Mitosis: Stages NEET Notes | EduRev

Biology Class 11

Created by: Sushil Kumar

NEET : Mitosis: Stages NEET Notes | EduRev

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CELL DIVISION 
Introduction: Rudolf Virchow proposed the cell lineage theory. Cell lineage theory states "omnis cellula e– cellula" i.e. new cells arise from pre-existing cells. Rudolf Virchow failed to prove the theory.
Mitosis: Stages NEET Notes | EduRevFig: Cell Lineage

Karl Nageli: New cells arise by division of pre-existing cells.
Starsburger: New Nuclei arise by division of pre-existing nuclei.

Cell division:
(1) Mitosis
(2) Meiosis
(3) Amitosis

Mitosis: Stages NEET Notes | EduRev

Fig: Cell division

MITOSIS
Mitosis name proposed by Flemming & its detail study was given by A. Schneider. Mitosis produced genetically identical cells, which are similar to mother cell.

Mitosis: Stages NEET Notes | EduRev

Fig: Mitosis

Cause of Mitosis
(I) Kern plasm theory
Hertwig proposed kern plasm theory. According to this theory mitosis is due to disturbance in Karyoplasmic Index (KI) of cell.
Karyoplasmic Index:

Mitosis: Stages NEET Notes | EduRev
Vn  = Volume of nucleus Vc   = Volume of cell Vc – Vn = Volume of cytoplasm
Karyoplasmic Index of small cell is high as they have less cytoplasm. Nucleus efficiently controls the activity of cytoplasm in small cells. In a large cell nucleus fail to control the activity of cytoplasm. To attain the control of nucleus on metabolism a large cell divides into two cells.

(II) Surface-volume Ratio

  • Surface-volume ratio of a cell plays an important role in starting cell division. 
  • A cell draws all the materials needed for its maintenance & growth from its surface. When a cell grows in size its volumes increases more than its surface. So a stage will reach when the surface area becomes insufficient to draw the material. At such critical stage, division of cell started.

CELL CYCLE

  • Complete life cycle of a cell is called as cell cycle. A typical eukaryotic cell is illustrated by human cells in culture. These cells divide once in approximately every 24 hours. 
  • Yeast can progress through the cell cycle in only about 90 minutes. The time period of cell cycle is varied from organist to organism.

Mitosis: Stages NEET Notes | EduRevFig: Cell Cycle


Cell cycle involves two stages
(1) Interphase and (2) Division phase/M-phase
1. Interphase
This is middle stage of cell cycle, because there occurs one interphase between two division phase. In interphase cell grows in size and prepares itself for next division. Interphase is most active phase of cell cycle.

  • In interphase, metabolism of cell increases. A series of metabolic changes occurs during interphase in cell.
  • These changes are not visible under microscope, So some scientist termed interphase as resting phase. But now it's known that it is the most active phase of cell cycle.
  • Howard and Pelc classified interphase into three sub stages:

(i) G1– phase or Pre DNA synthesis phase (Ist Gap phase) (Longest phase of cell cycle (12 hr)) During G1-number of cell organelles increases in cell and cell rapidly synthesizes different types of RNA and proteins. Due to availability of protein, synthesis of new protoplasm takes place in cell and it starts growing in size. Cell grows maximum in G1 stage.

(ii) S– phase (DNA Synthesis phase (6 – 10 hr)): 

  • Replication of nuclear DNA and synthesis of histone protein takes place in s-phase. Replication of cytoplasmic DNA may occur in any stage of cell cycle. Centriole replicates in late s-phase.
  • Cell cycle can remain arrested only in G1 phase. Then G1 is called as G0 phase in active stage or quiescent stage. G0 phase is found in cells of permanent tissue.
  • Cell cycle is running by a group of special proteins "Cyclins and Cdks (MPF). (Nurse, T.Hunt & Hartwell 2001 studies on saccharomyces (Baker yeast))

(iii) G2– phase (2nd Gap phase) or Post DNA synthesis phase (3 –12 hrs): (Pre mitosis phase) 

  • Number of cell organelles increases in cell. Actual preparation (Final preparation) of cell division occurs during this phase. Special materials required for cell division are synthesized in G2 phase. eg. Tubulin protein. –(Required for formation of spindle fibres).
  • Cell division involves enormous expenditure of energy thus cell stores ATP in G2 phase.
  • After G2 phase cell enters in division or M–phase.

