Chapter 18 Cell Division PDF

lOMoARcPSD|46396031 I. The Cell-Division Cycle Cell cycle- duplication and division A. Overview of the Cell Cycle a. The Eukaryotic Cell cycle Usually includes Four Phases M phase- mitosis and cytokinesis Interphase the period between one M phase and...

lOMoARcPSD|46396031 I. The Cell-Division Cycle Cell cycle- duplication and division A. Overview of the Cell Cycle a. The Eukaryotic Cell cycle Usually includes Four Phases M phase- mitosis and cytokinesis Interphase the period between one M phase and the next - S phase (synthesis) - cell replicates its DNA - G1 and G2- cells continue to grow. - Cell monitors internal and external environments to ensure suitable conditions for reproduction. In interphase, a cell continues to transcribe genes, synthesize proteins and grow in mass. b. A Cell-Cycle Control System Triggers the Major Processes of the Cell Cycle Cell-cycle control system- ensures cell cycle events occur completely before the next begins and has regulation at critical points of feedback cycle with checkpoints. From G1 to S, the control system checks on the environment (nutrient and signal molecules in EC) before duplicating DNA. If not, the cycle can be arrested in G0. From G2 to M, undamaged DNA is confirmed and fully replicated, so the cell enters mitosis when DNA is intact. Also, cell cycle control machinery checks that duplicated chromosomes are attached to the mitotic spindle before being pulled apart. c. Cell-Cycle Control is Similar in All Eukaryotes B. The Cell-Cycle Control System Cell-cycle control system- turns machinery for making and allocating new components of cell on and off. a. The Cell-Cycle Control System Depends on Cyclically Activated Protein Kinases called Cdks. Cyclically activating and inactivating key proteins and complexes of mitosis due to de/phosphorylation of proteins. Protein kinases aid in the end of G1 phase going into S phase, then another kinase before M phase and into mitosis. Cyclins- proteins in the control system that switch kinases on/off at appropriate times. No enzymatic activity but binds to cell-cycle kinases before the kinases are enzymatically active. Cdks- kinases of cell cycle control systems are cyclin dependent protein kinases which helps drive cyclic assembly and activation of cyclin-Cdk complexes, which trigger S phase into M... b. Different Cyclin-Cdk Complexes Trigger Different Steps in the Cell Cycle M cyclin- cyclin that acts in G2 to trigger entry into M phase and the active complex it forms is M-Cdk. S cyclin and G1/S cyclin- bind to a certain Cdk protein late in G1to form S-Cdk and Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 G1/S-Cdk--- these cyclin-Cdk complexes help launch S phase. G1 cyclins- act early in G1 and bind to other Cdk proteins to form G1-Cdks to drive cells through G1 toward S phase (depends on EC signal molecules that stimulate cells to divide). c. Cyclin Concentrations are Regulated by Transcription and Proteolysis Gradual increase in cyclin is due to transcription of cyclin genes and rapid fall is precipitated by a full-scale targeted destruction of the protein. Anaphase-promoting complex- large enzyme complex that abruptly degrades M and S cyclins part way through M phase by tagging the cyclins with ubiquitin which will direct to proteasomes where they are rapidly degraded. The ubiquitylation and degradation of cyclin returns its Cdk to an inactive state. - M-cyclin degradation and resulting inactivation of M-Cdk leads up to removing cells from mitosis. d. The Activity of Cyclin-Cdk Complexes Depends on Phosphorylation and Dephosphorylation Activity of associated cyclin-Cdk complexes switch on abruptly at the appropriate time in the cell cycle. The cyclin-Cdk complex has inhibitory phosphatases and Cdk needs to be dephosphorylated by a specific protein phosphatase to become active. e. Cdk Activity Can be Blocked by Cdk Inhibitor Proteins Cdk Inhibitor Proteins- binding modulates activity of Cdks. These inhibitors are used to block assembly/activity of the cyclin-Cdk complexes. - Some may help Cdks maintain inactive during G1 to give cells more time for favorable conditions before going into S phase. f. The Cell Cycle Control System Can Pause the Cycle in Various Ways At the G1 to S transition, Cdk inhibitors are used to stop cells from entering S phase and replicating their DNA. At the G2 to M transition, it suppresses the activation of M-Cdk by inhibiting the phosphatase needed to activate the Cdk. Can delay exit from mitosis by activating APC and preventing degradation of M cyclin. C. G1 Phase Metabolic activity, cell growth and repair and uses IC and EC signals. a. Cdks are Stably Inactivated in G1 If S-Cdks and M-Cdks are not disabled by the end of M phase, the cell will immediately replicate DNA and start another round of division without enough time in G1 or G2. Thus, S and M- Cdks need to be inactivated by eliminating all existing cyclins, stopping synthesis of new one and deploying Cdk inhibitor proteins. b. Mitogens Promote the Production of the Cyclins that