Chapter 11- Cell Cycle and Mitosis PDF

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Complete notes on chapter 11 of instructor-provided course manual (in place of Campbell Biology textbook)....

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CHAPTER ELEVEN 1. Prokaryotic cell division- occurs via binary fission. Bacteria have one circular chromosome. a. Common aspects in all cell divisions: i. DNA replication ii. Division of other cell parts in a similar manner b. Prokaryotes divide via binary fission i. Bacteria contain 1 circular chromosome (like the DNA in mitochondria) rather than linear chromosomes (like eukaryotes) ii. Bacteria can also contain plasmids (smaller, extra chromosomal circular DNA molecules) c. Binary fission process: i. Chromosomal DNA replicates once per cell division ii. Each daughter chromosome is than moved to 1 pole of the elongated cell iii. Plasma membrane grows inward to divide the cell in 2 iv. New cell wall forms between 2 daughter cells, which each has 1 copy of chromosomal DNA & a random number of plasmids

v. EUKARYOTIC CELL DIVISION 2. General points: definition of chromosome, sister chromatids and how they exist in different parts of the cell cycle. a. Chromosome: a segment of DNA, generally in a packed state with proteins i. Nucleoid region: where the chromosome is confined b. Origin of replication: a special area on the chromosome where DNA replication proteins assemble & begin DNA replication c. Eukaryotic cells have LINEAR CHROMOSOMES (contain genes & other regions of DNA)

d. e. f. g. h.

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j. k. l.

i. Have multiple ORIGINS OF REPLICATION Genes: specific DNA sequences in chromosomes i. May encode different kinds of RNA & a large variety of proteins CHROMOSOMES organized into chromatin (DNA + protein complexes in nucleus) Typically, all non-sexual cells of a species have the same number of chromosomes Gametes: reproductive cells i. Contain half the number of chromosomes that normal cells do Centrosome: a protein complex in the cytoplasm (the poles of the mitotic spindle???) i. Organizes the formation of microtubules in cell division ii. In animal cells, centrosome also contains a pair of structures called centrioles Sister chromatids: replicated strands of DNA (which are identical to each other & to the parental chromosome) i. 2 chromatids held together along most of their lengths by a protein called cohesin 1. When attached like this, still considered to be 1 chromosome Centromere: a region near the middle of 2 sister chromatids Mitotic spindle: composed of microtubules, associated proteins, & the 2 centromeres (of sister chromatids) Cleavage furrow: the pinched-in area of a cell that is formed during cytokinesis

MITOSIS 3. Understand names and sequence of stages of cell cycle, including the 3 subphases of interphase and how each phase is defined. Be able to describe the visual and biochemical changes that take place at each stage and how cytokinesis takes place in animal and plant cells that leads to two daughter cells. a. THE CELL CYCLE consists of 2 major phases: i. Interphase: divided into 3 subphases (G1, S, & G2) ii. Mitotic (M) phase: divided into mitosis (division of the nucleus) & cytokinesis (division of the cytoplasm) 1. Further subphases of mitosis: prophase, prometaphase, metaphase, anaphase, & telophase

iii. b. INTERPHASE: i. G1 (Gap 1): 1. Most cells spend the majority of time here 2. CENTROSOME duplicated 3. Cell grows in size ii. S (DNA Synthesis): 1. Chromosomes replicate (remain associated w/ proteins as CHROMATIN) → SISTER CHROMATIDS iii. G2 (Gap 2): 1. Growth & final prep 2. All organelles & membranes (including chloroplasts & mitochondria) duplicated by end of this c. M PHASE: i. Prophase: 1. NUCLEOLUS disappears 2. CHROMATIN become more tightly coiled 3. Most cohesin destroyed, except at CENTROMERE → 2 sister chromatids now visible 4. MITOTIC SPINDLE forms a. MICROTUBULES grow → moves centrosomes toward opposite poles ii. Prometaphase: 1. NUCLEAR ENVELOPE fragments → no longer visible 2. KINETOCHORE structures appear at centromere, 1 on each chromatid

a. Kinetochore microtubules attach to kinetochores → move chromosomes toward the middle of the cell b. 2 sister chromatids still considered 1 chromosome iii. Metaphase: 1. Chromosomes aligned midway (at the metaphase plate) between the CENTROSOMES (poles of the spindle???) 2. A PROTEASE breaks down cohesin at the centromere → chromatids separate a. Each chromatid now considered a chromosome iv. Anaphase: 1. Each chromosome moved by MOTOR PROTEINS in kinetochore along the spindle, towards the spindle pole it is facing 2. Kinetochore microtubules shortened at kinetochore ends by depolymerization 3. Nonkinetochore microtubules lengthen → move spindle poles further apart v. Telophase: 1. Chromosomes arrive at spindle poles & begin to decondense 2. Nuclear envelope reforms 3. Nucleoli form in new nuclei 4. Cytokinesis: a. IN ANIMALS: i. Cytoplasm division occurs by a contractile ring of ACTIN & MYOSIN that pinches the cell in 2 → CLEAVAGE FURROW b. IN PLANTS: i. Cytoplasm division occurs by laying down new plasma membrane & cell wall d. Cells then enter into interphase again & continue cell division process

e. REGULATION OF CELL CYCLE 4. Understand what is a checkpoint and examples of internal and external conditions that are monitored by cell to determine whether to progress from one stage to another. a. Checkpoints: critical stages where cell condition is assessed to decide if the cell is ready for further progress through cell cycle i. G1 checkpoint (controls G1 → S) 1. Checks for: a. Cell size b. Nutrients c. Growth factors d. DNA damage

ii.

