Virus

Viruses

Viruses are not cells, therefore, they are neither prokaryotes nor eukaryotes. They may be considered midway between the living and nonliving systems. In the free (extracellular) state, they are totally inert (inactive) and do not show any activity of life such as movement, growth, respiration, nutrition and reproduction. They may even be purified and crystallized much like the chemical substances as salt, sugar. In crystal form, they can be stored indefinitely without any change or damage. The crystals can be dissolved, and the vital particles are fully capable of infecting cells. When they get into a living host cell, they become active and multiply much like the living systems and may mutate also. Thus, the viruses resemble the living organisms in the intracellular state and the nonliving chemicals in the extracellular state. They have been defined as the living chemicals.

Since the viruses reproduce like living organisms, it is necessary to learn about them. In fact, their knowledge is essential as they are causative agents of many important plant and animal diseases.

The study of viruses is called virology. A specialist in virology is termed virologist.

Morphology: - the viruses are a heterogeneous group, showing a good deal of variation.

Size:- the viruses are too small to be seen with a light microscope, they can be photographed only with an electron microscope. They are scarcely larger than some very large single molecules of protein or nucleic acid. They vary in size from 50 Å (bactriophagus) to 2750 Å (psittacosis virus). Although individual virus particles are not visible under a light microscope, the latter often shows inclusion bodies in the virus-infected cells. These seem to be large colonies of virus particles.

Shape:- viruses vary in form too. They may be spherical, cuboidal, polyhedral, rod-shaped, and comma-shaped.

Structure: - a virus is far simpler than prokaryotes or eukaryotes cells. It exists in two very different states, one within a host cell and the other outside a cell. Outside the cell, the virus is a minute nucleoprotein particle, on virion, composed of a core of a single nucleic acid molecule, called viral chromosome, surrounded by a protein sheath termed capsid. It is also called nucleocapsid.

i.            Nucleic acid: the nucleic acid acts as the genetic material. It is DNA in some viruses, RNA in other. These viruses are respectively called DNA viruses and RNA viruses. The DNA is double-stranded but may be single-stranded as in ɸ viruses. It may be linear or circular. The RNA is single-stranded but may be double-stranded. The viral genetic material contains information for little more than production of more virus particles of the same kind. It is active in this function only when inside the host cell.

                                                           

forms of virus paticles

structure of a T-even bacteriophage

ii.            Capsid: The capsid is symmetrical or quasi-symmetrical. It is formed of a number of subunit or molecules termed capsomers. The latter vary in form, number and arrangement. The TMV virus has 2,200 capsomers in its capsid. The capsid may have molecules of one to over 50 different proteins. Certain highly specialized viruses, herpes virus, have around the capsid a membranous envelope derived from the plasma membrane of the host cell. The capsid protects the viral chromosomes during the extracellular state or its virus. It also helps in the recognition of the genetic material into it by contraction. The capsid contains certain ‘’ INTERNAL VIRUS PROTEIN’’ beside the genome. Viruses lack energy-yielding and biosynthetic enzymes. hence, they must necessarily be intracellular parasites. Viruses may have just 3 to as many as 500 genes. The fewer the genes, the more the virus depends on the host cell for materials it needs for  its multiplication.

                                                             

linear DNA molecule released from the ruptured head of a T2 bacteriophage.

Classification: - the virus are classified on the basis of the host they infect, the organ and tissue infected, the mode if transmission, and the disease symptoms. They are often divided into three main groups: bacterial viruses, plant viruses and animal viruses,

i.            Bacterial viruses: these viruses grow only within bacterial cells, which swell up and die. They are called bacteriophages or simply phages. They were discovered in 1917 by the French scientist d herelle. They are the most complex viruses. They may contains RNA or DNA in linear or circular form. The bacteriophagus occur in nature wherever bacteria are found, and are specially or strain of bacteria. Since the bacteriophagus kill bacteria, an attempt was made to treat patients suffering from bacterial disease such as dysentery and staphyloccus infections by using bacteriophagus. However, no bacteriophagus preparation proved successful to any significant extent. Bacterial viruses have played an important role in the development of molecular biology. They are widely used to investigate biochemical and genetic events.