How the cell cycle is controlled 

  • A cell reproduces by performing an orderly set sequences of irreversible events, In which it duplicates it's contents & then divides into two, these events are known as cell cycle.
  •  Molecular biologists, have made remarkable progress in identifying the biomolecules, that control or drive the cell cycle, many biologists, some of whom worked with invertebrate or frog egg's others with yeast cell or cell culture. Scientists concluded that the activity of enzymes, known as cyclin dependant kinases. (Cdk's) regulates the cell cycle. Kinase is an enzyme that removes a phosphate group from ATP & add to another protein. The kinases involved in the cell cycle are called Cdks because they are activated when they combined with  key protein called cyclin.
  •  At some check points Mitosis: Stages NEET Notes | EduRev kinase enzyme combines with cyclin & this moves the cell cycle forwardly.

S-kinase is capable of starting the replication of DNA after it combined with S-cyclin. After some time S-cyclin is destroyed & S-kinase is no longer active. M-kinase is capable of turning on mitosis after it has bind with M-cyclin. However certain characteristics are universal component of cell cycle control.

Genes 

CDC2, CDC28 

designated

Budding yeast

cdc2, cdc-28

designated

in fission yeast

Mitosis: Stages NEET Notes | EduRev

Fig Cell Cycle types

2. Division phase

  • Division phase or M–phase or mitotic phase (1hr). It is the phase of shortest time in cell cycle.
  • Karyokinesis: Division of nucleus. Name proposed by Schneider.
    Cytokinesis: Division of cytoplasm. Name proposed by Whitemen.
    Karyokinesis: Division of nucleus occurs by sequential change in nucleus and cytoplasm.

Division of nucleus in mitosis and meiosis is indirect.
(1) Prophase (Longest stage)
First stage of mitosis follows the S and G2 phases of interphase. Metabolism of cell decreases, cytoplasm becomes viscous, refractive and pale. Chromatin threads condenses to form chromosomes. In the S and G2 phases the new DNA molecules formed are not distinct but interwined. Centrioles start moving towards the opposite poles. Astral ray forms due to gelation of proteins around centrioles. In late prophase (Prometaphase) nuclear membrane and nucleolus disappear. Spireme stage of chromosome.

Mitosis: Stages NEET Notes | EduRev

Fig: Prosphase

Anastral and Amphiastral Mitosis
In plants, centrioles are absent and no asters are formed. Mitosis without asters is known as anastral mitosis. In animals, the asters are present and the mitosis is described as amphiastral, or astral mitosis.
Cell at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.

(2) Metaphase

  • Chromosomes arrange on equator to form metaphase plate (Metakinesis) or equatorial plate.
  • Chromosomal (discontinuous) fibres, (which run from pole to centromere) and supporting (continuous) fibres, (which run from pole to pole), arrange in cell.
  • Spindle fibre are composed of 97% tubulin protein and 3% RNA.
  • In metaphase plate large chromosomes lie towards periphery and small chromosomes towards centre.
  • Centromere lies at equator and arms remain directed towards poles.
  • Chromosomal fibres have polarity i.e. + end at equator and – end at the pole. In metaphase each chromosome splits lengthwise upto the centromere (division of matrix of chromosome). Thus replicated chromatids clearly visible at metaphase stage.
  • Two chromatids of a chromosome repulse each other and the arms of chromosomes are directed towards the opposite poles. Condensation of chromosomes is completed.