G2 checkpoint (controls G2 → M phase) 1. Checks for: a. Cell size b. DNA damage iii. Various checkpoints for stages w/in M phase 1. EX: Spindle Assembly Checkpoint a. Checks for: i. Chromosome attachment to spindle 5. Be aware that different cyclin-cdks serve as major components controlling different checkpoints. a. MPF serves to regulate M phase b. Other cdks & cyclins regulate other steps in the cell cycle 6. Be able to diagram and describe the molecular events at the G2 to M checkpoint as described below: a. What is the enzymatic activity of MPF and what are several processes in mitosis which it controls? i. M-phase promoting factor (MPF): a protein kinase which phosphorylates many target proteins 1. Activity detected in M phase only, not in interphase 2. Effects of MPF phosphorylation: a. Chromosome condensation b. Nuclear envelope breakdown i. By adding phosphate to lamins of the nuclear lamina c. Microtubule reorganization to form mitotic spindle 3. Has 2 subunits: a. Cyclin dependent kinase (cdk) i. Present at a constant amount through all stage of the cell cycle b. Cyclin i. Amount varies throughout cell cycle & peaks at M phase

ii.

iii.

Cyclin synthesis starts after M (phase?) → gradually accumulates → rapidly degrades in later part of M phase

1. iv. Activation of MPF causes rapid degradation of cyclin 1. MPF phosphorylates a target protein 2. Initiates (with built-in delay) the covalent linkage of ubiquitin (a small protein) to cyclin 3. Ubiquitin-cyclin complex recognized by the proteosome (a huge protein-degrading structure in the cytoplasm) 4. Cyclin degraded by proteases in the PROTEOSOME b. What is the structure and regulation of MPF, how does its activity sharply increase during the end of G2 and in the first part of M phase? Understand the roles of other proteins in this process as presented. What are the molecular events which terminate MPF activity? In a general way, understand that protein phosphatases remove phosphate groups off proteins as part of the checkpoint activity termination process. The specific names for subunits shown in figure (i.e. Cdc13) and enzymes (i.e. Wee1) are not important. i. The abrupt switch to dramatically increase MPF activity at the end of G2… 1. Cyclin binds to cdk subunit to form MPF a. MPF inactive 2. An inhibitory protein kinase adds P (phosphate) to the cdk subunit at the inhibitory site a. MPF inactive

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iii.

3. An activating protein kinase adds P to the cdk subunit at the activating site a. MPF inactive 4. At end of G2: an activating protein phosphatase removes the inhibitory P a. MPF active → dramatic increase in amount of active MPF What controls the phosphatase in step 4 so that it activates MPF at the right time… 1. Phosphatase activity controlled by its phosphorylation state: phosphorylated (active) OR dephosphorylated (inactive) 2. The balance between protein kinases & protein phosphatases acting on the enzyme determines which phosphorylation state predominates 3. At the end of G2, a small amount of active MPF exceeds the opposing protein phosphatase → rapid upswing of phosphatase activity (active form) 4. Mid-M phase, loss of MPF due to cyclin destruction changes the balance → inactive state predominates

CANCERS 7. Cancers: unregulated cell division. Difference between benign and malignant tumors. a. Tumors: rapidly multiplying clumps of cells (due to uncontrolled cell division) i. Benign: 1. Remain at original location 2. Non-spreading 3. Generally harmless ii. Malignant: 1. Can spread 2. May prove life-threatening 3. Too much cell division occurs + cells are mobile → detach from original neighbors → use proteases to digest through to a blood vessel and then move elsewhere & attach 8. Define oncogenes and tumor suppressor genes and how their protein abundance or activities are modified in cancer. But no need to learn the names nor the detailed Rb, p53 or RAS mechanisms. a. Cyclin-cdk controls disrupted in some cancers i. EX: important checkpoint occurs within G1 for initiation of cell division b. Transcription factors (proteins that control gene activity to make RNA in nucleus) which normally suppress cell division also disrupted in some cancers i. EX: Rb protein c. 2 major types of genes involved in cancer: i. Tumor suppressor 1. EX: Rb, p53 2. Can cause cancer when mutated to inactive form or too little is made ii. Oncogene 1. EX: cyclin, K-ras GTPase 2. Can cause cancer when mutated so it is excessively active or excessively made d. Most cancers require multiple gene mutations (6-8) before malignant tumors show up e. Main sources of mutations: i. Biological → mistakes in DNA replication or repair ii. Chemical → carcinogens iii. Physical → radiation/radioactive chemicals...