I.            ‘’T-even’’ E.coli Bacteriophage. This is the most widely studied bacteriophage. It infacts the colon bacillus,Escherichia coli. It is called colliphage. It has the form of a lollipop and Consists of a polyhedral head, a short neck with a collar, and a straight tail ending in a base plate. The head measures 90*60 nm. It consists of a linear, double-stranded, greatly coiled DNA molecule surrounded by a protein capsid. The latter is composed of about 2000 protein molecules or capsomers. 

                                                           

chromosomes of influenza virus(A), poliomycelitis virus(B), and tobacco virus (C)

    The neck connects the head with the tail. It is surrounded by a narrow collar at its middle. The tail has a hollow core enclosed by cylindrical sheath of several different proteins. The phage DNA passes into the host cell through the core of the tail. The base plate bears spikers, each carrying a long, hair-like tail. The tail, base plate and tail fibres together make the phage look like a landing module for the moon.

II.            Plant viruses: -  these viruses attack the plant cells, disturb their metabolism and cause severe disease. They usually have linear RNA as the genetic material. The tobacco mosaic virus  is a common plant virus. Other important examples are southern beet mosaic virus and turnip yellow virus.                                                                             Tobacco mosaic virus(TMV):  it a  much studied virus. It is rod-shaped measuring 300*15 nm. Its genetic material is a single-stranded , linear RNA molecule coiled into a regular spiral extending through the axis of the rod. The nucleic acid is surrounded by a protein capsid of about 2200 elliptical, spirally arranged capsomeres. TMV wasisolated and crystallized by W.M.Stanley in 1935. Since then many other viruses have been obtained as crystals. Plant viruses have revealed that RNA can act as a genetic material.

III.            Animal viruses: - these viruses attack animal cells, and may cause fatal diseases. They have DNA or RNA molecule of linear or circular form. Some animal viruses, e.g., influenze and herpes viruses have around the capsid a membrane derived from the plasma membrane of the host cells. The envelope consists mainly of lipids are similar to those in the plasma membrane of the infected host cell.

                                                           

structure of tobacco mosaic virus (TMV)

Poliomyelitis virus: it is spherical in form. It consists a single-stranded RNA molecule surrounded by a protein capsid of 60 capsomers. A few viruses can infect animals as well as plant cells. For examples, potato yellow dwarf virus can grow in leafhoppers and in plants.

Mode of infection

Life cycle of T-even bacteriophage illustrates the general pattern by which the virus particles infect their host cells. Free bacteriophage particles come in contact with bacterial cells by random collisions. When a phage collides with its specific bacterial cell host, a protein on the surface of the tail fibres binds or adsorbs to a specific receptor protein on the bacterial cell wall. This interaction determines the host range of a virus. An enzyme from the tail core digests part of the bacterial cell wall. The head and tail sheath then contract and inject the DNA molecule into the host cell. The protein sheath remains outside as empty shell. The internal viral proteins, if any, may also enter the host cell.

The animal viruses are often taken up by the host cells by phagocytosis and their protein coat is digested away.

Reproduction

There are two modes of the reproduction in viruses: lytic cycle and lysogenic cycle.

1.    Lytic cycle: - on entering a host cell, the virus immediately starts reproduction and exploits the biosynthetic machinery, raw materials and catalysts of the host cell. First the viral nucleic acid is replicated nucleic acids then directs the synthesis of proteins for their coats. They form viral mRNA, on which viral proteins are synthesized, using host cell’s ribosomes, amino acid, tRNAs and other substances. The first formed RNAs formed later. 

