Cell Cycles And Proliferation Essay

552 words - 3 pages

Cell cycle is a complex mechanism that governs the cell growth and proliferation. Cell proliferation contributes to the continuity of life by producing cells, replenishing cells which undergone to cellular differentiation to acquired specialized phenotypes (function and morphology) to carry out living mechanism and towards the end-point-cell-death. Cell proliferation is determined by both extracellular signals such as cytokines and mitogen, and intrinsic cellular factors. Interactions of extracellular signals with intrinsic cellular factors trigger the biochemical events of cell proliferation. In the case of acquired immunity, proliferation is the important state after lymphocytes encountered to antigen presentation, and then leads to their effectors functions. Cell cycle regulators control the appropriate entry and progression throughout the cell ...view middle of the document...

(Oshima and Campisi 1991; Schafer 1998) There are cells that have ceased proliferation, because lack of growth-stimulating signals or the present of anti-mitogenic signals, exited the cell cycle and entered to a state of quiescence (G0 phase) (Ivanchuk and Rutka 2004). These include the terminally differentiated cells, such as hepatocytes, muscle cells, neuron cells. However, cells in G0 phase still preserve the potential for cell division.

Progression of cell cycle is dependent on the integration of mitogenic signals, assessment of cell size, and DNA integrity (Lea et al. 2003). Upon stimulation of growth factors, cells are stimulated to enter into G1 phase of the cell cycle from a quiescent state (G0). The restriction point (RP) in the G1 phase serves as a marker for cell decision to commit to a new round of cell division. CDK4 and CDK6 are both active in early G1 phase bind to D-type cyclins to regulate RP progression (Draetta 1994). CDK4-cyclin D and CDK6-cyclin D complexes phosphorylate and inactive the retinoblastoma protein (pRb). In turn, pRb releases the E2F and Dp1 transcription factors which control the expression of genes that required for the G1/S transition and S phase progression. Additional phosphorylation of pRb by CDK2/cylin E complex is responsible for the G1/S transition and regulates the centrosome duplication. The onset of S phase is caused by phosphorylation of different substrate by CDK2/cyclin A complexes. This allows DNA replication and inactivation of G1 transcription factors. Around the S/G2 transition, cyclin A and B associate with CDK2 and CDK1 to form complexes that promote entry and progression of M phase (Ivanchuk and Rutka 2004). CDK1/cyclin B complexes are responsible for activation of the anaphase promoting complex (APC) which is important for transition to anaphase and completion of mitosis. Other than cyclin and CDK proteins, the progression of cell cycle is also affected by the tumor suppressor gene- the the cip/kip family (p21, p27, and p57) and the INK4a/ARF (p15, p16, p18, and p19).

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