved by cleavage and in the latter, cytokinesis takes place by phragmoplast formation.

 

 

Cytokinesis by cleavage:  In animal cells, cytokinesis sets in at late anaphase or early telophase.  The appearance of dense materials at the equatorial region of mitotic apparatus is the first indication of cytokinesis.  Gradually a number of membranous vescicles appear at this region and then a ring of depression further deepens into a constriction or deep furrow, finally it leads to the division of cytoplasm into two units.

 

 

 

However, during cleavage form of cytokinesis, at the equatorial region a ring of acto-myosin filaments appear in the cortical region of the cell.  Using ATP as the source of energy these acto-myosin filaments interact.  As a result the associated protein filaments contract and the membrane to which these protein filaments are bound is drawn inwards all-round till the membranes fuse in the middle.  Thus the cells get separated.  The exact mechanism of membrane contraction involving microtubules, actin and myosin and ATP is not clear.  Nevertheless the presence of these structures in and around the mitotic apparatus is known.  Their involvment in the cleavage is just a presumption, of course with valid reasons.

 

With the development of deep constriction, some of the spindle fibres disappear due to disassembly of microtubules into tubulin monomers.  Even after cleavage, some remnants of microtubules that are found at the centriole region, also disapperar at the end.

 

Cytokinesis by Phragmoplast:  Plant cells which do not possess centrioles, during cytokinesis produce numerous membranous vescicles derived from Golgi complex and endoplasmic reticulum (SER).  These vescicles appear at the interzonal region of the equatorial plate as well as at the microbutules found at this region.  With time lapse, some more microtubules are added to the peripheral mitotic spindle. Thus the mitotic apparatus appears to be bulged.  Such a bulged structure of mitotic apparatus is called phragmoplast.  Later, the vescicles found in the equatorial region within the mitotic apparatus fuse with one another and for a circular membranuous cisternae, which gradually extend laterally and reach the phragmoplast surface.

 

 

 

 

 

 

 

Finally both the cisternae and phragmoplast reach the lateral plasmamemrane and fuse with it.  Thus the cytoplasm gets divided into two compartments by the membranous cisternae which act as the newly formed plasmamembranes of the daughter cells.  The space found between these membranes will be soon filled up by calcium pectate which acts as the middle lamella.  Then the Golgi complex derived vescicles filled with cellulose fibres are directed with the help of microtubules towards the newly formed middle lamellae and a few cells wall, made up of cellulose fibres, is laid on either side of the middle wall.  Thus two daughter cells are produced.

 

Significance of Mitosis

All multicellular organisms, as well as unicellular organisms use mitosis as a mechanism for multiplication of cells.  During this process, chromosomes of parental cells duplicte and distribute equally to their daughter cells.  Here the term ‘equally’ denotes both quantitative as well as qualitative.  This process also helps in the growth of an organism.  In many organims where certain organs or cells are subjected to wear and fare, the cells are replaced continuously by mitosis, for example: in human beings there are about 2.5x1015 ; red blood cells and they have an average life of 420 days.  In order to maintain the constancy of the blood cells, the body produces about 2.5 million new cells every second to compensate the loss, which appears to be incredible, but human body does it with mitosis.

 

 

 

Mitosis and Cancer

Even though mitosis helps in the growth of an organism not only in the size but also in population, it is a highly related phenomenon.  By mutational studies in yeast cells, as many as 38 or more steps have been identified to take part in mitosis, of which some are highly crucial in the progression of mitotic stages.  If there are any mutations in the genome that control this process, cell division is completely inhibited or completely goes out of order or it may end up in an uncontrolled mitotic divisions.  Under normal conditions, particularly in multicellular organisms, mitotically derived cells undergo differentiation and perform specific functions. Instead, in an uncontrolled process, cells undergo continuous multipliation by repeated mitosis.  In these cases the cell derivatives do not undergo any differentiation, but they divide and redivide endlessly.  As a consequence of this, innumerable cells of the same kind are formed.  Such a group of cells which are endowed with a potentiality to divide and redivide ceaselessly, is called tumour cells and the disease thus produced is referred to as cancer.  This can be induced by various carcinogenic agents like drugs, X-ray irradiations and even some viruses.  Certain spontaneous mutations may also cause growth.

 

The analysis of cancer cells, indicates that the rapid and uncontrolled cellular divisions are due to some changes in the regulatory chromosomal proteins called non-histones.  Indentification of such causative non-histones is very essential and important to cure the cancer disease.