                                                       

(A) virus attaches to a bacterium. (B) viral DNA passes into the host cell through a hole made by an enzyme from the tail core. (C) new viral DNA and protein molecules are synthesized in the host cell. (D) host cell wall is dissolved by a viral enzyme lysozyme to let the new virus paricles escape

    These viral proteins are of two types: some proteins act as inhibitory factors which stop cell metabolism, majority of proteins are used in constructing new capsid components. As the head and tail portions accumulate in the host cell, the replicated nucleic acid molecules get into the heads. Then the heads and tails join to form complete viral particles. After completion of viral particles, a final viral protein (lysozyme enzyme) cause breakdown of the bacterial cell wall. This releases the newly formed viral particles into the surrounding medium. The above sequence of events is called lytic cycle. The entire cycles takes about half an hour and produces about 1000 progeny phages. The new viral particles about 1000 progeny phages. The new viral particles remain inert unless they contact and infect fresh host cells, when another cycle starts.

    Up to 10,000 viral particles may be produced in a single human cell           infected with the polo virus, this shows that the viruses multiply very       rapidly. In their power of reproduction, the viruses resemble the living   systems.

                                                

maturity of an enveloped virus.

In many plants and animal virus infections, the host cells are not lysed, the dead host cell releases the virions as it gradually disintegrates.

Viruses with an envelope bud from the host cell and thereby acquire and envelope on their outside. The viral encoded protein pass through the host-cell membrane and project from its surface. The virion containing nucleic acid and internal viral proteins escapes from the cell by budding through the plasma membranes, acquiring a phospholipid bilayer envelope having viral proteins on the surface. It may be added that the viruses do not really reproduce, but are reproduced by the biosynthetic machinery of their host cells. This is what the viruses do not grow on cell-free culture media.

Growth

There is never any kind of growth stage in viruses. They are assembled from the components directly into the mature-sized virion.

2.    Lysogenic cycle

The DNA of phage, on entering  and E.coli cell, may behave  in one of the two ways. It may undergo the lytic cycle and   produce more phages as described above. Alternatively, it may integrate with the help of the enzyme integrase, formation a prophage. The viral DNA does not exploit the host’s machinery to form more virus particles, but replicates along with the host’s DNA. the replicated prophages pass from the parent host cell into daughter cells during cell division. Existence of the phage DNA as a part of the host’s DNA is called lysogeny.

                                                            

lysogenic cycle of temperate bacteriophase such as lamda phage

 Such viruses, called  lysogenic or temperate phage, do not produce any visible effect on the host cell. Many animal viruses also show lysogenic cycle. The most important of these are retroviruses of eukaryotes.

Inheritance

Inheritance in viruses occurs by genes, and the nature and behavior of viral genes are the same as those of cellular genes. The phenotype of a virus is represented not only by the structure of the virus particle itself, but also by the effect it produces on the infected host cell.

Genetic crosses and recombination

If two or more different viruses simultaneously infect the same cell, recombination can occur between their DNA molecules. At some point during the formation of new virus particles in the infected cell, viral DNA molecules from the different parental types may pair and cross over by breakage and exchanges.The recombinant viral chromosomes are then packed in protein coats, and when the host cell ruptures, are released to the medium to infect fresh host cell. The viral recombinants are detected through biochemical’s changes, such as in the proteins of viral coats or the phenotype developed by the infected host cell. Like the cells or organisms, virus strains may be characteristics as wild type or mutant there is inherited by the progeny, and one type may be converted into the other by mutation. Mutation involving gene exchange enables viruses to undergo evolutionary process. This is another resemblance between viruses and living organisms.

Cell division in animal and plant

Cell division

Importance of cell division:-

Cell division is a means of multiplication in the unicellular organisms. In multicellular organisms, it begins about embroyonic development and growth, and also plays a role in repair and maintenance of the body, and also in reproduction, both sexual and asexual.

Modes of cell division:-

Cell division occurs in three ways: amitosis, mitosis and meiosis. In each case, division of the nucleus proceeds the division of the cytoplasm.