Stimulate Cell Division Mitogens- mammalian cells will only multiply as signaled by EC signals, made by other cells. If not, the cell cycle arrests in G1. Mitogens switch on cell signaling pathways that stimulate the synthesis of G1 cyclins, Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 G1/S cyclins and proteins for Na synthesis and chromosome duplication. To get out of arrest, cyclins need to be accumulated, which will trigger G1/S-Cdk activity relieving negative controls that would have blocked the progression from G1 to S phase. c. DNA Damage Can Temporarily Halt Progression Through G1 P53- transcriptional regulator that activates the transcription of a gene encoding a Cdk inhibitor protein called p21and operates at G1 to S transition. - P21 protein binds to G1/S-Cdk and S-Cdk stopping from entering the S phase. The arrest in G1 allows time for DNA to repair. If too severe, p53 can induce apoptosis. - If p53 is missing/defective, high rates of mutation and cancers can accumulate. d. Cells Can Delay Division for Prolonged Period by Entering Specialized Nondividing States Terminally differentiated cells- nerves/muscle cells where they permanently stop dividing when they differentiate. The control system is ended and genes encoding relevant cyclins and Cdks are irreversibly shut down. If appropriate signals are absent, cells can temporarily enter G0 to reassemble the system quickly and to divide again. D. S Phase a. S-Cdk Initiates DNA Replication and Blocks Re-Replication In G1, prepping is done for DNA to be made replication-ready by recruiting proteins to sites along each chromosome Origins of replication- nucleotide sequences where proteins are recruited along each chromosome where replication will begin. Origin Recognition complex (ORC)- is on top of replication origins in cell cycle. - In the first step of replication initiation, ORC recruits protein Cdc6, with concentrations rising early in G1. Both will make DNA helicases open helix and prepare the origin of replication. Replication signal comes from S-Cdk, cyclin-Cdk complex that triggers S phase. S-Cdk is assembled/activated at the end of G1. During the S phase, it activates helicases and promotes assembly of proteins that form the replication fork. S-Cdk also helps prevent re-replication by aiding with phosphorylating Cdc6 to mark it for degradation, so DNA replication will not occur again. b. Incomplete Replication Can Arrest the Cell Cycle in G2 System can delay entry into the M phase if errors in DNA replication occur or if replication is delayed. M-Cdk is inhibited by phosphorylation at certain sites, for entry into mitosis, inhibitory phosphates need to be removed by an activating protein phosphatase Cdc25. - When DNA is damaged/incompletely replicated, Cdc25 is inhibited and inhibitory phosphates can’t be removed. M-Cdk remains inactive and M phases delayed until DNA replication is complete and any DNA damage is repaired. Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 When the cell has replicated DNA in S phase and progressed through G2, it’s ready for M phase; here, the cell will divide nucleus in mitosis and then cytoplasm in cytokinesis. E. M Phase Cell recognizes components and distributes it equally into 2 daughter cells. a. M-Cdk Entry Into M Phase and Mitosis M-Cdk allows for arrangements in early stages of mitosis and helps prep duplicated chromosomes for segregation and induces assembly of mitotic spindle. M-Cdk complexes accumulate through G2 but isn’t activated until the end, when activating phosphatase Cdc25 removed inhibitory phosphates holding M-Cdk activity in check; once activated each M-Cdk complex can indirectly activate additional M-Cdk complexes by phosphorylating and activating more CDc25. - Activated M-Cdk shuts down inhibitory kinase Wee1 and promotes production of activating M-Cdk , ignites an increase in M-Cdk activity that drives cells from G2 into M phase. b. Cohesins and Condensins Help Configure Duplicated Chromosomes for Separation Condensins- protein complexes that condense the duplicated chromosomes in M phase in chromosome condensation, compacting bodies to be easily segregated. Assembly of condensin complexes into DNA is triggered by phosphorylation of condensins by M-Cdk. Sister chromatids- a single, double-stranded molecule of DNA along with associated proteins, both copies remaining tightly bound together during S phase. Sisters are held by cohesin’s- protein complexes that assemble along the length of each chromatid as DNA is replicated. Cohesins tie the sister chromatids together, condensins assemble on each individual sister chromatid at the start of M phase to help each helix coil up more. c. Different Cytoskeletal Assemblies Carry Out Mitosis and Cytokinesis The mitotic spindle carries out nuclear division in mitosis and contractile ring carries out cytoplasm division in cytokinesis. Mitotic spindle is composed of MTs, proteins and MT-associated motor proteins, spindle separates duplicated chromosomes and places one copy of each chromosome to each daughter cell. Contractile