In plants, however, callusing or callus formation is another example of uncontrolled, undifferentiated tumour formation.  Nevertheless the callus formation is known to be controlled by certain phytohormones like auxins.  The special feature of these hormones is that at particular concentration, they induce tumour formation in plant cells, but at a different concentration with other hormones like cytokinins, they may induce differentiation of shoots or roots.  The probable mechanism by which the hormones cause callus formation is again attributed to differential gene expressions or due to certain modifications of nonhistone proteins, which actually trigger off the cellular components to undergo such uncontrolled cell divisions.

 

Mitosis and cloning

Development of a multicellular organism always begins with the zygote which is nothing but the product of syngamy.  The zygote inturn undergoes repeated but controlled cell divisions which is followed by cell differentiation, where the cell derivatives develop into different types of cells which have their own characteristic structures and functions.  The overall growth of an organism, thus depends upon a controlled, determinate cell division and differentiation.  The molecular basis of such cell differentiation is not clear, though certain differential gene expressions in E.coli, Drosophila and others have been very well studied.

 

Using the property of cell’s totipotency, where a single cell could be induced to develop into a complete organism, biologists have succeeded in the clonal propagation of plant in general and animals in specific cases.  Normally, the production of off springs involves sexual reproduction, where two parents contribute the gene pool through gametes.  Such offspings possess the mixture of genes from their parents.  Instead, if diploid cells of one of the parents is induced to develop into an offspring, then such offsprings are referred to as clones.  Such clonal propagation is in vogue, particularly in plants, where the technique of tissue culture has been very well exploited.

 

In this process, a cell or a group of cells from any part of the plant body is explanted into a known solid or liquid agar based nutrient medium.  If the medium is appropriate and balanced with the required phytohormones, the cell or cells explanted develop into callus, from which numerous embryos can be induced at will.  Later, the embryos can be cultivated.  This method has been successfully employed in cultivating horticultural plants, crop plants and also plants which are difficult to multiply by vegetative propagation or by sexual reproduction.

 

It is important to note, that this process has employed mitosis as the most important mechanism for cell multiplication.  nevertheless, this process of cell division is always followed or preceeded by a regulated differentiation.  Inspite of recent technical innovations, the molecular mechanism of differentiation is not known.

 

Unlike plants, clonal propagation of animals has been successful only in certain cases like frogs and rats.  In these cases diploid cells from the somatic tissues, rather than germinal cells, have been successfully used.  Either by the transplantation of the somatic cell into the mother uterus, or by the transplantation of a nucleus taken out from the somatic cell into enucleated zygotic cell, complete animals have been grown in the laboratories.  Such animals have the genome of only one parent and such offsprings are called clones.  Though clonal propagation of human beings has been attempted, the moral, social and ethical problems have deterred him from doing any further experiments.  Nevertheless, with the time and change of attitude towards the fellow human beings, perhaps, one day he may resort to such clonal propagation of man to preserve himself.

 

Even though conal propagation of higher plants and animals has its own implications as well as limitations, cloning of genes by Genetic Engineering techniques has been the craze of the day; its application in the welfare of fellow human beings is unlimited.  Many genetic engineering industries have been set up in USA and other European countries.  The trials and tribulations in developing this elegant but sophisticated technique are unsurpassed in the recent history of molecular biology.  The pace of development in this field is phenomenal and it can be compared only to the space and computer technology.  Biologist have already succeeded in cloning of genes for insulin, growth hormone, interferon and work is in progress to clone nitrogen fixing genes (nif genes) into eukaryotic plants.  Hitherto, man has relayed on specific organisms as the source of gene products, unfortunately the labour, time and money spent to extract them was exorbitant.  Added to this, the recovery was extremely poor. But the cloning techniques have made life easy and these products can be synthesized on a large scale, thus the cost of production as come down which is a great boon for common man.

 

MEIOSIS

Plants like ferns and sweet pea reproduce by spores and gametes.  Higher animals produce gametes as a mode of reproduction. In all the above said caes and other innumerable organism, the plant or the animal body is diploid (2 n), such organisms resort to sexual reproduction by means of gamates.