      I.            Amitosis: amitosis was firstly described by Robert Remak in the red bloods cells of chick embryo.

Mechanism: Amitosis is very simple. It occurs without the formation of spindle and appearance of chromosomes. It is therefore often called the direct division. The nucleus of a cell elongates and develops a constriction round its middle. The constriction gradually deepens and finally cuts the nucleus into two daughter nuclei. A similar constriction appears in the cytoplasm between the two nuclei and divides the cell into two daughter cells, each with a nucleus. The daughter cells receive approximately equal amounts of nuclear and cytoplasmic materials.

                                             

stages in amitosis

Example.  Amitosis is rare, probably because it is not an exact method of cell division. It takes place in certain specialized cells, like those in the mammalian cartilage, in the degenerating cells of the diseased tissues, and in the old tissues. Foetal membranes of some vertebrates grow by amitotic cell divisions. The macronucleus of ciliates divides by amitosis. Its chromatin remains attached to the nuclear envelope throughout the division process.

   II.            Mitosis:                                                                                                             discovery:-  mitosis was first described by a German biologist Eduard Strasburger in 1875 and then by another German biologist Walther Fleming in 1879. It was termed ‘’mitosis’’ by Walther Flemming in 1882.

Occurrence: mitosis is the common method of cell division in eukaryotes. It takes place in the somatic cells of the body. Hence, it is also known as the somatic division. It occurs in the gonads also for the multiplication of undifferentiated germ cells. It takes place in the meristematic tissues in plant cells.

Duration:- mitosis often lasts on an average fro 30 minutes to 3 hours.

Definition: mitosis is the division of a parent cell into two identical daughter cells each with a nucleus having the same amount DNA, the same number and kind of chromosomes and the same heredity instructions as the parent cell. Hence, it is also known as the equational division.

Mechanism: mitosis is an elaborate process which involves a series of important changes in the nucleus as well as the cytoplasm; therefore, it is often called indirect division also. There are two main events in mitosis: karyokinesis or duplication of the nucleus, followed by cytokinesis or division of the cytoplasm. This is followed by separation of the daughter cells.

A.  Karyokinesis: karyokinesis in eukaryotes is complex due to the presence of many chromosomes. Through a continuous process, karyokinesis may be divided into four stages: prophase, metaphase, anaphase and telophase.

1.   Prophase: the DNA molecules combined with histone and nonhistone proteins from the chromosomes are greatly extended and spread throughout the space in the nuclear compartment. The nucleus of a human G₂ cell has approximately 4 meter of DNA organized into 46 duplicated chromosomes. In the extended state, the chromosomes are indistinguishable, and are together referred to as chromatin of the nucleus. The extended state of interphase of transcription and  replication, but not for processes of transcription and replication, but  not for division into two daughter cells. To facilitate the separation of duplicated chromosomes into different cells, evolution has provided a mechanism by which the chromatin is condensed and compacted into the so called mitotic chromosomes.

The transition from G₂ phase to M phase is induced by some stimulatory agent produced in the cell. This is indicated by the fact that fusion of a mitotic cell with a nonmitotic cell, induces premature condensation of the chromatin in the latter’s nucleus.

The prophase is long and complex. It lasts for about 50 minutes. It may be further divided into three sub stages: early, middle and late.

a.   Early prophase: the following events take place in the early prophase of mitosis-

      1.           The cell becomes more or less rounded, and its cytoplasm turns more viscous.

   2.            The centrioles, already duplicates in the interphase, lie close to the nucleus. Short radiating microtubules assemble around them by polymerization of the tubulin dimmers. The two pairs of centrioles start moving to the opposite ends of the cell. The microtubules surrounding each pair of centrioles (diplosome) look like a star-like body called the aster. The microtubules, termed the astral rays, are not in contact with the centrioles, but are separated from them by an amorphous zone of cytoplasm known as pericentriolar cloud. As the diplosomes move apart, the microtubules stretching between them increase in number and elongate by incorporating more tubulin dimmers. 