ring- made of actin filaments and myosin filaments in a ring around the cell equator just beneath PM pulling the membrane inward, dividing the cell in two. F. Mitosis a. Centrosomes Duplicate to Help Form the Two Poles of the Mitotic Spindle Centrosome- principal microtubule-organizing center and must be duplicated like the DNA, so each daughter cell can have their own centrosome. They duplicate at the same time as DNA and begin by the same Cdks-G1/S-Cdk and S- Cdk that begin duplication. When mitosis begins, both centrosomes separate and each nucleates a radial array of MTs called an aster. Both move to opposite sides of the nucleus to form both poles of the mitotic spindle in the centrosome cycle. b. The Mitotic Spindle Starts to Assemble in Prophase Prophase- spindles form and MT stability decreases b/c M-Cdk phosphorylates MT- Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 associated proteins, and MT rapidly growing/shrinking. Mitotic spindle- MTs from one centrosome interact with others from the other centrosome and stabilizes it, preventing depolymerization making two sets for the MSs bipolar shaped. Spindle poles- both centrosomes that made MTs , interacting MTs are interpolar microtubules. c. Chromosomes Attach to the Mitotic Spindle at Prometaphase Prometaphase- starts with disassembly of nuclear envelope breaking into small membrane vesicles due to phosphorylation/disassembly of nuclear pore proteins and IF proteins of nuclear lamina. Kinetochore- protein complexes on the centromere of each condensed chromosome during late prophase, where SMs grab onto chromosomes. (One of each sister chromatids in opposite directions). Chromosomes will not separate if DNA sequence at centromere is altered, kinetochore will not recognize it. Kinetochore MT links chromosomes to spindle poles and kinetochores on sisters have bi-orientation creating tension from opposite directional pulls. d. Chromosomes Assist in the Assembly of the Mitotic Spindle Chromosomes can stabilize and organize MTs, can nucleate MT assembly and motor proteins move/arrange MTs and chromosomes into a bipolar spindle. In cells with centrosomes, chromosomes, motor proteins and centrosomes form the mitotic spindle. e. Chromosomes Line Up at the Spindle Equator at Metaphase Metaphase- begins as duplicated chromosomes are attached to mitotic spindle, aligning at equator of spindle forming metaphase plate. - Continuous growth./shrinking of MTs and microtubule motor proteins, addition/loss of tubulin subunits are required to maintain metaphase spindle Duplicated chromosomes are suspended at metaphase under tension. f. Proteolysis Triggers Sister-Chromatid Separation at Anaphase Anaphase- breaking of cohesin linkages holding sisters together. - Done by protease, separase. Held inactive by inhibitory protein called securin (targeted for destruction by APC. When securin is removed, separase can sever cohesin linkages). Each chromatid is now a chromosome to be pulled to a spindle pole where it’s attached and segregates both identical sets of chromosomes to opposite ends of the spindle. g. Chromosomes Segregate during Anaphase Anaphase A- kinetochore MTs shorten and attached chromosomes move poleward. - Movement may be due to loss of tubulin subunits from both ends of kinetochore MTs. Anaphase B- spindle poles move apart, further segregating 2 sets of chromosomes. - Due to motor proteins kinesin and dynein. Kinesins act on long, overlapping Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 interpolar MTs, sliding MTs from opposite poles past one another at equator of spindle and pushing poles apart. Dynein anchored to the cell cortex pulls poles apart. h. An Unattached Chromosome Will Prevent Sister-Chromatid Separation A negative signal is sent when unattached chromosomes send a stop signal to the control system, signal inhibits progress by blocking activation of APC, (inactive APC can’t allow separation of chromosomes). Spindle assembly checkpoint- controls onset of anaphase and exit from mitosis. i. The Nuclear Envelope Re-forms at Telophase Telophase- mitotic spindle disassembles and nuclear envelope reforms. Nuclear pore proteins and nuclear lamins that were phosphorylated in prometaphase are now dephosphorylated and can reform. G. Cytokinesis Cytokinesis- cytoplasm is cleaved in two and completes M phase. a. The Mitotic Spindle Determines the Plane of Cytoplasmic cleavage Cleavage furrow cuts between two groups of segregated chromosomes, mitotic spindle dictates position of cleavage furrow b/c in anaphase overlapping interpolar MTs form the central spindle and proteins signal to cell cortex to initiate assemble of contractile ring. Most times it is in symmetrical division. If there is asymmetric division, daughter cells also differ in molecules they inherit and develop into different cell types. b. The Contractile Ring of Animal Cells Is Made of Actin and Myosin Filaments Contractile ring- made of overlapping array of actin filaments and myosin filaments. Made at anaphase. Has a strong force made by sliding AFs