Mitosis



 Mitosis or  (indirect nuclear division)

In this type of division chromosomes  replicate and become equally distributed  both quantitatively and qualitatively into two daughter  nuclei so that the daughter cells come to have the same number and type of chromosomes as are present in the parent cell. It is, therefore,  also called equational  division. Mitosis was first observed by Strasburger in plant cells, Boveri and  Flemming in animal cells. The term mitosis was coined by Flemming.Mitosis occurs in the formation of somatic body cells and is hence often named as somatic cell division. In animals, mitotic cell division  is only seen in the diploid somatic cells. Against this, the plants can show mitotic divisions in both haploid and diploid cells.The usual sites of mitosis in a plant are meristematic regions like stem tip, root tip, intercalary meristem, lateral meristem, growth of embryo leaves, flowers, fruits, seeds, etc. In animals, it is found in embryo development  and some regions in the mature form like skin and bone marrow. While the plant cell does not show much change, the animal cell becomes spheroid, more viscous and refractile at the time of mitosis. Depending upon the type of cell and the species, mitosis  takes 30 minutes to 3 hours for completion.It consists of two steps: (i) Karyokinesis 
(ii) Cytokinesis



Discovery:

Mitosis was  first  described by a Grerman biologist Eduard Strasburger in the plant cells in 1875  and later by another German biologist Walther Flamming in the animal cells in 1879. It was termed ' mitosis'  by Walter Flamming  in 1882.

The discovery of mitosis can be attributed to several scientists who made significant contributions to our understanding of cell division. Here are brief descriptions of their discoveries:

1): Carl Wilhelm von Nageli ( 1842):  Nägeli observed and described the process of cell division, but his observations were limited to plant cells and he did not provide a detailed understanding of the steps involved.

2): Edouard van Beneden  ( 1883):  Van Beneden furthered the understanding of cell division by studying the eggs of roundworms. He observed the separation of chromosomes during cell division and proposed the term "mitosis" to describe the process.

 3): Walther Flamming ( 1882): Flemming is often credited with first accurately describing the process of mitosis in animal cells. He identified the division of the nucleus into two daughter nuclei and the subsequent separation of the cytoplasm, and he discovered and named structures known as chromosomes.

Walther Flemming (1843-1905) was a German biologist and histologist who made significant contributions to our understanding of cell division and chromosome behavior. He is often credited with being one of the founders of modern cytology.

Flemming's most famous work was his detailed observations of cell division, specifically in animal cells. In the late 1870s and early 1880s, he conducted extensive studies on the process of mitosis (cell division) and described it in great detail. He observed the division of the nucleus into two daughter nuclei and the subsequent division of the cytoplasm. Flemming also discovered and named the structures known as chromosomes, referring to their ability to take up different staining dyes.

In addition to his work on mitosis, Flemming also investigated other aspects of cell biology. He conducted research on various tissues and cells, studying their structure and functions. Flemming is also known for his studies on cell membranes and cell movements.

Flemming's research laid the foundation for our understanding of cell division and the importance of chromosomes in inheritance. His detailed observations and descriptions of mitosis paved the way for further research in the field and influenced subsequent scientists in the study of cytology and genetics. Today, Flemming is regarded as one of the pioneers of modern cell biology.

 4): Theodor Boveri ( 1902):Boveri proposed the chromosome theory of inheritance, which states that genes are located on chromosomes and are transmitted from one generation to the next through cell division.

These discoveries collectively contributed to our current understanding of mitosis, a fundamental process in cell division that is involved in growth, repair, and reproduction of organisms.

Occurrence:

Mitosis is the common method of cell division.It takes place in the somatic cells ( body cells) in the animals. Here it is also known as the somatic division. It occurs in the gonads also for the multiplication of undifferentiated germ cells. In plants it takes place in the meristematic tissues and during  the growth  of leaves, flowers and fruits.

Duration:

Mitosis lasts on an average from 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 of DNA, the same number and kind of chromosomes,  and the same hereditary  instructions as the parent cell.It is also known as the equational  division. 








Mechanism of Mitosis:


Mitosis is an elaborate process which involves  a series of important changes in the nucleus as well as 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.