(3) Anaphase (Smallest stage)

  • In early anaphase interzonal fibres (small and contracted) appears at equator of cell.
  • Centromere of each chromosome splits lengthwise (division of centromere).
  • Number of chromosome becomes double in cell during mitotic anaphase.
  • Interzonal fibres expands and they push chromosomes towards the opposite poles. (Pushing) 
  • Chromosomal fibres contract and they pull chromosome towards opposite poles. (Pulling) 
  • By pulling and pushing mechanism chromosomes rapidly move towards the opposite poles.
  •  Approximately 30 ATP are required to carry a chromosome to pole. Chromosomes reach at poles in late anaphase.

Mitosis: Stages NEET Notes | EduRevFig: Anaphase

(4) Telophase (Reverse prophase)
During the telophase metabolism of cell increases. Nuclear membrane Golgi, ER and Nucleolus reappears. Chromosomes decondense to form chromatin net.

Mitosis: Stages NEET Notes | EduRev

Fig: Telophase

During telophage stage chromosome became decondense and lost their individuality.

Cytokinesis
Cytokinesis Starts in late anaphase. In animals cytokinesis occurs by constriction & furrow formation. Microtubules and microfilaments arrange on equator to form midbody. Contraction occurs in midbody and Plasma membrane starts constricting to form contratile ring. Thus a furrow forms from the out side to inside in cell. Furrow deepens continuosly and ultimately a cell divides into two daughter cells. In animals cytokinesis occurs in centripetal order.

Cytokinesis in plants– Takes place by cell plate formation because constriction is not possible due to presence of the rigid cell wall.

Mitosis: Stages NEET Notes | EduRevFig: Cytokinesis in Plants

  • Many golgi vesicles and spindle microtubules arrange themselves on equator to form phragmoplast. Fragments of ER may also deposit in phragmoplast. Membrane of golgi vesicles fuse to form a plate like structure called cell plate. Golgi vesicles secret calcium and magnesium pectate. Further cell plate is modified into middle lamella. In plants, cytokinesis occurs in centrifugal order (cell plate formation is from center to periphery).

SIGNIFICANCE OF MITOSIS
1. Development of an organism occurs by mitosis. Every organism starts its life from a single cell i.e. zygote. Repeated mitosis in zygote leads to the formation of the whole body.
2. Growth in body of an organism occurs by mitosis.
3. Repairing and regeneration in body occurs by mitosis. 5 ×  109 cells form per day in man.

MODIFICATION OF MITOSIS
1. Cryptomitosis or Promitosis: It is a primitive type of mitosis. In this type of division, nuclear membrane does not disappear but remain intact throughout the division. All the changes of karyokinesis occurs inside nucleus even the formation of spindle (Called as intranuclear spindle) Such division is found in some protozoans (Amoeba) during binary fission.
2. Dinomitosis: Dinomitoss is founds in dinoflagellates, which are mesokaryotes. In mesokaryotic cells histones are absent. Because of this, the chromosomes fail to condense properly and hence are not distinctly visible during cell division.
Nuclear membrane persists throughout the cell division and so spindle formed is intranuclear type. Normal mitosis is termed as Eumitosis. Spindle which forms during normal mitosis is termed as "Nuclear spindle".
3. Free nuclear division: Karyokinesis is not followed by cytokinesis so such divisions lead to coenocytic condition. eg. endosperm, fungi of phycomycetes group.
4. Endomitosis: This is duplication of chromosomes without division of nucleus. Endomitosis leads to polyploidy. i.e. Increase in number of sets of chromosome. Colchicine induces polyploidy in plants. Colchicine is a mitotic poison as it arrests the formation and arrangement of spindle fibres.
5. Endoreduplication: Endoreduplication is a modification of endomitosis. The polytene chromosomes form by process of endoreduplication. In endore-duplication, the chromonema replicate but do not get seperated.
This process is also known as polyteny.
Mustard gas and Ribonucleases are also mitotic poisons.

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