 

 

 

 

The gametes like sperms and eggs fuse to produce a zygote which is the first cell generation of a new offspring.  For example : human body is made up of diploid cells.  The reproductive organs are also diploid.  If the cells of these germ lines produce gametes of diploid nature, the fertilized product will be tetraploid.  And in successive generations the ploidy level goes on increasing.  But this does not happen the offsprings of the diploid parents will be always diploid.  This is achieved by a remarkable process called Meiosis.  The diploid reproductive cells produce haploid gametes by Meiosis, and such haploid gametes fuse to produce diploid offsprings thus the diploid chromosomal number is maintained between parents and offsprings by successive meiosis and fertilization.  Thus Meiosis is often considered as an antithesis for fertilizations.

 

Mechanism of Meiosis

Meiosis unlike mitosis takes place in spore bearing or gamete bearing cells. Most of the cells which are set to undergo meiotic division are quite large, distinct, rich in cytoplasm and possess large nucleus with a conspicuous nucleolus.  The cells show high rate of metabolic activity.  The duration of meiosis varies from organims to organism and from few hours to many days probably years.  It takes place in two successive stages (1) Meiosis I or Reduction state (2) Meiosis II or Equational division. Each of these stages further, shows substages like Interphase, Prophase, Metaphase, Anaphase, Telophase andCytokinesis.

 

Meiosis

Interphase: This stage is a preparatory stage for the subsequent events of karyokinesis.  This, like mitosis, is also subdivided into G1, S & G2 stages.

 

G1 Stage:  It occupies quite a large period of time at which cell prepares for the duplication of chromosomes.  The necessary precursors like nucleotides, proteins and many rich molecules are mobilized.  As a consequence of this accumulation, the nuclear volume increases.  The chromosomes found at this stage are extremely thin and they are not clearly visible except for certain heteropycnotic chromatin regions which appear as darkly stainable segments.

 

S-Stage:  At this stage, hitherto single stranded chromosomes undergo duplication (through DNA replication, histone and nonhistone assembly) into double stranded chromosomes.  Still the chromosomes are not clearly visible.  However the nuclear volume further increases.

 

G2 Stage:  This stage is comparatively of shorter duration and cell enters into prophase.  The details of molecular events are not clearly known.

 

Prophase I:  This stage is relatively longer and most complex; basing on the behaviour of chromosomes and appearance, this stage has been further divided into substages: sequentially these are referred to as leptotene, zygotene, pachytene, diplotene and diakinesis.  In all these stages, chromosomes continuously undergo condensation till metaphase.

 

Leptotene:  At this stage, the most invisible chromosomes gradually become visible.  This is due to condensation but still they appear to the single stranded structure, but are actually double stranded because of chromosomal duplication in the first interphase.  Here, most of the chromosomes appear to be looped into horse-shoe shaped structures, where the chromosomal ends are found to be assoicted with the nuclear membrane.  This association has been considered as very important for some proteins that are synthesized in the cytoplasm which get associated with chromosomes through the nuclear membranes.  This appearance of chromosomes has been referred to as “bouquet stage” at this stage, chromosomes show fine granular or bead like structures called chromosomes. They were equated to genes (Bellings) but later they were found to be nothing but coiled expressions of chromosomes.

 

 

 

 

 

 

Zygotene:  This is a stage which homologous chromosomes (derived from paternal and maternal side), start recognizing each other and initiate pairing.  The pairing may be initiated at any point, i.e. terminal, middle or any where.  However once the pairing is initiated it proceeds like a Zipper.  The process is so exact and precise, the pairing takes place– chromomere by chromomere and gene to gene.  In any one of the homologous pair of chromosomes, if there is any non-homologous segments get paired.  If there are any segments of chromosomes which remain non-replicated, they replicate at this stage and the DNA that is replicated is called Z-DNA which is rich in GC content.

 

 

 

Synapsis:  The pairing of homologous chromosomes, though dramatic and faseinating to observe, the forces and factors that are responsible for recognizing each homologous factors that are responsible for recognizing each homologous pair, and mechanists movement of chromosomes, have still remained unexplained.  Nevertheless, electron microscopic studies of some animal cells have revealed the presence of a ribbon shaped protein complex associated with the paired homologous chromosomes.  This complex has been named as Synaptonemal Complex.