(i) Karyokinesis or duplication of the nucleus,

(ii) Cytokinesis or division  of the cytoplasm 

A): Karyokinesis:

It is also called indirect nuclear division as the nucleus passes through a complicated sequence of event before forming two daughter  nuclei. Though the process is continuous one without any pauses,it has been divided into five phases* for the sake of convenience. They are prophase, prometaphase, metaphase, anaphase and telophase .Prophase is the longest while anaphase is usually  the shortest phase.

1): Prophase :

It has three substages  early, middle and later.In early prophase, the chromatin fibres shorten and thicken to form elongated  chromosomes. 

(a) Early Prophase:The  following events take place in the early prophase:

(i) The animal cell becomes somewhat rounded  due to depolymerization of skeletal microtubules into their protein subunits. The cytoplasm turns more viscous.The plant cell undergoes little change in form due to rigid cell wall around it. ER and Golgi apparatus  breakdown  into small vesicles . Gene expression, proteins  synthesis , secretion and cell motility stop. Cell's entire energy  is devoted to the process of division. 





(ii) The centrioles, already  duplicated  in the interphase, lie  close to the nucleus.They migrate in pairs to the opposite  ends of the cell.Short radiating microtubules assemble around them by polymerisation of tubulin subunits. The microtubules surrounding each pair of centrioles ( diplosome) look like a stat_ shaped body called aster³.The microtubules, called astral  rays, are not  in contact with the centrioles, but are separated   from them by a clear zone of 2, the pericentriolar cloud.As the centriole pairs move apart, the microtubules stretching between  them  increase in number and length. The role of the astral rays is to shift the centriole pairs to the opposite ends of the cell. In these locations,  the centriole pairs will pass into separate  daughter cells when cytokinesis occurs. The centrioles and asters have no role in the formation of a spindle. The latter can assemble  without asters and centrioles. 


(iii) Between  the separating asters, long microtubules assemble from tubulin subunits on side of the nucleus and form mitotic spindle.The Assembly of spindle microtubules starts at the centrosome, often called the microtubule organizing centre ( MTOC). The spindle is wide at the middle and tapers toward the  ends, hence the name, spindle.The middle thick part is called equator and the ends are termed poles.The microtubules are arranged  in bundles called spindle fibers. There may be 4 to 20 microtubules  in a spindle fibre.




(iv) The chromosomes first appear as long, thin threads in the nucleus. This thread__ like appearance of the chromosomes gives the cell division  its name mitosis¹. At this stage, the chromosomes overlap, have invisible ends and look like a loose ball of yarn. Because  of this appearance of chromosomes in early mitosis, the prophase was formerly termed spireme² stage.The chromosomes gradually change into short, thick rods, and become visible. The  change occurs by 


(i) Loss of water (condensation)and 

(ii) Progressive coiling of chromosomes  (spiralisation).

The progressive folding  and packing reduce them to 4% of their original  length. Due to the duplication  of chromosomes  that has taken place in the preceding  S phase, each chromosome appears  longitudinally double, consisting  of two identical sister chromatids.The chromatids of each chromosome may be coiled about each othe in the beginning.






 They are held together at the narrow region called primary constriction, or centromere.At this region, each chromatid has a disc_ like structure , the kinetochore³,where  the spindle microtubules join it. The chromosome's two kinetochores  face in opposite  directions. It may be noted that the chromosomes are fully replicated  and double at all point along their length, including  the centromere⁴. 






Earlier, centromere  was thought to be unreplicatef region of the chromosomes, joining  its chromatids together. 

(b) Middle  Prophase: The following events take place in the middle prophase:

(i) The chromosomes further shorter and thicken, and their chromatids uncoil. Finally, they assume their characteristic form and sizes, and become distinguishable individually. Besides form and size, the chromosomes  also differ in the location of primary  constriction ,in the length  of the arms, and in the presence  of secondary  constriction. 


(ii) The nucleoli start decreasing in size. They finally  disappear, their nucleic acid passes to certain pairs of chromatids.



