MCB 2210 1st EditionFinal Exam Study Guide Exam 4: The Cell Cycle Cell Cycle: ordered series of events that leads to cell duplication and divisiono M-phase (mitosis/meiosis)—small fraction of the cycleo Interphase—the rest of the cell cycle G1-Phase (Gap 1)—period between the completion of previous mitosis and the initiation of DNA synthesis for the next mitosis- Some cells exit the cell cycle into a post-mitotic, non-proliferating state—G0 (G zero) S-phase—period of DNA synthesis for chromosome duplication G2-phase (gap 2)—period between the completion of DNA replication and the initiation of mitosis The cell cycle is tightly regulated o Progression relies on cell growth and successful passage through several checkpoints Ensures the accuracy and fidelity of chromosome replication Chromosome replication and cell division must occur in the proper order and proper time in every cycleo In many cells, growth and division rates are coupled  The two processes can be regulated independently as well Cell size is determined by the rate of growth and duration of the cell cycle Depend on extracellular signals- Signaling relies on post-translational modificationso Phosphorylation of tyrosines, serines, and threonines--REVERSIBLEo Ubiquitination on lysines Triggers protein degradation—IRREVERSIBLE  Cell growth is driven by PI 3-kinase pathwayso Receptor tyrosine kinases (RTKs)—in response to growth factors, recruit and activate PI 3-kinases (PI3K) at plasma membrane PI3K is a lipid kinase that binds to phosphor-tyrosine residues in the cytoplasmic domains of activated RTKs—brings PI3K in proximity to phospholipids on the cystolic face of the membrane  PI3K phosphorylates the 3’ position of inositols to generate PI(3,4,5)P3 and other phosphatidylinositides These notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.o Accumulation of PI(3,4,5) in the membrane leads to activation of a serine/threonine kinase called Akt Once activated—phosphorylates many proteins in the cytoplasm and nucleus mTOR—one target, a kinase, that promotes cell growth - Activation of complexes containing mTOR results in phosphorylation of targets involved in protein synthesis and metabolism- S6-kinase—a substrate of mTOR that phosphorylates ribosomal protein S6o Results in increased translation of mRNAs- 4E-BP—inhibits translation initiation factor elF4Eo Phosphorylation releases elF4E from 4E-BP inhibition and allows it to initiate translation  Growth can be balanced by protein degradation in the proteasome o Two major protein degradation pathways: Lysosome pathways Proteasome pathways- Proteasome—very large macromolecular machine consisting of ~50 protein subunits that act collectively to degrade many cellular proteinso Hydrolyzes ATP to provide the energy needed for degradation- Protein lifespan is controlled by regulated protein degradation  Important functions of protein degradation:- Removes proteins that are misfolded, damaged, or potentially toxic to the cell- Controlled degradation of normal proteins provides a powerful mechanism to maintain appropriate levels of proteins and their activities - Regulated degradation also permits rapid responses to changing conditions  Ubiquitin—marks proteins for degradation by the proteasomeo Multiple ubiquitin molecules can be attached covalently to proteins via a multi-step process called poly-ubiquitin—recognized by the proteasomeo Process involves several enzymeso Specificity of poly-ubiquitination is achieved through E3 Ub-ligases Each recognizes a specific signal in a given protein for poly-ubiquitination Cell cycle progression is controlled by protein expression, phosphorylation, and degradation o Passage through the cycle is controlled by heterodimeric protein complexes consisting of: Regulatory subunit—cyclin Catalytic subunit—cyclin-dependent kinase (CDK)- Depend on cyclins for activation of their kinase activity and for access to substrateso Early G1= cyclin D-CDK4,6o Late G1/S= cyclin E-CDK2o S= cyclin A-CDK2o M= cyclin A A,B-CDK1o Together, cyclin/CDK complexes, protein phosphatases, and ubiquitin ligases regulate cell cycle transitions  The synthesis and degradation of cyclins control progression through the cell cycleo The concentrations of cyclin proteins oscillate during the cell cycle while the concentrations of the CDKs remain steady CDK activity is tightly regulated:o Activation by cyclin-binding and T-loop phosphorylation  Binding of a cyclin to a CDK alters the conformation of an area called the T-loop- Leads to partial activation of the CDK Phosphorylation by a CDK-activating kinase (CAK) of a specific T-loop residue induces an additional conformational change that allows substrate access and full CDK activationo Inhibitory phosphorylation of the active site by Wee1 kinases Wee1 phosphorylates and reduces the activity of CDKso De-phosphorylation of inhibitory sites by Cdc25 phosphatases The inhibitory effects of Wee1 can be antagonized by Cdc25-mediated removal of phosphates from a CDK- Thereby increasing the activity of the CDKo Physical inhibition by CDK inhibitors (CKIs) Binding of these molecules causes a large rearrangement in the structure of the CDK active sites—rendering it inactive  Cyclin-CDKs and Ub-ligases regulate transitionso Mid-G1—G1 cyclin-CDKs activate the transcription of genes required for DNA replication, and assemble pre-replication complexes at originso SCF ubiquitin ligase initiated passage through the restriction point by polyubiquitinating inhibitoes of S-phase cyclin CDKs Marking them for proteasome-mediated degradationo After this restriction point is passed S-phase cyclin-CDKs activate DNA replication origins Accurate DNA replication must occur once and only once during each cell cycleo Late G2—mitotic cyclin-CDKs trigger entry into mitosiso APC—polyubiquitinates securing to promote anaphase With the help of Cdc14 phosphatase—polyubiquitinates mitotic cyclins to promote telophase Mitotic cyclin-CDKs (cyclin B-CDK1)o Phosphorylate a number of cellular proteins and sets the stage for the dramatic changes in mitosis Condensins—chromosome condensation  Nuclear lamins—nuclear envelope breakdown Microtubule-associated proteins—chnages in MT dynamics ER- or