 

 

 

 

Synaptonemal Complex:  This complex appears at the late leptonema or early zygonema, establishes fully at zygonema and pachynema.  This structure starts disappearing at the diplonema. Synaptonemal complex is exclusively made up of protein units, arranged in between a pair of homologous chromosomes in the form of a paired ribbon.  It consists of a pair of axial, a pair of lateral and a single central filament is the overlapping structure extended from he axial filaments.  The space found between a pair of axial filaments, lateral filaments is 0.15µm to 2 µm. On the other side of the axial filaments, lateral filaments are present and these are associated with the homologous chromatin material.  Some of the chromatin material penetrates through these protein complexes and open out into the central region as free DNA segments.  These may consists of individual genes or a group of gene segments.  The genetic material that is extruded from both the homologous chromosomes align parallel to each other, such that recombination may be brought about.  Notwithstanding the above observation, the nature of the proteins, and their functional mechanisms in pairing and recombination has remained as an enigma. It has been speculated that the synaptonemal complex is made up of proteins, which not only recognize the homologous segments of chromosomes, but also bring about the pairing and recombination of genetic material between them.

 

Pachtene:  At this stage, if there are any parts of homologous chromosome unpaired, the paring will be completed.  Then, at certain points, the chromosomal materials between homologous pairs exchange.  This phenomenon is referred to as recombination or crossing over.  This involves breakage and reunion between crossing over.  This involves logous chromosomes.  Use of radio-active precursors have shown that the chromosomal material, get degraded and resynthesized.  The enzymes that are involved in this process have been recognized as specific endonucleases, DNA polymerases, ligases and exonucleases.

 

 

Mechanism of cross over:  Basing on the microscopic observation of meiotic chromosomes, Darlington has opined that homologous chromosomes pair and coil relationally at zygotene under great strain.  This coiling brings about great torsion and strain on chromonemal strands.  Because of this, sister chromatids of both homologous points.  This breakage releases the torsion and strain; the chromatids decoil and come to rest.  If the broken ends of the non sister chromatids of two homologous chromosomes are brought near to each other, then they join, thus crossing over is brought about, resulting in genetic exchange between the parental chromosomes.  Thus crossing over is often summed up as the phenomenon of breakage and reunion between two non-sister strands of two homologous chromosomes.  At the gross level this appears to be true.  But break down

 

and the reunion of chromosomes involve the break down of chromatin materials like proteins, DNA, and the rejoining involves the resynthesis and reassembly of the chromosomal proteins. 

 

Molecular Mechanism of crossing over:  When homologous chromosomes are brought together or paired by synaptonemal complex, some of the segments or chromosomal regions of homologus chromosomes get disassociated with chromosomal proteins and the DNA strands from the apposing chromosomes loop out with the central region found between two axial filaments of SC.

 

 

 

 

At various points, all along the length of chromosome start aligning with each other so that the homologous DNA segments are brought near as well as parallel to each other.  Then certain endonucleases found in synaptonemal complex cut open the DNA strands as shown in the figure. 

 

 

 

As a result, the DNA segments break open with long sticky ends of DNA belonging to homologous pair of chromosomes have complementary nucleotide sequences.  This leads to the cross pairing of the single stranded DNA ends.  This pairing may result either in the overlapping with some part of the segment sticking out or certain gaps may be left out.  In the former condition exonucleases cut out the extra DNA segment and in the latter case, gaps are filled up by DNA polymerases.  DNA synthesized at the time of genetic recombination is called P.DNA.  However, the strands are finally sealed by DNA ligases.  Thus crossing over or recombination is brought about.  Such recombination events, particularly in prokaryotic organims like bacteria have been observed through electron microscope.  But in Eukaryotic organisms the visibility, even under high voltage electron microscope is not clear.  However, indirect studies like the incorporation of radioactive DNA precursors into meiotic chromosomes, have added some evidence to this theory.  Further more, the presence of dense nodules of 100 nm size found in the central component of SC have been assumed to be recombination nodules, involved in genetic exchange.

 

Diplonema:  The duration of this stage varies from few hours to many years.  for example in human females, oocytes develop in the germ line, when the human embryo is 5 months old, the ooctyes start meiosis and reach upto diplonema. 

 

 

 

 

At this stage meiosis gets arrested till the females reach the puberty  then the oocytes one by one complete meiosis and eggs are released periodically till the age of 50 years.

 

In this stage of diplonema the pairs of homologous chromosomes start moving away from each other, probably because of the disappearance of synaptonemal complex.  As the chromosomes repel at each other parts of chromosomal segments are still held at the regions of chromosomal exchange, which show up as X configurations and they are called chiasmata.  As the chromosomes further open out, the chiasma starts slipping away along the chromosomal strands.  This phenomenon is called Terminalization.

 

Diakinesis: At this stage chromosomes complete the terminalization and whatever synaptonemal complex that has remained, disappears completely.  Meanwhile the chromosomes reach maximum condensation.

