Part 02 - Types of cell division - Amitosis

7. Cell Division 

Part 01 - Cell cycle to

• Life of all multicellular  organisms starts from single  cell  i.e. zygote.  
• Growth of every  living  organism depends on cell  division. 
• As stated  in the  cell  theory, every  cell  arises from the pre-existing cell.

Cell cycle :

• Sequential  events occurring in the life of a cell is called cell cycle.  
• There are two phases of  cell  cycle  as -

1. Interphase  and 
2. M-phase. 

• During interphase, cell undergoes growth or rest  as per  the  need.  
• During M-phase, the  cell undergoes division. 
• Interphase alternates  with the period of division.  

Interphase : 

• Interphase is the stage between two successive cell  divisions. 
• It is the longest phase of cell cycle during which the cell is highly active and  prepares  itself for  cell division.  
• The interphase is  divisible into  three sub-phases  as G-phase, S-phase and  G-phase. 

G1-phase :

• This is also known as first gap period or first growth period. 
• It starts immediately after cell division. 
• Cell performs RNA synthesis (mRNA, rRNA  and t-RNA), protein  synthesis and synthesis of membranes during this phase. 

S-phase :

• It is synthesis phase in which DNA is  synthesized  or  replicated,  so that  amount  of DNA per cell doubles.
• Histone proteins are also synthesized during this phase. 

G2  phase : 

• G2  is the second growth phase, during  which  nucleus  increases in  volume. 
• Metabolic activities essential for  cell division occur during this phase.  
• Various proteins necessary for cell  division are synthesized during this phase. 
• Besides,  RNA  synthesis also occur during this phase. 
• In animal  cells, a daughter pair of centrioles appear near the pre-existing pair.

M-phase or  period of division :

• 'M'  stands for mitosis or meiosis. 
• M-phase involves -

1. karyokinesis and 
2. cytokinesis. 

• Karyokinesis is the division of nucleus into two daughter nuclei.
• Cytokinesis  is  division  of cytoplasm resulting in two daughter cells. 

Part 02 - Types of cell division - Amitosis

Types of cell division : 

• Three kinds  of cell division are found in animal cells.  
• They are - 

1. Amitosis or  direct division
2. Mitosis or indirect  division and 
3. Meiosis or reductional division.

• Mitosis can be performed  by haploid as well as diploid  cells but meiosis can be performed by diploid cells only. 
• In honey bee, drones develop  from haploid unfertilized  eggs whereas  in  Marchantia, haploid spores form gametophyte by mitosis.

Amitosis :  

• It is the simplest mode of  cell division. 
• During amitosis, nucleus elongates and a constriction  appears somewhere  along its length.  
• This constriction  deepens  and divides the nucleus into two daughter nuclei.  
• This is  followed by  the division of  the cytoplasm which results in the formation of two daughter cells.  
• This division  occurs in unicellular organisms, abnormal cells,  old cells and in foetal membrane cells.

Part 03 - Mitosis

Mitosis :  

• This is a type of cell division in which a  cell divides to form two similar daughter cells  which are identical  to the parent cell. 
• It is completed in two steps as -

1. Karyokinesis and 
2. Cytokinesis. 

• Karyokinesis is nuclear division which is sub-divided into -  

1. prophase
2. metaphase
3. anaphase and 
4. telephase.  

• Although  for the sake of convenience  above mentioned  steps are used, it  must be  remembered  that  mitosis is a continuous process that starts with the disappearance of nuclear membrane  in prophase and ends with  separation  of two fully formed cells after cytokinesis. 

1. Prophase :

• This  phase  involves -

1. condensation of chromatin  material,
2. migration  of centrosomes, 
3. appearance of  mitotic apparatus and 
4. disappearance of nuclear membrane.

• Due  to  condensation,  each  chromosome becomes visible under light microscope which can be  seen with its sister-chromatids connected  by centromere.  
• The nucleolus starts to disappear. 
• Nuclear membrane disintegrates and disappeares gradually.
• Centrosome  which had undergone duplication during interphase begins  to move  towards opposite  poles of the cell. 
• Mitotic apparatus is almost completely formed.

 2. Metaphase :

• In  this  phase,  chromosomes are completely  condensed so  that they appear very short. 
• Sister-chromatids and centromere become very prominent.  
• All the chromosomes lie at equatorial plane of  the cell.  This  is called metaphase plate. 
• Mitotic spindle is fully formed. 
• Centromere  of each chromosome divides into two, each being associated with a chromatid.

3. Anaphase  :

• The chromatids of each chromosome separate and form two chromosomes called daughter chromosomes.
• The formed chromosomes  are pulled away in opposite direction by  spindle apparatus. 
• Chromosomes being pulled  away  appear like a bunch  of banana during  midway of anaphase. 
• Each set of chromosomes reach at opposite poles of the cells marks the end of anaphase.

