Bio Exam 4 Key Concepts (PDF)

Cytoskeleton major functions Scaffold gives structure and support to cytoskeleton Helps organize inside cell Resists forces Intracellular transport Network of tracks to traffick organelles Force generation Cell contracti...

Cytoskeleton major functions Scaffold gives structure and support to cytoskeleton Helps organize inside cell Resists forces Intracellular transport Network of tracks to traffick organelles Force generation Cell contraction Cell movement and migration Beating of the cilia Cell division Separation of chromosomes Splitting parents into daughter cells during cytokinesis 3 types of filaments Actin Plasma membrane Shorter and smaller Drives contraction movements (like muscle) Binds and hydrolyzes ATP 2 protofilaments polarized Microtubules Emanating out of a centrosome Building blocks are alpha and beta tubules Intermediate Cytoplasm and nucleus Flexible ropes that are resistant to stretching Non Polarized 8 tetramers (protofilaments) form filaments Has lamins that organize nuclear pores and structure Cytoplasmic filaments can bind with ATP or GTP (1 flavor) Filaments are formed from assembly of smaller subunits Through noncovalent interactions Subunits into protofilaments that interact lateral to form filaments Protofilaments stabilize interior of filaments Protofilaments are asymmetric and polarized BETA AT END AND ALPHA AT BEGINNING (MOVING UPWARDS) Elongation Adding more than removing Treadmilling Add to one end and remove at other end Staying the same length Occurs when 1 end is largely ADP/GDP bound and other is largely ATP/GTP bound Microtubules Rigid and hollow Made of 13 protofilaments Polarized Binds GTP Microtubule organizing centers Nucleation occurs here Polymerization of microtubules depends on nucleation and elongation Beta combines GTP and hydrolyzes it into GDP Concentrations Each end of filament will have a different value of critical concentration Critical concentration = rate of adding and rate of subtracting subunits is equal If subunit concentration goes above C, then there is filament growth If subunit concentration goes below C, then there is filament disassembly Filament formation and concentration of free subunits influence the rate of association Kon = association rate Koff = dissociation rate Plus end is more stable than minus end, greater than Kon and Koff value ATP/GTP hydrolysis Controls the Cc Beta binds GTP to GDP to become D form Subunits in filaments will hydrolyze quickly Hydrolysis causes small conformational changes that decrease filament affinity D forms more likely to Dissociate (remove from filament) T forms more likely to be added T form goes into filament, becomes D, then Dissociates the critical concentration for ATP/GTP subunits is lower than concentration for ADP/GDP subunits GTP Cap T form creates cap on plus end - very straight and stable Constraints the microtubule curvature that GDP subunits would have - round Catastrophe occurs when cap is lost and the filament then curves Motor proteins convert chemical energy into mechanical energy Have heads that bind to filament Tails bind to cargo (proteins) Carries organelles to destinations in cells Binds to hydrolyze ATP Causes the sliding of filaments 3 types of motor proteins Myosin = actin dependent Kinesin = microtubule dependent, usually plus end directed Dynein = microtubule dependent, usually minus end directed Velocity = rate of physical displacement Processivity = how long a motor protein stays active with a filament Regulation of filaments formation and polymerization Can be affected by proteins that bind free subunits Proteins bind monomers and expose only the plus end binding side of actin monomer, promoting plus end assembly ARP ⅔ Produces branched actin networks Act at minus end as nucleators Formin Functions at plus end of filament Linear, unbranched actin filaments Thymosin Binds subunits to prevent assembly and shut down Prolifin Binds subunits and speeds up elongation to add filament Regulation of filament after formation Cofilin Destabilizes bound actin filaments Induces a twist that loosens subunits Binds to ADP actin to promote turnover of older filaments  Capping proteins Stabilizes filaments Prevents ATP/GTP hydrolysis Capped on plus end after growth Skeletal muscles Skeletal muscles are made of myofibrils Myofibrils are made of contractile sarcomeres Actin on outside of Z disk, myosin is between these with plus heads attached to the actin Sarcomere and accessory proteins Actin filaments anchor and stabilize by capping proteins Nebulin binds actin Actin binding proteins stabilizes filament Stays actin bound fro short periods Sliding model Calcium is released and binds to