  3.  The role of the asters is to shift the duplicated centrioles to the opposite ends of the cell. In these location, the centriole pairs will pass into separate daughter cells when cytokinesis occurs. The centrioles and asters have no role in the formation of the spindle. The latter can assemble without asters and centrioles.

    The functions of centrioles in mitosis are not clear. They may be concerned with orienting the spindle.

      4.    Between the separating asters, long microtubules assemble on one side of the nucleus and form mitotic spindle. The latter is wide at the equator and tapers toward the poles, hence the name spindle. This is so because the microtubules are more closely spaced at the poles than at the equator. The microtubules are arranged in bundles called spindle fibres. At each pole of the spindle lies the mother-daughter centriole pair.

   5. The chromosomes first appear as long, thin threads in the nucleus. This thread-like appearance of the chromosomes gives the cell division its name mitosis (mitos=thread, osis=state).                                                                                                                

formation of asters and primary spindle

6. The chromosomes gradually change into short, thick rodlets and become visible. This change occurs by loss of water and progressive coiling of chromosomes. The progressive folding and packing of the chromatin fibres into thick chromosomes is called condensation. How this process occurs is not clear. According to the most widely accepted hypothesis, the chromosomal protein histon H1 forms crosslink between chromatin fibres that fold and hold them together. Due to the duplication of DNA and chromosomal proteins during the interphase, each chromosome appears longitudinally double, consisting of two identical sister chromatids. The chromatids of each chromosome may be coiled about each other in the beginning. They are held together at the narrow region called primary constriction or cenrtromere. At this region, each chromatid has a disc-like structure, the kinetochore, where the spindle microtubules join it. It may be noted that the chromosomes are fully replicated and double at all the point along their length, including the centromere. Earlier, centromere was thought to be unreplicated region of the chromosome, joining its chromatids together.

1.      

b.   Middle prophase: the following events take place in the middle prophase of mitosis:

                        i.            The chromosomes further shorten and thicken, and their chromatids uncoil. Finally, they assume their characteristic forms and sizes, and become distinguishable individually. The chromosomes differ in the location of primary constriction and in the presence of secondary constrictions, besides size and shape.
                                                  

A metaphase chromosomes with the kinetochores jointed by spindles microtubules

                     ii.            Nucleoli become progressively smaller and finally disappear. This happens because rRNA synthesis drops or stops, and the nucleolar materials (partially processed ribosomal subunits and processing enzymes) are dispersal into the nucleoplasm. It may be reminded that ribosomal subunits are formed in the nucleoli by association or rRNA and ribosomal proteins.

                  iii.            Nuclear envelope begins to breakdown into small vesicles which disperse into the cytoplasm, becoming indistinguishable from the ER elements. The lamina dissociates into its protein subunits.

c.    Late prophase: the following events take place in the late prophase of mitosis:-

                     i.            The nuclear envelope breaks down fully, releasing the chromosomes and other nuclear contents into the cytoplasm. The chromosomes looking like short double rodlets lie randomly at the site formerly occupied by the nucleus.

                  ii.            The spindle assumes its proper form and size. It may contain 500-1000 or more microtubules.

               iii.            The centriole pair are pushed to the opposite ends of the cell by the growing spindle.

The spindle and the asters are together referred to as mitosis apparatus, although a similar structure is formed in meiosis also.