4. Telophase  :

• The telophase  is the final stage of karyokinesis.  
• The chromosomes  with their centromeres at the poles begin to uncoil, lengthen and loose their individuality.  
• The nucleolus begins  to reappear. 
•  The nuclear membrane begins to appear around the chromosomes.
•  Spindle fibres break down and get absorbed in the cytoplasm.  Thus two daughter nuclei are formed in a cell.

Do you know ?

• Pulling away of daughter chromosomes is achieved by elongation and shortening of two types of spindle fibres. 
• Spindle  fibre present between centriole  and centromere,  called as kinetochore  fibres contract and  the spindle fibres  present between two opposite centrioles,  called  as polar fibres elongate.

Cytokinesis :

• The  division of the  cytoplasm into two daughter cells is called cytokinesis. 
• The  division starts with a constriction.  This constriction  gradually deepens and ultimately joins in the  centre  dividing into  two daughter cells.  
• This process of division of cytoplasm is perpendicular  to the spindle.  
• This mechanism  of cytokinesis is characteristic of animal cells. However, plant cells are covered by  a relatively  non-flexible  cell wall.
•  Due to this, furrow can not be formed. Instead, cell  wall/ partition starts to appear at the centre of the cell and grows outward to meet the existing lateral  walls.  
• The formation of the new cell wall begins with the formation of a simple precursor, called the 'cell-plate'  that represents the middle lamella between the walls  of  two adjacent cells. 
• At the time  of cytoplasmic  division, organelles  like mitochondria and plastids get distributed between the two daughter cells. 
  

Significance of mitosis :

• As  mitosis  is equational division, the chromosome number is maintained  constant. 
• It ensures equal distribution of the nuclear and  the cytoplasmic content between the daughter cells, both quantitatively  and qualitatively.  
• The hereditary material  (DNA)  is also equally distributed. 
• It helps in the growth and development  of organisms.
• Old and worn-out cells  are replaced through mitosis.  
• It helps  in the asexual reproduction of organisms and vegetative propagation  in plants. 
• The process of mitosis also maintains the  nucleo-cytoplasmic  ratio. Although mitosis is a very reliable  process for  preserving the genetic make-up of  cells or organisms, it cannot introduce variation or new combination of existing genes.

Death of cell  :

• You may think of it as a bad for cells in your body to die. In many cases, that's true:  it's not  good for cells  to  die  because of an injury (for example,  due to scrape or a harmful chemical),  which is called necrosis. 
• However, some  cells  of our body die;  not randomly but in a carefully controlled  way. 
• For example,  during the embryonic development, the cells between the embryonic fingers died in a process called apoptosis to give a definite shape to the fingers.  
• This is a common  form of programmed cell death where cells undergo "cellular  suicide" when they receive  certain signals.  
• Apoptosis  involves the cell death, but it benefits the organism as a whole (for instance, by letting fingers develop  or by eliminating potential cancer cells). 

Part 04 - Meiosis I

Meiosis :  

• The term meiosis was  coined by J. B. Farmer  in  1905. It  takes  place  only in reproductive cells during the formation of gametes.  
• By this  division, the  number  of chromosomes is reduced to half, hence it is also called  reductional division. 
• The cells in which meiosis take place are termed as meiocytes. 
• Meiosis produces four haploid daughter cells from  a diploid parent  cell. 
• Meiosis is of two subtypes :

1. First meiotic division or  Heterotypic division – (Meiosis I) 
2. Second  meiotic division or  Homotypic division (Meiosis II) A. 

Meiosis I /First meiotic division/ Heterotypic division :

• During 1st  meiotic  division, diploid cell is divided into two haploid cells.  
• The daughter cells resulting  from this division are different from the parent cell in chromosome number.
•  Hence this division is also called  heterotypic division. 
• It consists of the phases like -  

1. prophase-I
2. metaphase-I
3. anaphase-I
4. telophase-I and 
5. cytokinesis-I 

Prophase-I :  

• This phase has longer duration. 
• Significant features which are peculiar to meiosis occurs in this phase.  
• This phase can be sub-divided into five sub-stages  as 

1. Leptotene
   
2. Zygotene
3. Pachytene
4. Diplotene and 
5. Diakinesis. 

Leptotene : 

• The volume of  nucleus increases. 
• The chromosomes  become distinct, long thread-like and coiled.  
• They take up a specific orientation-  the 'bouquet stage' inside the nucleus.  
• This is characterised with the ends of chromosomes converged towards that side of the nucleus where the centrosome  lies. 
• The centriole  divides into  two and migrate  to opposite poles. 

