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 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 Cell division- results in genetically identical daughter cells Mitosis: division of the nucleus Mitosis: division of the nucleus Cytokinesis: division of the cytoplasm Cytokinesis: division of the cytoplasm Meiosis: non-identical daughter cells produced with only one set of chromosomes 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? Starting with a zygote, how many cell would result following five cell divisions? A chicken has 78 chromosomes in its somatic cells. A chicken has 78 chromosomes in its somatic cells. How many chromosomes are in the chicken’s egg? How many chromosomes are in the chicken’s egg? How many cells in the chicken zygote? How many cells in the chicken zygote? How many chromosome pairs are there in a chicken? How many chromosome pairs are there in a chicken?

Phases of the cell cycle Interphase Interphase G1 phase (first gap)- (5-6 hours) G1 phase (first gap)- (5-6 hours) S phase (DNA synthesis)- (10-12 hours) S phase (DNA synthesis)- (10-12 hours) G2 phase (second gap)- (4-6 hours) G2 phase (second gap)- (4-6 hours) M phase (<1 hour) M phase (<1 hour) Mitosis Mitosis Cytokinesis Cytokinesis

Mitosis Phases: Phases: Prophase Prophase Prometaphase Prometaphase Metaphase Metaphase Anaphase Anaphase Telophase & Cytokinesis Telophase & Cytokinesis Mitotic Spindle: fibers of microtubules & associated proteins; assembly starts at the centrosome Mitotic Spindle: fibers of microtubules & associated proteins; assembly starts at the centrosome

G 2 of Interphase 2 centrosomes have formed by replication 2 centrosomes have formed by replication In animal cells centrosomes have 2 centrioles In animal cells centrosomes have 2 centrioles

Prophase Chromatin condenses into chromosomes Chromatin condenses into chromosomes Nucleoli disappear Nucleoli disappear Duplicated chromosomes appear as 2 identical sister chromatids Duplicated chromosomes appear as 2 identical sister chromatids Mitotic spindle begins to form Mitotic spindle begins to form Centrosomes move away from each other toward opposite poles as spindle lengthens Centrosomes move away from each other toward opposite poles as spindle lengthens

Prometaphase Nuclear envelope fragments Nuclear envelope fragments Spindle invades nuclear area Spindle invades nuclear area Kinetochores of sister chromatids attach to spindle at the centromere Kinetochores of sister chromatids attach to spindle at the centromere

Metaphase Longest phase (~20 minutes) Longest phase (~20 minutes) Chromosomes arrange on metaphase plate Chromosomes arrange on metaphase plate

Anaphase Shortest phase Shortest phase Sister chromatids separate & move opposite poles as microtubules shorten Sister chromatids separate & move opposite poles as microtubules shorten

Telophase Daughter cell nuclei begin to form Daughter cell nuclei begin to form Nuclear envelope arises Nuclear envelope arises Chromosomes de-condense Chromosomes de-condense

Cytokinesis Occurs simultaneously with telophase Occurs simultaneously with telophase Animals- cleavage furrow pinches the cell in 2 Animals- cleavage furrow pinches the cell in 2 Plants- cell plate forms to separate cells in 2 (cell plate will become cell wall) 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 Circular DNA replicates beginning at the origin of replication As DNA replicates cell elongates As DNA replicates cell elongates When replication is complete plasma membrane grows inward separating the parent cell into 2 daughter cells 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? 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? 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? 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 Each stage is triggered by specific molecular signals present in the cytoplasm

Checkpoints 3 major checkpoints at G 1, G 2, & M phases 3 major checkpoints at G 1, G 2, & M phases G 1 checkpoint (restriction point)- appears to be most important G 1 checkpoint (restriction point)- appears to be most important If a cell is given go-ahead it completes S, G 2, & M phases If a cell is given go-ahead it completes S, G 2, & M phases If not given go-ahead, cell enters G 0 (non- dividing state) If not given go-ahead, cell enters G 0 (non- dividing state)

Regulating proteins for cell cycle control: kinases & cyclins Kinases enzymes that activate/inactivate other proteins through phosphorylation enzymes that activate/inactivate other proteins through phosphorylation Cause go-ahead signals at G 1 & G 2 checkpoints Cause go-ahead signals at G 1 & G 2 checkpointsCyclins Attach to & activate kinases (cyclin dependant kinases (Cdks)) Attach to & activate kinases (cyclin dependant kinases (Cdks))

Regulating proteins for cell cycle control: kinases & cyclins Cdk Example: Maturation promoting factor (MPF) Rises during G 2 to initiate mitosis Rises during G 2 to initiate mitosis Actions include- nuclear envelope fragmentation, chromosome condensation, spindle formation Actions include- nuclear envelope fragmentation, chromosome condensation, spindle formation Destroys own cyclin during anaphase to initiate end of mitosis 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) 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 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 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 Crowded cells stop dividing Determined by the amount of nutrients & growth factors available to cells 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 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 Cancer cells do not respond to regulating signals of cell cycle Divide as long as nutrients available while normal cells divide only times before death Divide as long as nutrients available while normal cells divide only times before death Ex. “immortal cells” – HeLa cells from Henrietta Lacks tumor removed in 1951 still dividing in culture Ex. “immortal cells” – HeLa cells from Henrietta Lacks tumor removed in 1951 still dividing in culture Transformation: normal cells convert to cancer cells Transformation: normal cells convert to cancer cells Tumor: transformed cell mass that evaded destruction by immune system Tumor: transformed cell mass that evaded destruction by immune system Benign tumor: remain at original site; easier to remove Benign tumor: remain at original site; easier to remove Malignant tumor: invades nearby tissues & organs Malignant tumor: invades nearby tissues & organs Metastasis: spread of cancer cells to locations in the body distant from original site Metastasis: spread of cancer cells to locations in the body distant from original site

Henrietta Lacks Source of HeLa cells Source of HeLa cells

Cancer Treatments Radiation cause more damage to cancer cells than normal cells cause more damage to cancer cells than normal cells Cancer cells lack repair mechanisms Cancer cells lack repair mechanismsChemotherapy Chemicals interrupt steps in cell division process of cancer cells 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? 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 is the purpose of Cdks? What features distinguish cancer cells & normal cells? What features distinguish cancer cells & normal cells? Distinguish between a benign & malignant tumor. Distinguish between a benign & malignant tumor. Why do you believe more money is invested in researching cancer treatments rather than researching cancer prevention? Why do you believe more money is invested in researching cancer treatments rather than researching cancer prevention?