2.   Metaphase: the metaphase is short and simple. It lasts for 2-100 minutes. It involves the following events:-

                     i.            The spindle moves into the region formerly occupied by the nucleus.

                  ii.            The chromosomes move to the equatorial plane of the spindle.

               iii.            Some spindle microtubules extend to and join the chromosomes. These are called chromosomal or kinetochore microtubules. Two chromosomal fibres (bundles of chromosomal microtubules) are attached to each chromosome, one from one pole of the spindle is jointed to the kinetochore of one chromatid and  from the opposite pole of the second chromatid. Other spindle microtubules extend from pole to pole of the spindle and pass right by the chromosomes, not attached to them in any way. These are termed polar or interpolar microtubules. The polar microtubules do not necessarily extend from pole to pole; they may begin at one pole and end at some distance or even exist free in the cytoplasm of the cell.

                iv.            The chromosomes soon get aligned at the middle of the spindle in the form of a plate called equatorial or metaphase plate. This plate is at right angles of the long axis of the spindle, and is actually formed by the kinetochores, the arms of the chromatids trailing away on the sides. The directed movement of the chromosomes into position at the metaphase plate is termed congression.

The events which connect the chromosomes to the spindle fibres and bring them to the metaphase plate are sometimes referred to as prometaphase. Metaphase represents the stage in which chromosomes have fully aligned into a plate and await the separation of their chromatids. The chromosomes are perhaps maintained in the equatorial plate by the balanced tension exerted by the two chromosomal fibres that connect to the sister kinetochore to the opposite poles.

The entire chromosomal complement of a cell or species a seen in metaphase pf mitosis is called its karyotype.

3.   Anaphase :  the anaphase is very short and simple. It lasts for only 2 to 3 minutes. It comprise the following events:-

                                             i.            The sister chromatids of each chromosome slightly separate at the primary constriction so that their kinetochores stretch toward the opposite poles of the spindle. Separation of chromatids poles of the spindle. Separation of chromatids occurs in all chromosomes almost simultaneously. The chromatids are now referred to as chromosomes because they are no longer held to their duplicates.

Behaviors of chromosomes in mitosis

                                          ii.            After a brief pause, the chromatids separate completely form their former mates, and start moving to opposite poles of the spindle. As each chromosomes is bring pulled by its attached microtubules, its kinetochore leads and arms trial behind. With the result, the chromosomes are pulled V, J and I shapes, depending upon the position of the kinetochore.
                                                            

Separation of chromatids in anaphase

                                       iii.            As the chromosomes move toward their respectective poles, the two poles move farther apart by elongation of spindle. The anaphase ends when all the chromatids reach the opposite piles. Each pole of the spindle receives one chromatid from every metaphase chromosome, the two groups of chromatids (chromosomes) have exactly the same hereditary information.
                                                   

Chromosomes, aster and spindle in mitotic anaphase

The movement of the chromosomes is called anaphase A, and the extension of the poles is termed anaphase B. the mechanisms of these movements are discussed below –

Chromosome movements:  the forces responsible for the movements of the chromosomes are not still clear. One view suggests that the chromosomal microtubules generate the force for poleward movement of chromosomes. These microtubules, progressively become shorter, and this separates and pulls the chromatids toward the poles of the spindle. Shortening may be brought about by active sliding between the chromosomal microtubules and the polar microtubules, or by reduction in the length of chromosomal microtubules through the separation of tubulin subunit at their tips (kinetochore ends) it has been experimentally shown that disassembly can generate sufficient force to pull the chromosomes to the poles. Also a spindle isolated form a cell about to divide contracts when ATP is added. May  be that both the mechanisms play a role in shortening of the chromosomal microtubules.

Moving apart of spindle poles: elongation of the spindle takes place as under –

Some microtubules extend from the poles to a little beyond the equator, but are not joined to the chromosomes. These microtubules overlap in the middle of the spindle. In the region of overlap, the microtubules activity slide past each other. This shortens the region of overlap and increase the overall length of the spindle by a distance equal to the decrease in overlap.
                                    

Anaphase movement. chromosomal microtubules shorten as anaphase proceeds. polar microtubules grow longer, increasing the length of the spindle

The spindle may also elongate by the addition of tubulin subunits to the ends of the polar microtubules, that is, by growth of microtubules makes the poles move farther apart.

Shortening of chromosomal microtubules and lengthening of polar microtubules by loss and addition of tubulin subunits occur at the same time in an anaphase cell.

