Friday, October 14, 2011
Wednesday, October 12, 2011
The phase between two mitoses is called INTERPHASE, during which the nucleus appears as it is normally observed in microscopic preparations.
The PROPHASE of mitosis is characterized by the gradual coiling of nuclear chromatin (uncoiled chromosomes), giving rise to several individual rod-or hairpin-shaped bodies (coiled chromosomes) that stain intensely.
During METAPHASE, chromosomes, due to the activity of microtubules, migrate to the equatorial plane of the cell, where each divides longitudinally to form two chromosomes called sister chromatids. The chromatidsattach to the microtubules of the mitotic spindle.
In ANAPHASE, the sister chromatidsseparate from each other and migrate toward the opposite poles of the cell, pulled by microtubules. Throughout this process, the centromeresmove away from the center, pulling the remainder of the chromosome along. The centromereis the constricted region of a mitotic chromosome that holds the two sister chromatidstogether until the beginning of anaphase.
TELOPHASE is characterized by the reappearance of nuclei in the daughter cells. The chromosomes revert to their semidispersedstate, and the nucleoli, chromatin, and nuclear envelope reappear. While these nuclear alterations are taking place, a constriction develops at the equatorial plane of the parent cell and progresses until the cytoplasm and its organelles are divided in two. This constriction is produced by microfilaments of actinassociated with myosin that accumulate in a beltlikeshape beneath the cell membrane.
The first phase within interphase, from the end of the previous M phase until the beginning of DNA synthesis is called G1 (G indicating gap). It is also called the growth phase. During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate. This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication. Duration of G1 is highly variable, even among different cells of the same species.
The ensuing S phase starts when DNA synthesis commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids. Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidyof the cell remains the same. Rates of RNA transcription and protein synthesis are very low during this phase. An exception to this is histoneproduction, most of which occurs during the S phase.
The cell then enters the G2 phase, which lasts until the cell enters mitosis. Again, significant protein synthesis occurs during this phase, mainly involving the production of microtubules, which are required during the process of mitosis. Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.
MITOSIS (M PHASE)
The relatively brief M phase consists of nuclear division (karyokinesis) and cytoplasmicdivision (cytokinesis)
In cells that are not continuously dividing, the activities of the cell cycle may be temporarily or permanently suspended. Cells in such a state (eg, muscle, nerve) are referred to as being in the G0phase.
CLINICAL :Growth factors stimulates increase cell cycle rate thus increase the cell production e.g.. Erythropoietin.In Cancers the cell cycle is abnormally increased thus uncontrolled cell production.Some Anticancer drugs act by controlling this uncontrolled rate.