(iii) Now the nuclear envelop begins to breakdown  and disperse into the cytoplasm as small vesicles  that become indistinguishable from the elements  of endoplasmic  reticulum.


(c) Late Prophase: The following  events take place in the late prophase.

(i) The spindle assumes its proper form and size.


(ii) The nuclear envelope breaksdown fully, releasing  the chromosomes  and other nuclear contents into the cytoplasm.

(iii) Central  cytoplasm  becomes fluid  to let the chromosomes  move freely toward the equator.

(vi) The centriole pairs are pushed to the opposite  ends of the cell by the growing spindle. 

Mitotic Apparatus:The asters and the spindle  are together referred  to as the mitotic apparatus although a similar  structure  is formed in meiosis also.

Spindle Types:
Two  types of spindle are formed in the cells. In the animal cells,  the spindle has an aster at each pole, and is said to be amphiastral¹.There are no centrioles in plant cells and no asters are formed. A spindle without asters is described as anastral ².


2): Prometaphase:

The beginning  of prometaphase is marked by the disappearance  of the nuclear membrane .When the  nuclear membrane dissolves, there is no differentiation  between  cytoplasm and nucleoplasm. The chromosomes are attached to the  spindles through their centromeres. This type of mitosis is called extra _ nuclear mitosis or eumitosis.In many protozoans and some animals  cells, however,  the nuclear  membrane does not disappear during cell division. The mitosis takes place within the nuclear membrane. This type of mitosis is called intranuclear mitosis or promitosis.In some protists, the centriole is present within the nucleus. In such instances, mitosis  is both intra nuclear and centric. When the centriole is outside the nucleus, mitosis is extranuclear and centric. When the nuclear membrane dissolves,a fluid area is observed in the centre of the cell. The chromosomes move freely through  this area as they proceed  towards the equator.

 The spindle apparatus( mitotic  apparatus) consists of numerous fine fibres. Each fiber is made up of four to twenty  microtubules.  The spindle fibres converge towards  th two ends, which are known as poles. In animals  cells, the poles are formed  by asters. Since there are two asters in animal cell, its spindle apparatus is called amphiaster. The spindle of plant cell, on the  other hand, is called anastral as it lack the asters at the poles. Anastral  spindle is also called acentric while amphiaster is known as centric spindle.

The spindle apparatus  has the maximum  diameter  in the middle. The area is called equator .The fibres of the spindle, without associated  chromosomes,  are of two types 
Continuous ( which run from pole to pole) and discontinuous ( which radiate out from one pole but do not reach the other pole). Chemically the spindle  consists of 90__ 95% proteins ( mostly  tubulin rich in sulphur containing  amino acids), 3.5__ 5% RNA and traces of lipids and other substances. 

3): METAPHASE³:

Metaphase is short and simple. It lasts for only 2__ 10 minutes.It involves the following  events:

(i) The spindle moves into the region formerly  occupied  by the nucleus, that is, middle of the cell.

(ii) The chromosomes, for sometime, lie irregularly in the middle of the cell. Soon they move toward the equatorial plane of the spindle. 

(iii) Now some spindle fibres extend to the chromosomes and become attached  to them at their kinetochores. These spindle fibres are called chromosomal, or kinetochore, or tractile fibres.Two chromosomal fibres are joined to each chromosome, one from each pole of the spindle.Other spindle fibres extend  from pole to pole of the spindle  and pass right  by the chromosomes, being  not attached  to them in any way. These are called the polar, or interpolar, or nonkinetochore, fibres. Some of them overlap at the equator .

(iv) At the equator of the spindle, the chromosomes get arranged in the form of a plate, called the equatorial, or metaphase plate.Metaphase is the best time to count the number and study  the morphology  of chromosomes. This plate is at right angles to the axis of the spindle, and is actually  formed of the kinetochores, the arms of chromatids trailing away.The centromeres are perhaps drawn to the equator pull of two chromosomal  fibres that connect the sister kinetochores to the opposite poles. The process of drawing  the chromosomes into the equator of the spindle is known as congression.

The events which connect the chromosomes to the spindle fibres and bring them to the metaphase plate are sometimes referred  to as prometaphase. 