Advantages of chromosome shortening:

Fictional significance of chromosome condensation, that occurs in prophase, become clear in anaphase. It is physically easier for a Shorty, compact chromosome to move through the cytoplasm than it is for a ling, twisted interphase chromosome.

4.   Telophase :- the telophase is long and complex. It lasts for an hour or so. In this phase, nucleus is reconstructed from each group of chromosomes. It involve the following events:

                        i.            The chromosome at each pole unfolds, and become long and slender. Finally they become indistinguishable as in an interphase cell.

                     ii.            Nuclear envelope is reconstructed around each group of chromosomes. This occurs gradually. First, membrane vesicles associate with the individual unfolding chromosomes, partially enclosing each chromosomes. Then they fuse to form an envelope surrounding the entire set of chromosomes (by now almost changed to interphase chromatin) at each pole. The lamina proteins reassociate simultaneously with the reconstruction of nuclear envelop and form a complete lamina within nuclear envelope. How the nuclear pores are formed is not known.

                  iii.            Nucleolar material (partially processed ribosomal subunits and processing enzymes) dispersed into the cytoplasm in the prophase return to the nucleolar organizer site and form a small nucleolus. Processing of this preexisting material then continues. Transcription of new rRNA also begins at this time, it gradually picks up until it attains the high level characteristics of interphase cell. With this, the nucleolus reformed at telophase, thus, contains both old and new rRNA and ribosomal proteins. If nucleoli form at more than one secondary constrictions, these nucleoli may fuse together or remain separate, depending on the cell type and the species. In humans, five pairs of chromosomes have nucleolar organizer sites and form 10 separate nucleoli during telophase. These usually fuse into a single nucleoulus during the subsequent interphase. Presumably , the dispersal of nucleolar material in prophase is an adaption aimed at reducing the chromosome mass required to be moved in anaphase. It probably also reduces the chance that the chromatids bearing nucleoli will tangle or fail to separate during anaphase.
                                                   

Breakdown and reconstruction of nuclear envelope and lamina in mitosis

                   iv.            With the transformation of chromosomes into chromatin and reconstruction of nucleoli, transcription of all the three RNA types gradually becomes normal.

                      v.            The spindle begins to disappear. This occurs by depolymerization of microtubules. The asters become small till only a few and short microtubules are left. This also occurs by depolymerization of microtubules. The centrioles then take up their characteristics interphase position close to the side of the nucleus. Short spindle microtubule persists for some time at the spindle equator. These tubules mark the region where the cytoplasm will later divide.
                                                         

Stages of mitosis in an animal

Accuracy of karyokinesis:- the two daughter nuclei formed in telophase are identical because they are formed form identical sets of chromosomes. The accuracy of karyokinesis depends on two basic features of the process:

                                    i.            The arrangement of spindle microtubules to form two distinct poles in the cell, and

                                 ii.            The connection of two chromatids microtubules chromosome to the opposite poles of the spindle to ensure their delivery to opposite poles.

B.   Cytokinesis: cytokinesis, the division of cytoplasm, encloses the daughter nuclei formed by karyokinesis in separate cells, thus completing the process of cell division. Cytokinesis is signaled at the metaphase by cytoplasmic movements that bring about equal distribution of microchondria and other cell organelles in the two halves of the cell. Division occurs differently in animal cells and plant clls. However, the spindle remains function similarly in both.

1. cytokinesis in animal cells: animal cell typically divide by a process called furrowing or cleavage.Short spindle microtubules that persist at the spindle midpoint, become surrounded by patches of dense, apparently structure less material. This material forms a layer called midbody, which soon extends completely across the cell. 
                                         

Cytokinesis by furrowing in an animal cell

Then a constriction of furrow appears in the plasma membrane all round the cell at the level of the midbody. The constriction is caused by a peripheral band of microfilaments that are formed by polymerization of actin subunits just within the cell membrane. The microfilaments have their long axes oriented parallel to the plane of thr furrow gradually deepens, following the plane of the midbody, until the opposite edges meet at the centre of the cell. Then the membranes fuse. With this, the original cytoplasm and the two daughter nuclei form two independent daughter cells. The latter are about half size of the original mother cell. They enter the G₁ phase of the next cell cycle.