Zygotene :  

• Intimate pairing of non-sister chromatids  of homologous chromosomes takes place by formation of synaptonemal complex.  
• This pairing  is called  synapsis. 
• Each pair consists of a maternal  chromosome and a paternal chromosome. 
• Chromosomal pairs are called bivalents or tetrads. 

Pachytene :  

• Each individual chromosome begins  to split  longitudinally  into  two similar chromatids.  
• At this stage, tetrads become more clear in appearance because of presence of four visible chromatids.  
• The homologous chromosomes of each pair begin to separate from  each  other. 
• However, they  do not completely  separate but remain attached together at one or more points.
• These points appear like a cross (X) known as chiasmata.  
• Chromatids break at these points and broken segments are exchanged between non-sister chromatids of homologous chromosomes.  This is called as crossing-over or recombination. 

Diplotene :  

• Though chiasmata are formed in pachytene, they become clearly visible in diplotene  due to the beginning  of repulsion between  synapsed homologous chromosomes. This is called desynapsis.  
• It involves disappearence of synaptonemal complex.

Diakinesis :  

• In this phase, the chiasmata  beings to move along the length of chromosomes from the centromere towards  the ends of chromosomes.  
• The displacement  of chiasmata is termed  as terminalization.  
• The terminal chiasmata exist till the metaphase. 
• The nucleolus  disappears and the nuclear  membrane also begins  to  disappear. 
• Spindle fibres starts to appear in the cytoplasm.

Metaphase-I :  

• The spindle fibres become  well developed. 
• The tetrads move towards the equator and they orient themselves on the equator in such a way that centromeres of homologous tetrads lie towards  the poles  and arms towards the  equator.
• Due to increasing repulsive  forces between homologous chromosomes, they are ready to separate from each other.

Anaphase-I :  

• In this  phase, homologous chromosomes are pulled away  from each other and carried towards opposite poles by spindle  apparatus. This is disjunction.  
• The  two sister chromatids of each  chromosome  do not separate in meiosis-I.
• This is reductional division.  
• The sister chromatids of each chromosome  are connected  by a common  centromere. 
•  Both sister  chromatids  of each  chromosome  are  now different in terms of  genetic content as  one of them has undergone the recombination

Telophase-I :  

• The haploid number of chromosomes after reaching their respective poles, become uncoiled  and elongated.  
• The nuclear  membrane  and the nucleolus  reappear and thus two daughter nuclei are formed.

Cytokinesis-I :  

• After the karyokinesis, cytokinesis occurs and two haploid cells are formed.
• In many cases, these daughter  cells pass through a short resting phase or interphase / interkinesis. 
• In some cases, the  changes of the telophase  may not occur.  
• The anaphase directly leads to the prophase of meiosis II.

Part 05 - Meiosis II

Meiosis II / Second meiotic  division / Homotypic Division :

• During this division,  two haploid  cells formed during first meiotic  division divide further into four  haploid cells.  
• This division is similar to mitosis.  
• The daughter cells formed in second meiotic  division are similar  to their parent cells  with respect  to the chromosome number formed in meiosis-I. Hence this division is called homotypic division.
• It consists of the following  phases : 

1. prophase-II
2. metaphase-II
3. anaphase-II
4. telophase-II and 
5. cytokinesis-II. 
   

Prophase-II :  

• The  chromosomes  are distinct with two chromatids.  
• Each  centriole  divides into two resulting  in the formation  of two centrioles  which  migrate  to opposite  poles and form asters. 
• Spindle fibres are formed between the centrioles.  
• The nuclear membrane and nucleolus disappear. 

Metaphase-II : 

• Chromosomes gets arranged at the equator.  
• The two chromatids of each chromosome are separated by the division of the centromere. 
• Some spindle fibres are attached to the  centromeres and some are arranged end to end between two opposite centrioles. 

Anaphase-II : 

• The separated chromatids become  daughter chromosomes and move to opposite poles due to the contraction  of the spindle fibres attached to centromeres. 

Telophase-II : 

• During this stage the daughter chromosomes uncoil.  
• The nuclear membrane surrounds each  group of  chromosomes  and the nucleolus reappears.

Cytokinesis-II : 

• Cytokinesis occurs after nuclear division.  
• Two  haploid cells are formed from each haploid cell. Thus, in all, four haploid daughter cells  are formed.  
• These cells undergo further changes to develop into gametes. 

Significance of Meiosis :  

• Meiotic division produces gametes.  
• If it  is absent, the  number of chromosome would double or quadruple resulting in the formation of monstrosities (abnormal forms).  
• The constant number of  chromosomes in a  given species across generations is maintained  by meiosis. 
• Because of crossing over, exchange  of genetic  material takes place leading to genetic variations, which are the raw materials for evolution.

Source from Intertnet