Cell Cycle and Cell Division M Phase

BIOLOGY CELL CYCLE AND CELL DIVISION M PHASE M-PHASE (MITOTIC PHASE): IT DIVIDES INTO TWO STAGES (A) Karyokinesis (B) Cytokinesis Mitosis It occurs either in diploid (animal) cells or both in diploid and haploid (plant) cells. A. Karyokinesis : It can be divided into four stages for sake of convinience (I) Prophase (II) Metaphase (III) Anaphase (IV) Telophase (I) Prophase: It is longest phase. It involves three phases. (a) Early prophase:  Chromatin undergoes dehydration to form long thin & coiled chromosomes that are called spiralization and their ends are indistinct and this stage is called spireme stage.  Centrosomes move away to each other and move towards opposite poles. (b) Mid prophase:  Chromosomes are thickened and shifted in the peripheral part of nucleus.  Formation of Astral rays starts. Nuclear membrane starts breakdown. Nucleolus reduces in size. 1 BIOLOGY (c) Late prophase:  Chromosome are comparatively thickened.  Centrosomes are reached to the opposite poles.  In this phase nuclear membrane is disappeared.  Nucleolus, ER & GB are also disappeared.  Aster is formed at each pole by a pair of centrioles and astral rays.  Centrioles start formation of spindle fibres.  Centrosome helps in the initiation of cell division in animal cells by forming spindle.  In plant cells centrosome is absent.  The spindle fibre arise from tubulin protein by gelation in the cytoplasm thus in plant cell centriole and asters have no role in the formation of spindle apparatus.  The middle broad part of spindle apparatus is called equator. (II) Metaphase :  The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis hence the chromosomes are spread through the cytoplasm of the cell.  By this stage condensation of chromosomes is completed and they can be observed clearly under the microscope.  This then is the stage at which morphology of chrmnosomes is most easily studied.  At this stage metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres (formed by the microtubules) to the chromosomes that are moved into position at the centre of the cell. 2 BIOLOGY Hence the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.  Chromosomal fibres (discontinous/kinetochore which run from pole to centromere) and supporting fibres (continous/non-kinetochore, which run from pole to pole) arrange in cell.  Centromere lies at equator and arms of chromosomes remain directed towards poles. The key features of metaphase are:  Spindle fibres attach to kinetochores of chromosomes.  Chromosomes are moved to spindle equator and get aligned along metaphase plate through spindle fibres to both poles. (III) Anaphase:  It causes equatorial division in each chromosome as a result the number of chromosomes becomes doubled.  Now half chromosomes move towards one pole and remaining half chromosomes move towards opposite pole by the constriction of chromosomal fibre. It is called Anaphasic movement.  At this time centromere of each chromosome lies towards poles and its arms behind it.  The rate of anaphasic movement of each chromosome is 0·2 m to 5 m / min. 30 ATP are consumed by a chromosome during anaphasic movement.  Each chromosome has single chromatid & it is V-shaped / L-shaped / J-shaped / I-shaped. (IV) Telophase (Reverse prophase):  At the beginning of the final stage of karyokinesis, i.e. telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and chromatin material tends to collect at each of the two poles. This is the stage which shows the following key events:  Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements. 3 BIOLOGY  Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.  Nucleolus golgi complex and ER reform. B. Cytokinesis: It is a division of cytoplasm. It starts in late anaphase & Completed at the end of telophase. Cytokinesis comprises two types. (i) Cell furrow or cell cleavage method  A viscous fibrous dense mid body is formed in the centre.  Constriction of peripheral microfilaments takes place toward centre (Centripetal) and finally both sides meet and two daughter cells are formed e.g. Animal cell. i. ii. (ii) Cell plate method  In case of plant cell, some spindle fibres exist that form phragmoplast.  Golgi vesicles and some ER vesicles deposite in the central part of phragmoplast and their deposition proceeds towards peripheral part (centrifugal) and finally a cell plate is formed followed by the formation of middle lamella. 4 BIOLOGY  Now primary wall deposites at each side of middle lamella and ultimately both cells get seperated to form two daugther plant cells.  In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of coenocytic (e.g.liquid endosperm in coconut). Note: As per NCERT - Liquid endosperm in coconut is syncytium. 5