CELL DIVISION

Cell of Class 9

GLYOXYSOMES

  • A beaver (1961) was the first person to discover these organelles and were described later by R.W. Briedenbach (1967).
  • They are about 0.5 to 1 µm in size and are surrounded by a single unit membrane.
  • They are found in plant cells, particularly, in germinating fatty seeds e.g. Ricinus (castor) and groundnut where fat is being converted into carbohydrates by a process called glyoxylate cycle.
  • Glyoxysomes contain several enzymes that initiate the breakdown of fatty acids.
  • Structure of glyoxysomes is similar to peroxisome.

CELL DIVISION

Cell division and cell development occur simultaneously in the organisms in which former is more significant and takes place continuously throughout life to certain extent.

Cells divide to replenish the degenerating cells so that protoplasm is continuously renovated by the accumulation of food material and growth. This process of replacement and repairing of cells was first suggested by ‘Nageli’ in 1846 and later ‘Remark’ described the process of cell division.

Cell division can be clearly divided into two phases, first is nuclear division or karyokinesis and second is cytoplasmic division or cytokinesis. Three Types of Cell Division are : 

  • Amitosis
  • Mitosis
  • Meosis

AMITOSIS

It is the process of cell division in which unequal division of chromatin materials or chromosomes takes place. Amitosis occurs in lower plants, bacteria, some protozoans, some bone cells, in some white blood cells and pathological cells.Its a simple division and active chromosomes do not take any active part in it. So it is also known as vegetative division. 

The process of division takes place as follows :

  • Nucleus of the cell constructs in the middle of the cell and enlarges to form a dumbbell shaped structure.
  • The construction deepens and the nucleus divides into two daughter nuclei.
  • At the same time near the middle of the daughter nuclei, a cytoplasmic constriction appears that also deepens and divides the mother cell into two daughter cells with one nucleus in each daughter cell.
  • The chromosomes are not visible at any time and achromatic spindles are not formed.

MITOSIS

Definition : Mitosis involves the exact replication of parent cell followed by its division into two daughter cells which are identical and contain the same number of chromosomes as found in parent cell. If the parent cell is haploid (N), then the daughter cells will be haploid.   If the parent cell is diploid, the daughter cells will also be diploid. 

N → N

2N → 2N

Interphase :It is the stage where cell prepares for cell division. The cell is engaged in metabolic activity and performing it’s prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants). The DNA replicates so the diploid condition becomes tetraploid (2N to 4N) or from haploid to diploid (1N to 2N) depending upon cell’s chromatin material.

Process : Replication and distribution of chromosomes is known as karyokinesis while cytoplasmic division is called as cytokinesis.

Karyokinesis: It comprises of four phases namely prophase, metaphase, anaphase and telophase.  

CELL DIVISION

I. Prophase (pro: first) is the beginning of mitosis. Events, which take place during this phase, are:

Nuclear changes 

  • Chromatin gets condense by losing water. 
  • Chromatin threads get coiledand become shorter and thicker. They become 1/25 of their size in early prophase.  
  • In this phase the chromatin looks like threads. (mitos:threads)
  • Nucleolus and nuclear membrane disappear during the end of prophase. 

Cytoplasmic events     

  • The centriole divides into two and each one occupies opposite pole.
  • Astral rays come out from each centriole.

Prometaphase.

  • Nuclear membrane completely disintegrates (eumitosis or extranuclear mitosis). Whereas in some protists, fungi and algae nuclear envelope does not disintegrates (premitosis or intranuclear mitosis). 
  • A clear zone known as equator appears in between the mid line of spindle and nucleus. 
  • The chromosomes move towards equator.
  • In many protozoans and some animal cells, the nuclear membrane does not disappear and whole mitosis takes place within the nucleus. Such phenomenon is called as intranuclear mitosis or premitosis. 

 Metaphase

During metaphase (Meta: after, phases: appearance) following events takes place: 

  • (Microtubules in the cytoplasm orient in between the centrioles of opposite poles and form spindle. 
  • Each chromosome reaches the equator and arranges itself at the periphery of the spindle. Chromosomes aligned around the equator constitute metaphase plate.
  • Spindle fibres which attach with the chromosomes are called chromosomal fibres. 

Anaphase 

  • Centromere divides and allows the separation of sister chromatids into two daughter chromosomes.     
  • Daughter chromosomes move apart and move towards opposite poles. 
  • Centromere leads the chromosome towards pole. 

Telophase 

  • The chromosomes, which reaches at the opposite poles of the cell now elongates, the coils of DNA protein fibres loosen and the chromosomes become thread-like.
  • The nucleolus reappears from nucleolar organizer regions of satellite chromosomes. 
  • The endoplasmic reticulum forms the new nuclear envelope around the chromosomes and the nucleolus.
  • The microtubules of the aster and mitotic spindle rearrange and disappear.

CYTOKINESIS

Cytokinesis is the division of cell cytoplasm into two separate cells. It usually occurs along with the formation of daughter nuclei after the nuclear division. It differs in plant and animal cells. At times, cytokinesis do not follow karyokinesis, as a result a multinucleate condition arise such cell is called coenocyte or syncytium. 

Animal cells: A shallow groove appears in cytoplasm because of constriction of microfilaments, leading to cell division. 

Plant cells: In plants, a plate appears at the equator followed by cell division, it is also called as cell plate. 

SIGNIFICANCE OF MITOSIS 

  • Mitosis is required for growth of animal as because of mitosis the number of somatic cells increase and the animal grows. 
  • It is because of mitosis that, the dead cells are replaced by new cells. Hence, maintaining the number of cells in body. 

