BIOL 3510 1st EditionExam # 4 Study Guide Lectures: 22 - 26Lecture 22 (November 18)Roles of Actin Networks include (A) Stable: microvilli, (B) Stable: contractile bundles, (C) Temporary: protrusions needed for cell movement, (D) Temporary: cytokinetic contractile ring.Microfilaments (actin filaments) are composed of globular actin proteins aligned into two twisted protofilaments. Like MTs, microfilaments have a distinct polarity. Actin is more likely to be added to the plus end. Microfilament (MF) Growth and Disassembly: Actin is an ATPase, Actin-ATP is added to both ends, faster growth at plus end; After incorporation to a MF, ATP is hydrolyzed to ADP causing a conformation change that destabilizes the MF. Treadmilling and dynamic instability are dependent on nucleotide hydrolysis. Some microfilaments are nucleated from actin nucleating proteins near the plasma membrane. Microfilament behavior is modified by actin-binding proteins. Actin in action: the three steps of cell crawling are actin dependent. #1 Actin polymerization at the leading edge of the cell results in the protrusion of lemellipodia and filopodia. Branched actin networks in lemellipodia are nucleated by ARP complexes. Growth of unbranched microfilaments in filopodia (andother structures) is promoted by formins. Formins promote the addition of actin monomers. #2 Integrinsbind to the extracellular proteins and internal microfilaments. Integrins: transmembrane proteins that interact with the crawling cell’s environment, connected to actin filaments. #3 Internal contraction of thecell is mediated by myosin II, an actin-dependent motor protein. All actin-dependant motor proteins belong to the myosin family (plus end directed): two main types, myosin I and myosin II; bind and hydrolyze ATP. Different myosin I proteins bind to different cell components. Actin in action: How does a cell know what type of microfilament network to form? Cell signals converge on a family of Rho GTP binding proteins (GTPases). Rho – promotes contractile bundles, Rac – promotes lamellipodia, -Cdc42 – promotes filapodia.Actin in action: Mucle Cell Contraction. Myosin II filaments are critical for muscle cell contraction. Movement of myosin II along opposite oriented actin filaments creates a contractile force. Muscle cells contain myofibrils which are chains of repeating sarcomere units. Muscle contraction occurs when the myosin and actin filaments slide past each other. Transverse tubules convey the excitatory signal (action potential) from an activating neuron to the sarcoplasmic reticulum. In response to the action potential, Ca2+ is released from the sarcoplasmic reticulum. Resting muscle cell: tropomyosin covers MFs and prevents myosin from binding. Contracting muscle cell: Ca2+ released from the sarcoplasmic reticulum bind to the troponin complex changing its conformation. This alters the tropomyosin – actin interaction allowing myosin to bind and initiating a contraction.Lecture 23 (November 20)Cell cycle: duplication and division of a cell and its contents. The cell cycle has four phases: 1) G1 phase (Gap) = Cell growth, 2) S phase (synthesis) = DNA replication. 3) G2 phase (Gap) = Cell growth, 4) M phase = mitosis and cytokinesis. Progress through the cell cycle controlled by the cell-cycle control system. Cyclin-dependant kinases (Cdks) are major components of the cell cycle control system. Cdk = kinase that activates cell machinery – constant amount, cyclical activation. Cyclin = activates a Cdk – cyclical amounts. Cyclin concentrations are regulated by transcription and proteolysis. Transcription: gradual increase in cyclins. Proteolysis: quick decrease in cyclins. APC = anaphase promoting complex. In addition to cyclins, Cdks are regulated by phosphorylation, dephosphorylation, and inhibitory proteins. Different Cdk-cyclin combos trigger different cell cycle events.G1 Phase: At the end of M, all S-Cdks and M-Cdks are inactivated by cyclin degradation and inhibitor protein activation. In mammalian cells, mitogens promote cyclin transcription needed for entry into S phase. Mitogens promote cell division by activating G1-Cdk and G1/S-Cdks which inactivate Retinoblastoma (Rb) proteins. Cell cycle progression is paused if the DNA is damaged (or incompletely replicated): Inactivation of G1/S and S-Cdks during G1 and S by p53 activation pauses the cell cycle (DNA repair or cell death), then No p53 leads to the replication of damaged DNA promoting cancer. Centrosome cycle: centrosome duplication initiated in G1 is triggered by G1/S Cdk and S-Cdk.Control of S phase – initiation of replication: 1) DNA synthesis is initiated at origins of replication, 2) Pre-existing origin of recognition complexes (OCRs) recruit Cdc6 to origins of replication during G1, 3) Helicase binding generates the prereplicative complex (pre-RC), 4) Activation of S-Cdk: Activates helicases causing the assembly of the remainder of the replication fork, and Phosphorylates Cdc6 targeting it for degradation preventing re-replication. Sister chromatids are held together by cohesion ring complexes until anaphase., Activation of M-Cdk by Cdc25 leads to entry into M phase. M-Cdk is activated at the end of G2.6 Stages of Mitosis: Prophase: phosphorylation by M-Cdk triggers formation of condensing complexes that condense chromosomes. Increased MT dynamic instability and spindle formation is triggered by phosphorylation of MAPs by M-Cdk. Motor proteins and MAPs crosslink interpolar microtubules forming the mitotic spindle. Prometaphase: breakdown of the nuclear envelope begins with the M-Cdk dependent phosphorylation of nuclear pore proteins and lamins. Microtubules connect to the centromeres of sister chromatids via kinetochore protein complexes. There are three types of microtubules in a spindle. Bi-orientation of kinetochore microtubules generates tension. Chromosomes line up on the metaphase plate via the action of dynamic instability and motor proteins. Prometaphase: alignment period. Metaphase: begins when the chromosomes are aligned on the metaphase plate.Lecture 24 (November 25)Aurora kinase B detects tension and promotes kinetochore microtubule stability. Anaphase begins with the separation of sister chromatids; mediated by the anaphase promoting complex (APC). Sister chromatids are pulled to opposite spindle poles during anaphase A and anaphase B. Sister chromatids are pulled to opposite spindle poles during anaphase A and anaphase B.