Lec.7
Active MPF induces the downstream events of mitosisMPF has to bring about many radical changes in the cell to drive it into mitosis ,the chromosomes must be reorganized to form a mitotic spindle .MPF induces of these essential events through its protein kinase activity .It brings some changes directly by phosphorylation components of the cell .Other may be induced indirectly by phosphorylations that activate other protein kinases to alter the state of the cell .
The breakdown of the nucleus requires the disassembly of the nuclear lamina ,MPF catalyzes this process directly ,forcing the lamina molecules to disassemble by phosphorylating then one key serine residues .
MPF changes the behavior of microtubules in mitosis by phosphorylating microtubule associated proteins .In interphase the centrosome nucleates long microtubules that extend throughout the cytoplasm ,this disassembles and centrosoms nucleate a large number of shorter ,less stable microtubules ,which interact with one another to form a mitotic spindle .This transformation reflects a chemical change in the centrosome ,the microtubules ,or both.
Cell-division controls in multicultural animals
Mammalian cells in the resting state or non growing state or called G0 for a cell nutrient are not enough to it in order to grow and divide ,it must receive specific positive signals from other cells .Many of these signals are protein factors which bind to complement are receptors in the plasma membrane to stimulate cell proliferation .Thus the absence of appropriate growth factors sends cells into a sort of cell-cycle sleep ,where the cell-cycle control system is disabled from progressing past the G1 checkpoint. Others competition for growth factors are the shape of a cell also affects into ability to divide .The mechanics of cell division
The process of cell division {M phase} consist of :nuclear division {Mitosis} .
cytoplasmic division {cytokinesis} the splitting of cell as a whole into two cells.
These process are carried out by :
form a bipolar mitotic spindle ,composed of microtubules {are hollow cylinders structure found in the cytoplasm } and their associated proteins .
contractile ring of action filaments {forms beneath the plasma membrane in a plane perpendicular to the axis of the spindle .As the ring contracts it pulls the membrane inward so as to divide the cell into to two cells.
Centrosome divided before mitosis begins {during the S and G2 phase } into a pair of barrel-shape organelles called centrioles and the duplicated separate and move to opposite sides of the nucleus at the onset of {M} phase to form a two poles of mitotic spindle.
Plant cells lack centrioles {suggested that centriole are not required for spindle formation }.
Organelles such as Golgi apparatus and endoplasmic reticulum ,break up into many smaller fragments during {M phase} ,to distribute it into daughter cells during cytokinesis .
Mitosis divided into phases:
Prophase
The chromatin ,which is diffused in interphase slowly condensed by coiling and folding into well defined chromosomes .Each chromosome has duplicated during the preceding {S phase } and consist of two chromatids held together at a centromere ,which is required for proper segregation .Toward the end of prophase ,the cytoplasm microtubules that are part of the interphase cytoskeleton this assemble and the main components of the mitotic apparatuse ,the mitotic spindle ,begin to form nucleus disappear ,nuclear envelop fragments ,kinetochores {a structure } develop on other side of each centromere ,short microtubules radiate out in a star like from the pair of centrioles .
Metaphase
At early metaphase {prometaphase} . Spindle consist of poles a star and fibers which are bundle of parallel microtubules ,kinetochore of sister chromatid capture spindle microtubules {which are then called kinetochore microtubules } coming from opposite pole .They remaining microtubules in the spindle are called poler microtubules ,while these out side the spindle are called astral microtubules.
At late metaphase the chromosomes attached to kinetochore fibers at the metaphase plates {equatorial plates }.
Anaphase
At early anaphase daughter chromosome each with a centromere and single chromatid begin to move toward opposite pole. The spindle fibers shorten one third to one fifth they organelle length {presence of dynein } in kinetochores and its thought that these motor molecular dynein assist chromosome movement along the microtubules of kinetochores fibers .
Telophase
The separated daughter chromosome arrived at the poles in the kinetochore microtubules disappear , the polar microtubules elongated still more ,and a new nuclear envelop reforms around each group of daughter chromosome .Chromosome again uncoil nucleus reappear ,cytokinesis is nearly completed {by a process known as cleavage },and soon their will be two individual daughter cells .
There are variation between different organism ,particularly animal and plant cell in cells division ,these are :
in higher plants they spindle has no centrioles or aster at the poles.
Cytokinesis
In animal cells the cleavage furrow deepens when a band of actin filaments ,called they contractile ring ,slowly forms construction between the two daughter cell then a narrow bridge between the two cell can be seen during Telophase.While in plant cell its involves the formation of a cell plate .
Meiosis
Meiosis is a special type of cell division that is found in organism in which there is sexual reproduction cycle .This cycle involved an alteration of haploid {n} generation of cells , each carrying single set of the chromosome ,the diploid {2n} generation of cells {by fusion of two haploid cells } each carrying a double set of chromosomes .New haploid are generated by the process of meiosis .During this process the chromosome of the double chromosomes set ,exchange DNA by genetic recombination before being shared as into single chromosome set .
Haploid cell that are specialized for sexual fusion are called gametes .Two types of the gametes are formed by two successive cell division following one round of DNA replication give rise to one large and non-motile {egg or ovum } and three polar bodies by process called oogenesis and for motile bodies {sperm or spermatozoa } by process called spermatogenesis ,by their fusion in zygote {2n} ,while originated the complete multicellular organism in process called fertilization .If the mitosis where they only type of the cell division ,then each gamete would have 46 chromosomes and the zygote 92 chromosomes .This would be repeated in the next generation leading to un-increase in the number of the chromosomes.
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