Related Topics 

MEOSIS

Definition : Reproduction is one of the most important characteristics of the living system by which we can make a distinction between living entities from a non-living system. There is a need of dissimilar types of gametes (male gamete and female gamete) to achieve the process of reproduction. If the gametes were diploid i.e., 2n number of chromosomes, the resulting zygote or new individual would have twice the diploid chromosomes number. But every organism contains a definite number of chromosomes and to maintain this, there is a special type of division in the cells responsible for the production of gametes. This special type of cell division in the gamete producing cells is referred to as meiosis or meiotic division.A cell undergoing meiosis will divide two times; the first division is meiosis 1 and the second is meiosis 2. The phases have the same names as those of mitosis. A number indicates the division number (1st or 2nd):

Process :

I. Prophase I : It is further divided into six phases namely Proleptotene, Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis. 

  • Proleptotene: In this stage the chromosomes are extremely thin, long, uncoiled, longitudinally arranged, single and cylindrical thread like structures. Movement of centrioles and formation of astral rays is similar to mitosis.
  • Leptotene or Leptonema: More uncoiling of the chromatin takes place and the chromosomes become more thread like. The nucleolus is well marked, increases in size during both Leptotene and Zygotene. 
  • Zygotene or Zygnema: During this stage, homologous chromosomes come near to each other and starts pairing to form a bivalent. This pairing is known as synapsis. This pairing is intimate and precise chromomere to chromomere. Synapsis results into formation of bivalents whose number is half of total chromosomes numbers. 
  • Pachytene or Pachynema:  During this stage, crossing over of chromosomes of same pair takes place due to which genetic recombination takes place. It is because of this recombination that genetic diversity is evolved. During this stage each homologue starts splitting resulting into formation of two sister chromatids as a result the bivalent becomes tetrad.  Breaks during this stage are because of intensive coiling of chromosomes. Chromatids belonging to different chromosomes of same homologous pair are called non-sister chromatids.   
  • Diplotene or Diplonema:  During this stage, homologous chromosomes start separating. However, the separation is not complete the chromosomes remain attached to each other in those places where crossing over has taken place during Pachytene. These points of attachment are called as chiasmata (meaning crosses). This is X-shaped structure.
  • Diakinesis: During diakinesis the chiasmatas moves from centromeres towards the ends of chromosomes and the intermediate chiasmata disappear. At this stage the nucleolus and nuclear membrane disappears and the formation of nuclear spindle starts. 

CELL DIVISION

Diagram of chromosomal exchange in the four-stranded stage and of terminalization during first meiotic division

  • Metaphase I : During this stage, chromosomes align themselves at the equator with their centromeres attached to spindle fibres. 
  •  Anaphase I : During this stage, the four chromatids, which were formed during prophase I gets separated into two diads due to separation of maternal and paternal chromosomes of the bivalent. It means that, chromosome will get two sister chromatids attached to common centromere. This is called as disjunction. During this time, the chromatids also start separating. However, they remain attached to each other at centromere. 
  •  Telophase I : The endoplasmic reticulum forms the nuclear membrane around the chromosomes and uncoiling of chromosomes starts. Nucleolus reappear, cytoplasm also gets segmented between two cells each of which contain haploid number of chromosomes. 
  • Interphase I : Interphase I remains present in some of the animal species whereas absent from others. Sometimes even telophase remains absent so the cell directly enters into prophase II after anaphase I due to this nuclear membrane do not form.
  •  Prophase II: During this stage, spindles are once again formed but in reverse orientation or we can say that the plane of spindle formation is at right angle to that of prophase I. 
  •  Metaphase II:  The chromosomes align themselves around the center of spindles and the centromere divides into two as a result of which two daughter chromosomes are formed. 
  •  Anaphase II: During this stage, chromosomes which are attached to the microtubules (spindles) starts sliding towards the poles by the same mechanism as described earlier in anaphase (mitosis).
  • Telophase II: The chromatids move towards opposite poles, endoplasmic reticulum forms nuclear membrane around the chromosomes and nucleolus reappears because of synthesis of RNA’s and also due to accumulation of ribosomes proteins. 

After this, cytokinesis takes place resulting into formation of four daughter nuclei with haploid number of chromosomes. 

CELL DIVISION

SIGNIFICANCE OF MEIOSIS 

  • As meiosis takes place in gametes, so it maintains the chromosomes number constant. 
  • As crossing over and recombination of chromosomes takes place during meiosis so variations take place resulting into origin and evolution of species.

DIFFERENCE BETWEEN MITOSIS AND MEIOSIS

The differences between mitosis and meiosis are as follows:

S. No

Mitosis

Meiosis

1.

Mitosis takes place within somatic cells (cells that make up the body).

Meiosis takes place within gamete cells (sex cells).

2.

One single division of the mother cell results in two daughter cells.

Two divisions of the mother cell result in four meiotic products or haploid gametes.

3.

A mitotic mother cell can either be haploid or diploid.

A meiotic mother cell is always diploid.

4.

The number of chromosomes per nucleus remains the same after division.

The meiotic products contain a haploid (n) number of chromosomes in contrast to the (2n) number of chromosomes in mother cell.

5.

It is preceded by a S-phase in which the amount of DNA is duplicated.

In meiosis, only meiosis I is preceded by a S-phase.

6.

In mitosis, there is no pairing of homologous chromosomes.

During prophase I, complete pairing of all homologous chromosomes takes place.

7.

There is no exchange of DNA (crossing-over) between chromosomes.

There is at least one crossing-over or DNA exchange per homologous pair of chromosomes.

8.

The centromeres split during anaphase.

The centromeres do separate during anaphase II, but not during anaphase I.

9.

The genotype of the daughter cells is identical to that of the mother cells.

Meiotic products differ in their genotype from the mother cell.

10.

After mitosis, each daughter cell has exactly same DNA strands.

After meiosis, each daughter cell has only half of the DNA strands.

CELL DIVISION

 

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