against MFs. Transient b/c it assembles for cytokinesis and gets smaller and disassembles fully when the cell is cleaved in two. Reorganization of filaments in the cell cortex causes changes in shape and less adherence of cells to other cells and to ECM. Substratum- fibroblasts spread out flat during interphase b/c of strong contact with the surface they grow on. Integrins- cells change shape b/c of PM proteins to attach cells to the substratum. c. Cytokinesis in Plant Cells Involves the Formation of a New Cell Wall New cell wall forms due to contractile ring and is guided by a phragmoplast- formed by remains of interpolar MTs at equator of old mitotic spindle. d. Membrane-Enclosed Organelles Must Be Distributed to Daughter Cells When a Cell Divides Fragmentation and duplication with spindle MTS via motor proteins. Membrane- enclosed organelles, ribosomes and all soluble proteins are inherited randomly when the cell divides. H. Control of Cell Numbers and Cell Size Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 Cell growth, division and death determine organ and body size a. Apoptosis helps Regulate Animal Cell Numbers Apoptosis- programmed cell death via activation of an IC death program. Cells die due to structures they form being no longer needed, and need to balance cell division (unless growing or shrinking in size). b. Apoptosis Is Mediated by an Intracellular Proteolytic Cascade Cell necrosis- dead cells due to acute injury and spill contents on neighbors, can be a damaging inflammatory response Caspases- family of proteases responsible for apoptosis with their enzymes made as inactive precursors (procaspases). Both caspases work to take cells apart. - Initiator caspases- cleave/activate downstream executioner caspases which activate more executioners, for amplifying, proteolytic cascade and dismember key proteins in cells (such as lamin proteins). c. The Intrinsic Apoptotic Death Program Is Regulated by the Bcl2 Family of Intracellular Proteins Bcl2 family- IC proteins that regulate activation of caspases and are held in cells until signaled. Some can promote or inhibit caspase activation. Bax and Bak- promotes cell death by inducing release of electron transport protein cytochrome c from mitochondria in the cytosol. Other members of the Bcl2 family and Bcl2 itself inhibit apoptosis by preventing Bax and Bak from releasing cytochrome c. Cytochrome c activate initiator procaspases and induce cell death by assembling protein complex apoptosome, which recruits/activated an initiator procaspase, then a caspase cascade, then apoptosis d. Extracellular Signals Can Also Induce Apoptosis EC signals from neighboring cells can activate another cell's death by affecting Bcl2 family of proteins. Some may stimulate apoptosis by activating a set of cell-surface receptor proteins (death receptors). Fas- death receptor on cell surfaces and is activated by a membrane-bound protein, Fas ligand on the surface of specialized immune cells, killer lymphocytes which regulate immune responses by inducing apoptosis in other immune cells no longer needed. - Fas ligand binding to its receptor triggers death-inducing signaling complex with initiator procaspases when activated launch a caspase cascade leading to death. e. Animal Cells Require Extracellular Signals to Survive, Grow and Divide To replace cell loss or to allow tissue growth, they need EC signals, which are soluble proteins secreted by other cells or proteins on other cell surfaces or to ECM. 1. Survival factors- promote cell survival by suppressing apoptosis 2. Mitogens- stimulate cell division by overcoming IC braking mechanisms that Downloaded by Areeba Khan ([email protected])  lOMoARcPSD|46396031 block cell cycle progression. 3. Growth factors- stimulate cell growth in size and mass by promoting synthesis/inhibiting degradation of proteins/macromolecules. f. Survival Factors Suppress Apoptosis By activating cell-surface receptors, receptors turn on IC signaling pathways to suppress apoptotic pathways by regulating Bcl2 proteins. g. Mitogens Stimulate Cell Division by Promoting Entry into S Phase Secreted signal proteins bind to cell-surface receptors which initiate IC signaling pathways to stimulate cell division which releases molecular brakes blocking transition from G1 phase of cell cycle into S phase. - Platelet derived growth factor (PDGF)blood platelets in clots release PDGF which binds to receptor tyrosine kinases in surviving cells at wound, cells proliferate and help wound heal h. Growth Factors Stimulate Cells to Grow Cell growth and cell division depend on signals from other cells and pathways lead to accumulation of proteins and other macromolecules with growth factors increasing rate of synthesis and decrease of degradation. i. Some Extracellular signal Proteins Inhibit Cell survival, Division, or Growth EC signal proteins can also inhibit tissue growth - Myostatin is a secreted signal protein inhibiting growth/proliferation or precursor myoblasts that fuse to form skeletal muscle. Downloaded by Areeba Khan ([email protected])