 

The disappearance of nuclear membrane and nucleolus marks the end of Diakinesis and Prophase.

 

Metaphase I:  With the disappearance of nuclear membrane, mitotic apparatus appears.  The tactile fibres get attached to the kinetochore regions of the centromere of the double stranded chrosomes.  Unlike mitotic metaphase chromosomes in this stage each of the homologous chromosomes will have only one kinetochore, though centromere appears to be double.  The tactile fibre drag the respective homologous chromosomes will have only one kinetochore, though centromere appears to be double.  The tactile fibre drag the respective homologous chromosomes on the metaphasic plate or equatorial plate and orient the homologous chromosomes in such a way, one of the homologous chromosomes is directed towards one pole and the other is to the opposite pole. Thus a pair of homologous chromosomes found on the equatorial plate will have one group of tactile fibres emanating from one kinetochore of one set of chromosomes and another group of tactile fibres from the other chromosomes, which are directed to opposite poles.  This particular orientation is remarkable, for it is this that ensures reduction division.  Furthermore, the centromeres of the metaphasic chromosome do not divide and double strands of each homologous chromosomes remain intact.

 

Another interesting, as well as important aspect is that during the orientation of homologous pair in the equatorial region, the maternal and paternal chromosomes are at random.  This results in the random and indepdent assortment of genes.

 

Anaphase:  The respective homologous chromosomes are pulled to their respective poles.  During this movoement chromosomes undergo decondensation slightly.  The mechanism of chromosomal movement is same as that of mitoitic Anaphase movement.

 

Telophase:  All the chromosomes that move towards their respective poles are pulled together and chromosomes undergo further decondensation resulting in the formation of chromatin network.

 

 

 

 

Interphase II and Prophase II:  The interphase is the shortest stage.  Some times the meiotic cells skip the interphase and they directly move on the metaphasic stage.

 

Metaphase II:  The mitotic apparatus appears at this stage with a different polarity.  Chromosomes are brought to the equatorial region and the chromosomes are oriented in such a way that the tactile fibres originate from Kinetochrores that develop on either side of the centromere.  The tactile fibres are oriented in the opposite direction.  The centromeric region divides horizontally into two, which makes the end of metaphase.

 

 

 

 

 

Anaphase II & Telophase II: This single stranded chromosomes are now pulled to their respective poles.  During these stages, chromosomes undergo further decondensation.  Finally all the chromosomes in their respective poles get organized into a net work of chromatin.  Later nuclear membranes and their nucleoli appear.

 

Thus, starting from the single diploid nucleus, four nuclei with haploid chromosomal sets are formed.

 

Cytokinesis:  In the case of animal cells the cytokinesis is by cleavage formation, on the contrary phragmoplasts are formed in plant cells.  However, the cytokinesis always takes place after complete karyokinesis.  This may be successive or simultaneous.  The cour cells they derived from meiotic division are called tetrads.  These may further develop either into haploid spores or haploid gametes, which again depends upon the organims and the stage of life cycle.

 

 

 

 

 

 

 

 

 

 

 

Importance of Meiosis

1.     It acts as an antithesis for fertilization.  Genetic recombination takes place between parental homologous chromosomes.

 

2.    As parental homologous chromosomes arrange randomly at first metaphasic plate, chromosomes segregate randomly but independently.

 

3.    Genetic recombination and random segregation result in the production of gametes with variable genotype.

 

4.    Variable genotypes help in bringing about variation result in the production of gametes with variable genotype.

 

5.    This variation plays a significant role in evolution.

Mitosis

Meiosis

1.     It is an equational cell division

It is a reductional cell division.

 

 

2.    Chromosome number is maintained between cell generation.

Chromosome number is reduced to half the original number

 

 

3.    If helps in the growth of the population of cells and it takes place during growth and development

It takes place during gameto genesis.

 

 

4.    This process takes place at 5 different stages viz. Interphase, prophase, Metaphase, Anaphase and Telophase.

This process takes place at two stages called Meiosis I and Meiosis II.  Prophase I is further sub-divided into Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.

 

5.    Homologous chromosomes do not pair, do not exchange chromosomal segments by crossing over.

Homologous chromosomes pair and undergo crossing over.

 

 

6.    Homologous chromosomes do not undergo segregation.

They undergo random but independent segregation.

 

 

7.    The daughter cells produced are qualitatively and quantitatively similar and do not exhibit any variation

The daughter cells produced are qualitatively ad quantitatively dissimilar and exhibit variation.