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Cell Cycle and Cell division





CELL CYCLE:-

Cell cycle is an orderly sequence of events or a set of stages by which a cell duplicates its Genome, synthesizes the other constituents of the cell & eventually divides into two daughter cells.

CELL DIVISION:-

The process by which new cells are formed from pre- exisiting cells is called cell- division.
 

PHASES OF CELL CYCLE:-

A typical Eukaryotic cell (Human cell) divides one in every 24 hrs. The two major phases in cell- cycle are:-

 

Interphase:- (Resting stage, Preparatory phase)

  • It is the period between the end of one cell division to the beginning of the next cell division.
  • It is the most active stage in the cell cycle, when the cell prepares for the division phase.
  • It consists of three sub- stages:-
  •  
G 1 phase (Gap 1 Phase):
  • It corresponds to the duration between the mitosis (M- phase) and initiation of replication of DNA.
  • The cell becomes metabolically active & grows in size during this period.
  • Also called Post-mitotic or Pre- Synthetic phase.
S- phase/ Synthesis Phase:
It is the period during which DNA replication takes place & the amount of nucleic acid doubles, but no increase in no. of chromosomes is seen during this stage.
G2/ Gap- 2phase:
  • During this stage more RNA and proteins are synthesized in preparation of mitosis & also for the growth of the cells.
  • Also called pre- mitotic or post-synthetic phase.
 
NoteGo phase/ Quiescent Phase:-
Go phase represents an inactive stage in the cell- cycle where cells are metabolically active but don’t undergo division, unless it becomes necessary to replace any worm act or injured cells. In such cases cells from the G1 phase enter the Go phase.

M- phase:-

Following the Interphase, the cell enters the M- phase or mitosis phase

Mitosis:-

  • In this type of cell division the chromosomes replicate themselves & get equally distributed into daughter nuclei i.e. the chromosome number in the parental & progeny cell become the same. Therefore, it's also known as equational division.
  • Mitosis is also known as somatic cell division because it results in the formation of somatic cells.
  • Mitosis is known to be the phase of actual cell division which starts with the division of nucleus, followed by separation of daughter chromosomes i.e. Karyokinesis & terminates with the cytoplasmic division i.e. Cytokinesis.

Karyokinesis:-

In mitosis, it is completed in four stages:-
  1. prophase
  2. Metaphase
  3. Anaphase
  4. Telophase

Prophase:-
The characteristics or key- features are:-
  • Chromosomal material (chromatin) condenses to form compact, mitotic chromosomes composed of two chromatids attached together at centromere.
  • Initiation of assembly of mitotic spindle by the microtubules of the cell cytoplasm.
  • During late prophase, the nucleolus disintegrates gradually & the nuclear envelope disappears.

Metaphase:-
  • Condensation of chromosomes is complete & chromosomes are thickest & shortest in this phase. The spindle formation is complete & all the chromosomes lie at the equator known as metaphase plate.
 
The key features of Metaphase are:-
  • Spindle fibres attached to kinetochore of chromosomes.
  • Chromosomes are moved to spindle equator & get aligned along metaphase plate through spindle fibres to both poles.

Anaphase:-
  • Centromere splits & chromatids separate.
  • Movement of chromatids towards opposite poles.

Telophase:-
  • Chromosome s clustered at opposite spindle poles & their identity is lost (decondensation starts)
  • Nuclear Envelope assembles around the chromosome clusters.
  • Nucleolus, Golgi bodies & ER are formed.

Cytokinesis:-

  • It is the process of division of cytoplasm where the two daughter nuclei become separated into two daughter cells.
  • The process is different in plant & animal cells.

Cytokinesis in Animal cells:-
  • In animal cell cytokinesis occurs by cell furrow which appears in plasma membrane due to the contraction and development of micro- filament.
  • The furrow starts deepening gradually & finally gets joined in the centre by dividing the cytoplasm into two.
Cytokinesis in Plant cells:-
  • Plant cells divide by the cell plate method.

