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.
Fig: Cell Lineage
Karl Nageli: New cells arise by division of pre-existing cells.
Starsburger: New Nuclei arise by division of pre-existing nuclei.
Fig: Cell division
Mitosis name proposed by Flemming & its detail study was given by A. Schneider. Mitosis produced genetically identical cells, which are similar to mother cell.
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.
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
Fig: Cell Cycle
Cell cycle involves two stages
(1) Interphase and (2) Division phase/M-phase
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.
(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)):
(iii) G2– phase (2nd Gap phase) or Post DNA synthesis phase (3 –12 hrs): (Pre mitosis phase)
How the cell cycle is controlled
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 After some time S-cyclin is destroyed & S-kinase is no longer active.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.
in fission yeast
Fig Cell Cycle types
2. Division phase
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.
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.
(3) Anaphase (Smallest stage)
(4) Telophase (Reverse prophase)
During the telophase metabolism of cell increases. Nuclear membrane Golgi, ER and Nucleolus reappears. Chromosomes decondense to form chromatin net.
During telophage stage chromosome became decondense and lost their individuality.
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.
Fig: 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.