    Time for Chromosomes Study: Metaphase is the most suitable time to determine the  number of chromosomes and study their morphology. 

4): ANAPHASE⁴:

Anaphase is very short and simple. It lasts for only 2__ 3 minutes. It comprises the following  events:

(i)  The sister chromatids of each chromosome separate from their respective  mates and begin to move toward the opposite  poles⁵.The separation of the chromatids starts at the kinetochores and the  arms trail behind. With the result, the  chromosomes are pulled into V.L,J, and I shapes depending  upon the position  of the kinetochores. 






(ii) As  the chromatids move apart, the polar fibres of the spindle elongate and the cell becomes  longer. The anaphase ends when all the chromatids reach the opposite  poles. The chromatids are now referred  to as chromosomes because they are no longer held to their duplicates.

Forces for Chromatid Movement:

The forces responsible  for the movement of the chromatids are uncertain. One theory suggests that the chromosomal fibres of the spindle shorten in the presence  of ATP and pull the chromatids to the poles. Shortening  of chromosomal fibres occurs  by disintegration of microtubules into tubulin subunits at their kinetochores ends. A chromatid shifts poleward as its kinetochore somehow keeps hanging on to the remaining  tip of microtubule just ahead of the zone  of depolymerization. There is evidence  to show that the kinetochore has motor proteins  which shift a chromosome along the shortening  microtubules. The chromosomal fibres finally  disappear. 






   A spindle isolated from a cell about to divide contracts when ATP is added. Electron  microscopy  shows cross_ bridges between  the polar and chromosomal spindle fibres, and this suggests  that the fibres slide past each other during chromatid movement, using ratchet_ type mechanism similar to that associated with the movement  of muscle fibres.

   Movement  of chromosomes has been called " ballet of chromosomes " by some authors.




Advantage of Chromosome Shortening:

Functional significance of shortening  of chromosomes, that occurs in prophase, becomes clear in anaphase.It is physically  easier  for a  short, compact chromosome to move through the cytoplasm than it is for a very long, twisted interphase chromosome. The interphase chromosome must be uncoiled and extended for proper error_ free replication  of DNA double strands.

In their extended from during interphase,the chromosomes are involved in controlling the synthesis  of all materials in the cell, but during cell division this function ceases. 

5): TELOPHASE¹:

Telophase in long and complex like the prophase.It lasts for an hour or so. In this phase, nucleus is reconstructed from each group of chromosomes. It involves the following  events: 


(i) Nuclear envelope  slowly appears  around each group of chromosomes  by  coming together  and fusion of small  vesicles  formed in prophase by breakdown of  parent cell's  nuclear envelope  and some portions of endomembrane system. Simultaneously,  the nucleoplasm gathers around the chromosomes. 

(ii) The chromosomes  gradually  uncoil and become long and slender. Finally, they become indistinguishable as in an nondividing cell. Two nuclei are thus formed  from one. The daughter  nuclei are identical  because they are formed from identical  sets of chromosomes. 

(iii) The nucleolar chromosomes  in each new nucleus  form new nucleoli at the nucleolar organizing regions from the nucleolar material dispersed in the prophase.The nucleoli start synthesizing ribosomes. 

(iv) The spindle fibres and the astral  rays gradually become indistinct, and finally disappear  by depolymerization of tubulin subunits  of microtubules. 

(v) Each pair of centrioles  remains surrounded  by centrosome. The  latter takes up its characteristic  interphase position close to the nucleus. 


(vi) The centrioles  may duplicate in the telophase in some cases. Frequently,  however, their duplication occurs in the interphase.

( vii) Viscosity of cytoplasm  decreases  and the cell assumes  its normal  form.



B): Cytokinesis¹ ( D _ phase):

Cytokinesis is the division of protoplast of a cell into two daughter cells. Normally  it starts towards the middle anaphase and is completed simultaneously with the telophase. In some organisms  karyokinesis is not followed  by Cytokinesis as a result of which multinucleate conditions arises leading  to the formation  of syncytium ( e.g., liquid  endosperm in coconut). 

