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CELL CYCLE AND CELL DIVISION

                                               

INTRDUCTION

All living beings  are made up of cells. The structural and functional unit of life is a cell which is the building block of the body. New cell arises from the pre-existing cells by the process of cell division.

Cell division occurs  in all living organisms. In unicellular organisms,cell division directly produces two individuals. In multicellular organisms or higher level organism , life begins from a single cell,as  zygote, whIch divides and redivides mitotically  into a number of cells to form a complete organism.

In multicellular organisms , there are two types of cells.

a)The somatic cells or the body cells- They form the body of organism.

b)The reproductive cells or sex cells- They  are gamete-producing cells.

                                                      WHY DO CELLS DIVIDE

The cells divide to produce new cells. The new cells are produced for the following reasons:-

i)GROWTH: During cell division, a single cell divides to produce new cells which form a cluster of similar cells to form tissues and organs.

ii)REPLACEMENT OF DEAD CELLS:- The existing cells in our body are destroyed regularly. These  dead cells should be replaced by the new cells by regular cell division for the normal functioning of the body.

iii)FORMATION OF RBC:- Cell division in the bone marrow helps in replacement of old RBC by new blood cells.

iv)REPAIR OF TISSUE:- Cells divide and new cells fill up the broken cut ends to heal wounds for repairmen of tissues.

v)REPRODUCTION:-New cells arise from pre-existing cells. The sex cells like sperms and eggs are formed because of  meiotic cell division. These sex cells contain half the normal number of chromosomes. During fertilization , these sex cells combine to form zygote. Thus cell division is essential for reproduction.

TYPES OF CELL DIVISION

There are two types of cell divisions in higher organisms:-

i)Mitosis- It occurs in somatic cells leading to growth and development.

Ii)Meiosis– It occurs in reproductive cells or sex cells leading to gamete formation.

Here is one doubt: If the sex cells or gametes undergo mitosis instead of meiosis will the offspring  be affected? 

Solution: certainly, because by meiosis gamete number is maintained by reductional division. But  by mitosis, number of cell is increased as well as number of chromosome is same as parent. So by mitosis in gamete whole living world will be destroyed by increase of gamete numberin each generation.

( n–>2n–>4n–>8n–>16n….. and so on) 

 

AANUVANSHIK JAANKAREE is the hereditary information present in the sex cells or gametes  of the parent in hindi ( It may be X  or Y in father.  X or X in mother)

PHASES OF CELL DIVISION

The cell division has two phases . First is the nuclear division or karyokinesis where parent nucleus divides to form daughter neclei. This is followed by the division of cytoplasm or cytokinesis where parent cell divides into daughter cells.

 

BRIEF EXPLANATION OF MITOSIS

Mitosis  or mitotic cell division is an equational division in which one parent cell divides to form two daughter cells. The daughter cells formed are identical to each other and also to the parent cell in every respect . In mitosis , the same normal chromosome number of the parent cell is maintained at each stage of mitotic division of the cell.

CELL CYCLE

All cells go through a basic cell cycle. These cells  may vary in the time period  in different stages of cell cycle. For example, bacteria divide once in every 20 minutes, epithelial cells every 8-10 minutes and onion root tip cells every 20 hours. There are three main stages in a cell cycle:

i)INTERPHASE

ii)MITOSIS OR KARYOKINESIS

iii)CYTOKINESIS

*Out of 18-20 hours that an epithelial cell takes to divide in the tissue culture

*Here are some stages common  in mitosis . Plant cells donot have centriole . Except some few changes in cytokinesis , rest all the stages are the same.

INTERPHASE

It is a very active phase in the cell cycle. In which DNA is duplicated . Although the cell looks to be restive, physiologically it is a very active stage. It is a preparatory phase in which a number of molecules needed for the mitotic phase or division of cell are synthesized, it is further divided into three stages:-

 

a)G1 PHASE OR GAP ONE:- Cell grows in size and RNA , proteins and enzymes needed for the next synthetic  phase are synthesized.

b)S PHASE OR SYNTHETIC PHASE:- DNA synthesis takes place.

c)G2 PHASE OR GAP TWO:- Spindle and aster rays are formed.

 

EXPLANATION

G1 PHASE– I) Cells grow in size, cytoplasm of cell is increased.

