MITOSIS
It is completed in two parts—
1.Karyokinesis 2. Cytokinesis
Karyokinesis has four stages—
Prophase , Metaphase , Anaphase , Telophase
PROPHASE — It is completed in three sub stages early , middle & late. It is longest phase of division.
•EARLY PROPHASE 1.Depolarization of skeletal microtubules into their protein subunits in animal cell .
•2.ER & Golgi apparatus breakdown into small vesicles.
•3. Cell motility stops.CENTRIOLE.ASTER RAYS,PHRAGMOPLAST,
•4.Centriole migrated to the Poles of cell ,microtubule (tubulin subunit) assemble around them giving a star shape appearance called Aster. Centriole and aster rays separated from each other by a clear zone called pericentriolar cloud.Role of aster rays is to shift the centriole pairs to the opposite ends of cell.
•5.Spindle can be form without aster & centriole.
•6. As the centriole move apart the microtubule stretching between them increase in number and length called mitotic spindle (SPINDLE FIBRES).The asembly of spindle starts at centrosome called microtubule organizing centre MTOC.
•7. The chromosome appear as a long thin threads in the form of ball hence named spireme stage.
•8. Chromosome undergo various changes 1. loss of water i.e cndensation.2.coiling that reduces their length to 4%.
•9. Now chromosome consisting of two identical sister chromatids held together at centromere at this region they have disc like structure called Kinetochore where spindle microtubule join it.
•10. Chromosomes are fully replicated at all points including centromere
Middle prophase—-Chromosome become shorten & thicken chromatids uncoil they distinguishable individually, nucleoli decreases in size & Nuclear envelop begin to breakdown and disappear.
Late prophase—-Spindle fully develop, nuclear envelop breaksdown fully releasing the chromosomes and other nuclear content into cytoplasm & centriole pair pushed to the ends.
•METAPHASE—-Spindle fibres attached to the chromosomes at their kinetochore called chromosomal or kinetochore or tractile fibres.And those spindle which do not attached to the chromosomes are called polar or interpolar or nonkinetochore fibres.
•At the equator of the spindle the chromosome get arranged in the form of plate called equatorial plate or metaphase plate.
•Metaphase is most suitable time for chromosome study.
•Anaphase—-The sister chromatids separate from each other at their kinetochore and move towards opposite ends. Cross bridges between polar and chromosomal spindle form and fiber slide past each other during chromatid movement using ratchet type mechanism for movement
Telophase—-Nuclear envelop reappear, chromosome gradually uncoil, nucleoplasm appear , two nuclei are formed , nucleoli start synthesizing ribosomes , spindle fibers astral rays disappear by depolarization of microtubules into tubulin. Centrioles form centrosomes.
•CYTOKINESIS—-
•Animal cell— Animal cell divide by furrowing also called cleavage . Constriction is brought about by a peripheral band of actin– myosin microfilaments just within the plasma membrane & involve enlargement of cell membrane. Constriction deepens and completely divide into two daughter cells.
•Plant cell—Plant cell form cell plate , the formation begins during late anaphase or early telophase . Small vesicle from Golgi apparatus or ER line up right across the middle of the spindle here they fuse to form phragmoplast which form plasma membrane of two daughter cells.The content of phragmoplast form cell plate which form middle lamella then primary and secondary wall by the deposition of cellulose hemicellulose and pectin.