  Division  of cytoplasm  is signalled  at the metaphase by cytoplasmic movements  that bring about equal distribution  of mitochondria and other organelles  in the  two halves of the cell.Division occurs differently in animal, plant and bacterial  cells.

1): Animal Cell or Animal Cytokinesis:

Animal cells typically divide by furrowing, also called cleavage .In early anaphase, a midbody  of  dense material  appears at the middle of  the sti persisting  spindle microtubules. Then a circular  constriction appears around the middle of the cell between  daughter  nuclei at the level of the midbody.Contriction is brought  about by a peripheral  band of actin __ myosin microfilaments  just within  the plasma membrane, and involves  enlargement of cell membrane. The band contracts and with this the constriction gradually  deepens till the cell completely  division  into two daughter  cells. Simultaneously, the telophase is also completed. The two  daughter cells are about half the volume of the original  mother cell. They enter 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 formed.


  Cytokinesis  also divides the cell organelles  into two cells. Each cell receives  at least some of every component  it needs.


  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  amiotic cells in some flowering  plants divide by furrowing. 


2): Plant Cell  or Plant Cytokinesis:

A plant cell, due to the presence  of  a rigid cell wall, cannot undergo  Cytokinesis  by an invaginating  cleavage  furrow,Hence, plant cells divide by cell plate formation. Formation of cell plate usually  begins during late anaphase or early telophase. Small vesicles from Golgi apparatus  or ER line up right across  the middle of the spindle. They contain polysaccharide precursors of pectin for the middle lamella. At the centre of the cell, the vesicles fuse together,  forming  a structure called phragmoplast².Their membranes  form the plasma membranes  of the two adjacent daughter  cells and their contents form a film which soon solidifies to form the cell plate between  the plasma membranes.  The plasma membranes and cell plate grow peripherally  by fusion of more vesicles till they meet the side walls of the cell. The fully formed cell plate is called middle lamella. In certain  areas, the vesicles of the cell plate fail to fuse and cytoplasmic contact remains between  the daughter  cells. These cytoplasmic  channels  are lined by  plasma  membrane and form Plasmodesmata. 



Formation of Primary Cell Wall: Cellulose, hemicellulose and pectin  are then deposited by the cytoplasm  of the daughter  cells on either side of the  middle lamella to form the primary Cell Wall. This divides the parent cell into two daughter cell.




Formation  of Secondary  Cell Wall: The primary  cell wall is flexible to allow for the growth of  the  cell. A full _ grown cell later forms a rigid secondary  cell wall. The primary  and secondary  cell walls are formed of secretions  produced  in the cell.

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


3): Bacterial  Cell:

In prokaryotes  ( bacteria) and also in some eukaryotes ( algae and fungi), 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 membrane 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.



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


C): Cell Separation: 

The daughter  cells soon separate by secreting intercellular  substance between  themselves.  To begin with, this substance is jelly _ like hyaluronic acid , but later other materials may permeate it.

Significanc of Mitosis:

Mitosis has a manifold Significance :

(i) Maintenance  of Cell Size: Mitosis helps maintain  the size of the cell. A cell, when full_ grown, divides by mitosis instead of growing further. This also restores the surface/ volume ration and the nucleo_ cytoplasmic ratio, both of which are important  for proper functioning  of the cell.


(ii) Growth: A fertilized egg  develops into an embryo , and finally  into an adult by repeated  mitotic divisions. About 6× 10¹² cells an average human  body has result from the divisions  of a single zygote  cell.Hyperplasia,i.e.,increase in tissue mass, results from increase in cell number. 


(iii)  Maintenance of Chromosome Number ( Genetic Stability): Mitosis keeps the number of chromosomes equal in all the cells of an individual. In other words, mitosis provides a complete set of genetic  information  to each cell, since DNA is duplicated  in the S phase prior to mitosis.

(iv) Repair: Mitosis provides new cells to replace the old worm out and dying cells.



(v)   Healing and Regeneration: Mitosis produces new cells for the healing of wounds and for regeneration. 


(vi) Reproduction:  Mitosis brings about multiplication in  the acellular organisms. In multicellular organisms also, it plays an important  role in reproduction, asexual as well as sexual. 


