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Chapter 12: The Cell Cycle Cell cycle: life of a cell from its formation from a dividing parent cell until its own division into 2 cells Cell division- results in genetically identical daughter cells Mitosis: division of the nucleus Cytokinesis: division of the cytoplasm Meiosis: non-identical daughter cells produced with only one set of chromosomes Concept Check Starting with a zygote, how many cell would result following five cell divisions? A chicken has 78 chromosomes in its somatic cells. How many chromosomes are in the chicken’s egg? How many cells in the chicken zygote? How many chromosome pairs are there in a chicken? Phases of the cell cycle Interphase G1 phase (first gap)- (5-6 hours) S phase (DNA synthesis)- (10-12 hours) G2 phase (second gap)- (4-6 hours) M phase (<1 hour) Mitosis Cytokinesis Mitosis Phases: Prophase Prometaphase Metaphase Anaphase Telophase & Cytokinesis Mitotic Spindle: fibers of microtubules & associated proteins; assembly starts at the centrosome G2 of Interphase 2 centrosomes have formed by replication In animal cells centrosomes have 2 centrioles Prophase Chromatin condenses into chromosomes Nucleoli disappear Duplicated chromosomes appear as 2 identical sister chromatids Mitotic spindle begins to form Centrosomes move away from each other toward opposite poles as spindle lengthens Prometaphase Nuclear envelope fragments Spindle invades nuclear area Kinetochores of sister chromatids attach to spindle at the centromere Metaphase Longest phase (~20 minutes) Chromosomes arrange on metaphase plate Anaphase Shortest phase Sister chromatids separate & move opposite poles as microtubules shorten Telophase Daughter cell nuclei begin to form Nuclear envelope arises Chromosomes de-condense Cytokinesis Occurs simultaneously with telophase Animals- cleavage furrow pinches the cell in 2 Plants- cell plate forms to separate cells in 2 (cell plate will become cell wall) Cell division in Prokaryotes: Binary Fission Circular DNA replicates beginning at the origin of replication As DNA replicates cell elongates When replication is complete plasma membrane grows inward separating the parent cell into 2 daughter cells Evolution of the mitotic process Concept Check In which stages of the cell cycle would a chromosome consist of two identical chromatids? What are the similarities & differences in the cytokinesis of plant & animal cells? Why would it be a disadvantage for a cell to divide first then replicate its DNA? Cell Cycle Control Each stage is triggered by specific molecular signals present in the cytoplasm Checkpoints 3 major checkpoints at G1, G2, & M phases G1 checkpoint (restriction point)- appears to be most important If a cell is given go-ahead it completes S, G2, & M phases If not given go-ahead, cell enters G0 (nondividing state) Regulating proteins for cell cycle control: kinases & cyclins Kinases enzymes that activate/inactivate other proteins through phosphorylation Cause go-ahead signals at G1 & G2 checkpoints Cyclins Attach to & activate kinases (cyclin dependant kinases (Cdks)) Regulating proteins for cell cycle control: kinases & cyclins Cdk Example: Maturation promoting factor (MPF) Rises during G2 to initiate mitosis Actions include- nuclear envelope fragmentation, chromosome condensation, spindle formation Destroys own cyclin during anaphase to initiate end of mitosis Cell Cycle Control: internal signals at checkpoints Growth factors Proteins that cells release to cause other cells to divide- (promote mitosis= mitogen) Over 50 different growth factors that trigger cell division have been identified Example: platelet-derived growth factor (PDGF) Release by platelets to cause fibroblast (connective tissue cell) division to repair wound Cell Cycle Control: external signals at checkpoints Density-dependant inhibition Crowded cells stop dividing Determined by the amount of nutrients & growth factors available to cells Anchorage dependance Cells must be attached to a substratum (culture dish or tissue matrix) in order to divide Loss of cell cycle control: Cancer Cancer cells do not respond to regulating signals of cell cycle Divide as long as nutrients available while normal cells divide only 20-50 times before death Transformation: normal cells convert to cancer cells Tumor: transformed cell mass that evaded destruction by immune system Ex. “immortal cells” – HeLa cells from Henrietta Lacks tumor removed in 1951 still dividing in culture Benign tumor: remain at original site; easier to remove Malignant tumor: invades nearby tissues & organs Metastasis: spread of cancer cells to locations in the body distant from original site Henrietta Lacks Source of HeLa cells Cancer Treatments Radiation cause more damage to cancer cells than normal cells Cancer cells lack repair mechanisms Chemotherapy Chemicals interrupt steps in cell division process of cancer cells Concept Check If a drug is administered that prevents DNA synthesis of a cancer cell which phase of the cell cycle is the cell trapped in? What is the purpose of Cdks? What features distinguish cancer cells & normal cells? Distinguish between a benign & malignant tumor. Why do you believe more money is invested in researching cancer treatments rather than researching cancer prevention?