As the furrow deepens, the midbody is compressed and becomes smaller, and finally disappears as the two cells are fully demarcated.

The above mechanism of cytokinesis is called contractile ring theory.
                                         

Cytokinesis in a plant cell by cell plate formation

It is clear from the above description of furrowing that the orientation of the spindle determines the plane of cytoplasmic division. Generally, the spindle lies with its midpoint at the cell equator so that daughter cells formed by cytokinesis are of equal size. In some cases, such as developing animal’s eggs, spindle is formed on one side of the cell and the furrow formed opposite the spindle midpoint divides the cell into two unequal daughter cells. What determines the alignment of the spindle is not known.

The division of the cytoplasm by furrowing is not confined exclusively to the animal cells. It occurs in a few kinds of plant cells and in some groups of protists. Pollen-forming meiotic cells in some flowering plants divide by furrowing.

2. cytokinesis in plant cells: a plant cell, due to the presence of a rigid cell wall, cannot divide its cytoplasm by an invaginating cleavage furrow. Hence, plants cells divide by cell plate formation. Short spindle microtubules persist in the midpoint of the spindle in telophase. Membrane-bound vesicles appear among the microtubules in the central region of the cell during late anaphase or early telophase. They arise from the ER or golgi apparatus. They contain a dense material that represents polysaccharides precursors of cellulose and pectin for the cell wall. The vesicles gradually increase in number and form a continoues layer cross the cell at the former spindle midpoint. This layer of vesicles is called phragmoplast. It looks like the midbody formed in animal cell cytokinesis.
                                                

Stages of mitosis in a plant cell

In the central part of the phragmnoplast the vesicles fuse together. Their membranes from the plasma membranes of the two adjacent daughter cells and the cell grow towards the periphery (lateral walls of the cell) by the formation and fusion of more vesicles till they meet the side walls of the cell. The fully formed cell plate is called the middle lamella. Cellulose is then deposited on either side of the middle lamella to form the primary cell wall.

The primary cell wall is flexible to allow the growth of the cell. A full-grown cell later forms a rigid secondary cell wall internal to the primary cell wall. The primary and secondary cell walls are formed of secretions produced in the cell. At places, the new cell wall retains canals through which cytoplasmic strands interconnect the adjacent daughter cells. These connections are called plasmodesmata. They allow movement of materials between the adjacent cells.

Orientation of the spindle in metaphase determines the plane of cell division in plant cells as in animal cells. Generally, the spindle is formed at the middle of the cell so that the daughter cells formed by cytokinesis are equal in size. In certain plant tissue, the spindle is formed on the side in the cell and the daughter cells formed are unequal. As in animal cells, the factors that determine the alignment of the spindle in plant cells are not known.

It should be noted that the cytokinesis in an animal cell begins at the periphery and proceeds inwards, whereas in a plant cell it starts centrally and proceeds outward.

In prokaryotes and some eukaryotes, cytokinesis occurs in a different way. The plasma membrane invaginates around the middle of the cell, and new cell wall forms alongside. As the plasma membranes pinches in closer toward the centre of the cell, the new wall extends till both join at the centre and completely separate the daughter cells. As the separation of the daughter cells proceeds from the outside inward, the process resembles the animal cell furrowing.
                                                 

Cytokinesis in bacteria, algae and fungi

The mitotic process described above is for diploid cells, but the process is similar in haploid cells such as those the gametophytic generation of plants.

C.   Cell separation: the daughter cells soon separate by secretion intercellular substance between themselves. To begin with this substance is jelly-like hyaluronic acid, but later other materials may permeate it.