troponin This moves tropomyosin and allows myosin to connect to actin (bridge) Myosin heads hydrolyze ATP and allows the actin to slide and contract Myosin head binds with ATP and detaches from the actin Calcium regulates contraction Ca+ regulates the positions of tropomyosin in actin b By binding to move it out of way Flagella and Cilia Can propel cells through liquid media Flagella is long filaments that cause wave movement Cilia is short filaments that drive swimming power and recovery stroke Axoneme Inside of flagella and cilia Connected in a circle Activation of dynein has motor protein arms that reach and attach to neighbor and tries to walk/slide to other, cross linking proteins stops this so it just ends up bending Basal bodies Anchors the flagella and cilia ECM A network of secreted proteins and polysaccharides that form a solid substrate for cells to anchor and crawl on Glue that holds cells together  Collagen Trimers and rod-like triple helix Rigid fibrils that pack into thick filters Resistant to pulling forces Proteoglycans Assembled into gigantic complexes from core linkage Has complex carbs attached Fibronectin 2 polypeptides linked by disulfide bridges Binding sites for ECM proteins, receptors, brings together important components Laminin 3 polypeptides linked by disulfide bonds Binds to cell surface adhesion receptors and other components of ECM Integrins mediates cell-ECM attachment Occurs at ECM binding sites and hemidesmosomes Capable of inside-out signaling - active and inactive Outside-in signaling - affects cell differentiation Focal adhesions Sites that anchor cells to substratum (ECM) Large clusters of integrins ○ Signaling hub ○ Capable of creating and responding to mechanical forces Hemidesmosome Anchor basal surface of epithelial cells to basement membrane (anchors hemidesmosomes) Large clusters of integrins ○ Signaling hub ○ Resist forces ○ INTERMEDIATE FILAMENTS Cadherins Require Ca++ for binding Mediate cell-cell interactions Catentin tether cadherins to the cytoskeleton Cell-Cell anchoring junctions Adherens junction  Made of cadherin clusters Provides a pathway for signals to be transmitted from the cell exterior to the cytoplasm Links to actin cytoskeleton Desmosomes Links intermediate filaments Tight junctions In epithelial tissues form impermeable barrier Limits lateral diffusion within plasma membrane Claudins and occludins Gap junctions Allows passage of small molecules Link cell cytoplasms Connexins proteins assemble to form transcellular pores Adhesion and traction Protrusion is when cell extends plasma membrane and will attach to substratum Bundles of actin and myosin contract and squeeze cell body forward Adhesion on back breaks Filopodia Spike out of plasma membrane Tight bundle of unbranched actin filaments Lamellipodia ARP ⅔ nucleates actin at leading edge Branched network leads to broad, flat membrane extension Cell cycle Cells duplicate the genome and divide to create more cells When to divide? A slow buildup of factors hit a threshold and signal (the health of cell) S Phase Replication of entire genome Starts at origin of hundreds of different located genomes Initiation watch a vid confuseed Initiator proteins Pre-RC load into origins by binding Pre-RC proteins recognized the origin and binds to it M - phase mitosis Spindle formation Microtubules and motor proteins Made from 2 centrosomes Separates chromosomes Astral microtubules -contact cell cortex from centrosome Interpolar microtubules - each centrosome that interacts in an equatorial region Kinetochore - attach to chromosomes and mediates movement Mitotic dynein binds to actin and pulls centrosomes away from each other Kinesin 5 walks towards plus end on 2 filaments to push them apart Kinesin 14 walks towards minus ends and pulls centrosomes together This balance determines spindle length Chromosomal attachment Nuclear envelope must break down Kinetochores on center of chromosome mediate attachment to mt Can subtract or add subunits Tension and alignment When both chromatids are bound to same centrosome, low tension ○ Kinetochore will signal to loosen plus end microtubule bonding When chromatids are bonded to different centrosomes, high tension ○ Kinetochore will signal to add microtubules to strengthen bonds Chromosomal separation Microtubule disassembly and motor proteins ○ Plus end disassembly at kinetochore drives 1st movement towards centrosomes ○ Curvature of disassembly microtubules slides collar Strengthens dynein pull of centrosomes towards the cortex Cytokinesis Occurs through actomyosin contraction and membrane insertion Microtubule spindle signals where contraction will occur 2 sisters being pulled in opposite directions