(vii) Evidence of Basic Relationship  of Organisms:Mitosis being essentially similar  in many kinds of organisms,  supports the basic relationship of all living things.

Types ( Modification) of Mitosis:

There are many modifications of mitosis.

1): Intranuclear and Extranuclear  Mitosis:

In unicellular  organisms, such as Amoeba and yeast, mitotic events usually occur within the nuclear envelope, which remains intact, and the nucleus divides  by furrowing  alongwith the  cytoplasm. Such a mitosis is called intranuclear, or premitosis. In multicellular  organisms, the nuclear envelope  breaks down and the mitosis is termed extranuclear  or eumitosis.

  
  Some protists  have centriole within the nucleus. Their mitosis is intranuclear  and centric. If the centriole lies outside the nucleus, mitosis is extranuclear  and centric.


2): Anastral and Amphiastral Mitosis:

In plants, there are no centrioles and no asters are formed. Mitosis without asters  is known as anastral.In animals, the asters are present and the mitosis is described  as amphiastral, or simply  astral.Form of the spindle varies in species with and without asters. It is spindle  __ like in the former and barrel__ like in  the latter.






3):Endomitosis:

In some kinds of cells, DNA and chromosomes replicate but nuclei do not  divide. This increases the number  of chromosome sets per cell. The process is called endoduplication, or endopolyploidy, or endomitosis.Cells of  some tissues, such as mammalian liver, commonly  have endopolyploid nuclei with 4 or 8 sets of chromosomes instead of the usual 2 sets present in the diploid  cells of other tissues  of the organism. In plants, polyploidy is achieved  by application of colchicine to form new varieties. 

4): Free Nuclear Division:

In  certain  cases, nucleus divides repeatedly  without the division  of cytoplasm. This process is called free nuclear division. It produces multinucleate cells, or syncytia ( singular  is syncytium). Opalina, a protozoans,  fungi  and endosperm  cells of some seed plants become multinucleate in this manner.

Control of Mitosis:

What initiates cell division  is not definitely  known.Many factors are through to induce mitosis. 

1): Usually  it is held that the surface  _ volume ratio of a cell plays an important  role. As a cell grows in  size, its volume increases more than its surface¹. Since a cell draws all the materials needed for its maintenance  and growth  through its surface, a stage will reach when the surface area becomes insufficient to supply the large volume. It has been suggested  that there is critical  point in the surface_ volume ratio at which the division  of a cell starts.


2): It has also been suggested  that  a change in nucleocytoplasmic  or kern_  Plasma ratio in a growing  cell, or  doubling of the DNA content in the interphase may trigger  cell division. 

  Cyclic  changes in regulatory proteins are though to function  as a mitotic clock. The proteins are kinases which get activated by joining  to another protein cyclin.The complex is called cyclin_ dependent  kinases or CdKs. Some internal and external signals also help control mitosis in a cell, e.g., a protein from unattached kinetochores and a growth factor protein from  other cells.

Cancer :

Uncontrolled mitosis leads to cancer. Certain radiations, smoking , toxic chemicals,  some viruses, etc., cause cancer . It has been found that specific genes ( oncogenes) are  associated  with cancer. No cure for cancer  has yet been developed. 

Differences between Animal and plant Cytokinesis 


Animal Cytokinesis :

1): It occurs by cleavage  .

2): Spindle  starts disappearing in early telophase. 

3): Cleavage  begins at the periphery  and proceeds inward.

4): Cleavage  is started by contraction  of peripheral ring of microfilaments. 


5): A midbody  of dense material  is formed at the middle of  the cell.

6): It usually  takes place by cell plate method.

7): The spindle  persists during  Cytokinesis. 

8): Central part of spindle  grows in size and forms an interdigited complex called phragmoplast.

9): Vesicles  from  Golgi apparatus  reach the equator of  the phragmoplast and form cell plate and cell membranes. 

10): Cell plate grows centeifugally.

11): The new cell membrane  is derived from Golgi apparatus. 


plant Cytokinesis :


1): It commonly occurs by cell plate formation. 

