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Transcript
THE CELL CYCLE Chapter 8 A. The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: Interphase Karyokinesis (mitosis) Cytokinesis 1. Interphase Cell is not dividing. G1 Phase - carries out basic functions & performs specialized activities. duration is extremely variable contains restriction checkpoint ~ cell “decides” to: - divide - enter a quiescent phase (G0) - die G0 Phase - cell maintains specialized characteristics, but does not divide. Ex. neurons & muscle cells S Phase - cell replicates chromosomes & synthesizes proteins. [animal cells replicate centrioles as well] G2 Phase - cell synthesizes additional proteins (ex. tubulin) & assembles/stores membrane material. 2. Karyokinesis (mitosis; M phase) Equal distribution of replicated genetic material. Prophase replicated chromosomes condense* centrosomes separate & migrate toward opposite sides of cell mitotic spindle forms (microtubules grow out from centrosomes) nucleolus disappears Prometaphase nuclear membrane breaks down spindle fibers attach to centromeres of chromosomes Metaphase chromosomes are lined up single-file along equator of mitotic spindle. Anaphase centromers part, sister chromatids (now called chromosomes) separate chromosomes move toward opposite poles Telophase mitotic spindle breaks down chromosomes decondense nuclear membranes reform around two nuclei nucleoli reappear 3. Cytokinesis Distribution of cytoplasm to daughter cells. begins during anaphase or telophase differs in animal & plant cells Cytokinesis in animal cells: cleavage furrow (slight indentation) forms around equator of cell. actin & myosin microfilaments act like a drawstring to pinch the cell in two. usually an equal division. Cytokinesis in animal cells Cytokinesis in plant cells: phragmoplast (microtubule structure) forms in cytoplasm & traps vesicles containing cell wall material. vesicles fuse, forming a cell plate across midline of cell. cell plate gives rise to two primary cell walls. Does cytokinesis always accompany karyokinesis? Karyokinesis in the absence of cytokinesis results in a syncytium (mass of multinucleated cells). Control of the Cell Cycle Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence. Shortening of telomeres - loss of telomere DNA signals cell to stop dividing. Some cells produce telomerase (enzyme that continually adds telomere DNA). Contact Inhibition - healthy cells stop dividing when they come in contact with other cells. Hormones - stimulate cell division. Ex. Estrogen stimulates uterine cell division Growth factors - proteins that stimulate local cell division. Ex. Epidermal growth factor (EGF) stimulates epithelial cell division filling in new skin underneath a scab Interaction of kinases & cyclins activate genes that stimulate cell division. B. Apoptosis Programmed cell death; part of normal development. Steps of Apoptosis: C. Cancer (loss of cell cycle control) Condition resulting from excess cell division or deficient apoptosis. Characteristics of Cancer Cells: can divide uncontrollably & eternally are heritable & transplantable lack contact inhibition readily metastasize exhibit angiogenesis exhibit genetic mutability Causes of Cancer: Over-expression of oncogenes Oncogenes are genes that trigger limited cell division. Inactivation of tumor suppressor genes Tumor suppressor genes prevent a cell from dividing or promote apoptosis. Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors: carcinogens radiation viruses diet exercise habits