Significance of Mitosis:-

  1. In multicellular organisms, body growth occurs by mitotic division of the cell.
  2. Replacement of worn out tissues/cells and repair out injured tissues occur by mitosis.
  3. In unicellular organisms, mitosis is involved in asexual reproduction.
  4. In plants, vegetative propagation involves only mitotic division.

MEIOSIS:-

  • It is a type of cell- division where the number of chromosomes is reduced to half in the daughter cells & hence it is also known as reductional division.
  • Meiosis occurs in any life- cycle that involves the process of sexual reproduction.

Meiosis- I:

  • During this division, the chromosome number reduces from diploid to haploid i.e. from 2n to n. It is also known as heterotypic division.
Karyokinesis:-
It consists of Prophase-1. Metaphase- 1, Anaphase- 1 & Telophase- 1
Prophase- I:-
  • It is the most complicated & prolonged phase which is further sub- divided into five sub- phases on the basis of chromosomal behavior.

LEPTOTENE:-
  • Condensation & coiling of chromatin fibre lead to the formation of distinct chromosomes.
  • Centriole starts moving towards opposite ends & each centriole develops astral rays.

ZYGOTENE:-
  • Homologous chromosomes starts pairing, the process of association is known as synapses.
  • It is done by the formation of Synaptonemal complex & chromosomes occur as bivalent or tetrads.
PACHYTENE:-
  • The two sister chromatids of each Chromosome become visible & bivalent become a tetrad with four chromatids.
  • Recombination nodules, the site of crossing over between the two non- sister chromatids appear.
  • Crossing- over is catalyzed by the enzyme Recombinase.

DIPLOTENE:-
  • In this the synaptonemal complex appears to get dissolved & the Homologous chromosome starts separating from each other except at the sites of crossing over. i.e. chiasmata. (X- Shaped structure)

DIAKINESIS:-
  • The charismata move to the hip (terminalisation) & may slip off or remain at the tips.
  • Chromosome become fully condense
  •  Nucleolus disintegrates.
 
 Metaphase-I:-
 
  • The bivalents are arranged at the equator of the spindle.
  • The centromere are aligned in two rows.

Anaphase-I-

  • The member of every homologous pair of chromosomes separate from each other and start moving to opposite poles of the spindle, the centromere do not split.

Telophase-I:-

  • The chromosomes have reached the poles.
  • A nuclear membrane forms around the chromosome cluster at each pole.

Cytokinesis:-

It generally follows the first nuclear division so two daughter cells (dyades) are formed.


Interkinesis:-
  • It is similar to the short interphase between meiosis I and meiosis II.
  • There is no replication of DNA. Sometimes this phase may be absent.
 

Meiosis II:-


  • It is similar to mitosis and it is also known as equational division.
Karyokinesis:
 
Prophase II:-
  • Chromosomal material (chromatin) condenses to form compact, mitotic chromosomes composed of two chromatids attached together at centromere.
  • Initiation of assembly of mitotic spindle by the microtubules of the cell cytoplasm.
  • During late prophase, the nucleolus disintegrates gradually & the nuclear envelope disappears.

Metaphase II:-
  • Spindle fibres attached to kinetochore of chromosomes.
  • Chromosomes are moved to spindle equator & get aligned along metaphase plate through spindle fibres to both poles.

Anaphase II:-
  • Centromere splits & chromatids separate.
  • Movement of chromatids towards opposite poles.

Telophase II:-
  • Chromosome s clustered at opposite spindle poles & their identity is lost (decondensation starts)
  • Nuclear Envelope assembles around the chromosome clusters.
  • Nucleolus, Golgi bodies & ER are formed.

Cytokinesis:-
  • It is the division of cytoplasm.
  • The nuclei are separated into two cells by cytokinesis.
  • Hence, each daughter cell (dyad) of meiosis I divided to form two daughter cells so at the end of meiosis of diploid cells (meiocyte), 4 haploid daughter cells are formed.
 

 

Significance of Meiosis:-


  • This ensures the maintenance of constant chromosome number, characteristic of particular species generation after generation.
  • Crossing over results in variations of genetic characters in the offsprings, variations forms the raw material for evolution.


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