2): Spindle persists till cytokinesis is nearly half through , and it expands to form a barrel _ like phragmoplast.

3): Cell plate appears at the centre and extends outward. 

4): Fusion  of vesicles  begins  cell plate formation. 

5): Midbody is not formed.

6): It takes place by cleavage. 

7): The spindle begins  to degenerate  after anaphase.

8): A mid body of dense is formed in the middle.

9): Such an event is absent.

10): Cleavage  progresses centripetally

11): The new cell membrane  is derived from endoplasmic reticulum. 

Differences between  Animal  and plant cells Regarding  Mitosis 


Animal cells  

1): Centrioles present at spindle poles.

2): Asters are formed ( amphiastral).

3): Cytokinesis by furrowing of cytoplasm. 

4): Furrow extends centripetally. 

5): Microfilament ring brings about cleavage. 

6): Cell does not change from or nature at the time of at the time of mitosis. 

7): Cell does not change form or nature at the time of at the time of mitosis. 

8): Midbody is formed at the equator of the spindle. 

9): Intercellular spaces appear  between  the daughter  lamella. 

10): Animal mitosis is controlled  by certain mitogens. 


plant cells

1): Centrioles  lacking at spindle poles.

2): No asters  are formed  ( anastral).

3): Cytokinesis mostly  by cell plate formation. 

4): Cell plate grows centeifugally. 

5): Microfilaments have no role in cytokinesis. 

6): Occurs mainly at meristems.

7) : Cell becomes rounded  and its cytoplasm  more viscous  mitosis. 

8): Equator of the spindle  changes into phragmoplast. 

9): Daughter  cells remain  adhered  together  by middle  lamella. 

10): Plant mitosis  is usually controlled by a hormone  cytokinine.

Differences between plant and Animal  Mitosis

plant Mitosis

1): It occurs  usually  in the region  of a meristem. 

2): It is controlled by hormone cytokinin.

3): The cell does not change shape before starting  division. 

4): Spindle is anastral. 

5): The equatorial region of spindle forms phragmoplast. 

6): Cytokinesis generally occurs by cell plate method.

7): Cell plate grows centeifugally. 


8): Microfilaments do not have much role in cytokinesis.

9): Cell plate cements  the daughter  cells.

Animal  Mitosis

1): It occurs  at several  places.

2): It is controlled by a number  of mitogens  like lymphokines, epidermal growth factor, platele derived  growth factor, etc.

3): The cell often become spherical  prior to division .

4): Spindle  is amphiaster. 

5): The equatorial  region of spindle forms midbody. 

6): Cytokinesis occurs by cleavage. 

7): Cleavage proceeds centripetally. 

8): Microfilaments bring about cleavage. 

9): Cleavage  creates an intercellular  space between the daughter cells. 

QNo:  Why is mitosis also known as the equational division?

Mitosis is the division of  a parent cell into two  identical  daughter cells, each with a nucleus having the same amount of DNA, the same number and kind of chromosomes, and the same hereditary  instructions as the parent cell. Hence, it is also known as the equatorial division. 

Qno: You are viewing a dividing  cell under a microscope.How can you determine whether  it is  an animal cell or a plant cell?

Ans:In the animal cells, centrioles are present. Hence,,  the spindle has an aster at each pole in the dividing  cell. On the other hand, there  are no centrioles in plant cells and no asters are formed.

Differentiate between  intranuclear and extranuclear mitosis. In  which organisms such types occur?


In uni organisms  ( e.g., Amoeba and yeast) mitotic events events usually  occur  within the nuclear envelope, which remains  intact, and the nucleus divides by furrowing alongwith the cytoplasm. Such a mitosis is called intranuclear mitosis.

In multicellular  organisms ( e.g., man), the nuclear envelope breaks down and the mitosis  is termed extranuclear mitosis. 

What is its other significance?

Colchicine, an alkaloid. 

 Colchicine is also used to  stop cell division  at metaphase so that condensed chromosomes may be obtained for studying  their morphology and prepare a karyotype 

What is the source  of the above mentioned  chemicals?

Colchicine  is obtained  from the  corms  of the  autumn crocus plant, Colchicum autumnale.






































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