2)Mitochondria, chloroplasts( in plant), lysosomes, endoplasmic reticulum, golgi complex, vacuoles and vesicles are produced.

3)Structural and functional proteins are formed.

4)Nucleolus produces rRNA,mRNA  and tRNA.

5)Ribosomes are synthesized.

6)Metabolic rate of the cell becomes very high.

At this stage one of the two ways is taken up by the cells. Some cells withdraw and go to resting phase and others continue to divide and proceed to the next S-phase

S PHASE-1) Replication of DNA takes place. Chromosomes are duplicated.

2)Protein molecules called histones are synthesized that cover each strand of DNA.

3)Each chromosome is in the form of two chromatids.

G2 PHASE- 1)Synthesis of RNA and proteins continues.

2)Centriole replicates in animal cells. Plant cells do not have centriole.

3)Mitotic spindle and aster rays begin to get formed

4)Energy store is increased.

5)Increased metabolic state.

MITOTIC PHASE OR KARYOKINESIS

Technically it is a precise division of nuclear ,material especially of chromosome called karyokinesis. There is equal division of genes and DNA in the daughter cells. The daughter cells contain chromosomes which are identical to the mother cell.

Nuclear division or karyokinesis occurs in four distinct stages: PROPHASE, METAPHASE, ANAPHASE, TELOPHASE.

 

 

CYTOKINESIS

After the mitotic phase or karyokinesis, the cytoplasm is ready to divide. This stage is known as cytokinesis or cytoplasmic division. In this phase the cytoplasm divides,  into two  new daughter cells. The cell membrane gets furrowed  in the middle of a cell and divides the cell organelles and the cytoplasm in such a way that the two daughter cells formed are identical in all aspects.

IMPORTANT FEATURES OF MITOSIS

NAME OF THE STAGE DESCRIPTION DIAGRAM
a)Prophase i)Longest phase of division.

ii)Chromosomesms are shortend by thickening

iii)Nuclear membrane disintegrates.

iv)nucleolus is started to disappear.

v)Centrioles move to opposite  poles of the cells.

vi)Astrer rays radiate from centriole.

 

b) Metaphase i)Formation of spindle fibre

ii) Chromosomes line up at the center or equator forming equatorial or metaphase plate.

iii)Chromosomes are attached  to spindle fibres at the centromere region.

iv)centrioles at the opposite poles.

v)Nuclear membrane absent,

vi) Nucleolus is absent.

 

c)Anaphase i)It is a very rapid stage and the shortest stage.

ii)Centromere  of each chromosomes splits into two.

iii)Two chromatids of  a chromosome  separate and get pulled to opposite  sides by the spindle fibres.

iv) Shrinking of spindle fibre takes place.

 

 

d)Telophase i)Chromosomes (now called ) reach the poles of the cells.

ii)Chromosomes uncoil, lengthen and form chromatin network again.

iii)Spindle fibre stars to disappear.

iv)A constriction appears at the centre  of the cell.

v)Nuclear membrane reappears and two nuclei are formed.

vi) Nucleoli reappear in each daughter nucleus.

vii)Leads to cytokinesis.

 

 

In animal cells, the cell membrane  begins to form furrow or  constriction  at the equator during telophase. The constriction deepens and eventually the cell membranes of opposite side meet at the centre. The cell finally divides into two daughter cells.

In plant cells, the presence of rigid cell wall does not perform  cytokinesis by cleavage formation by  cell membrane. Instead of that , cytokinesis begins by the  cell plate formation  in the middle of the cell during late anaphase and continues through telophase. The spindle fibre does  not disappear at the equatorial plane and form a cell plate. As cell late gradually becomes distinct and develops into a new cell wall, it divides a plant cell into two.

SIGNIFICANCE OF MITOSIS

1)Identical gene composition

2)Helps in growth

3)Healing of wounds and replacement of cells

4)Regeneration and asexual reproduction

5) Mitotic division without control may give rise to tumours and cancerous growth in the body

 

TWO  QUESTIONS WHICH YOU WOULD ASK YOUR TEACHER TO KNOW ABOUT CELL DIVISION:

(a) How do living organisms grow?

(b) By which process living organism produces sperm and egg?

 

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