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Cell Biology Ch. 17
Cytoskeleton
| Question | Answer |
|---|---|
| cytoskeleton | system of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and the capacity for directed movement. |
| the cytoskeleton is built on a framework of three types of protein filaments: | actin filaments, microtubules, and intermediate filaments |
| Each type of filament has a distinct mechanical properties and is formed from a different protein subunit. What are intermediate filaments formed from? | a family of fibrous proteins |
| Each type of filament has a distinct mechanical properties and is formed from a different protein subunit. What are microtubules formed from? | globular tubulin subunits |
| Each type of filament has a distinct mechanical properties and is formed from a different protein subunit. What are actin filaments formed from? | globular actin subunits |
| Intermediate filament | fibrous cytoskeletal element that forms rope-like networks in animal cells |
| Intermediate filaments: What is the main function? | enable cells to withstand the mechanical stress that occurs when cells are stretched (they are great tensile strength) |
| Intermediate filaments: Why are these filaments called "intermediate"? | in the smooth muscle cells, where they were first discovered, their diameter is between that of the thinner actin filaments and the thicker myosin filaments |
| Intermediate filaments: They are the _____________ and most ____________ of the cytoskeletal filaments | toughest, most durable |
| Intermediate filaments: form a network throughout the cytoplasm, surrounding the nucleus and extending to the cell periphery where often anchored at cell-cell junctions called ______________, where the plasma membrane is connected to that of another cell | desmosomes |
| Intermediate filaments: They are also found in the nucleus of all eukaryotic cells where they form a meshwork called the __________ ___________, which underlies and strengthens the nuclear envelope | nuclear lamina |
| Intermediate filaments can be grouped into 4 classes: | (1) keratin filaments in epithelial cells (2) vimentin /vimentin-related filaments in connective-tissue cells, muscle cells, supporting cells of the nervous system (3) neurofilaments in nerve cells (4) nuclear lamins, which strengthen the nuc. env, |
| Intermediate filaments: Where are these four classes of filaments found? | keratin, vimentin, and neuro-filaments are found in the cytoplasm while nuclear lamins are found in the nucleus |
| Intermediate filaments: How are these filaments formed? | by the polymerization of their corresponding intermediate filament subunits |
| Intermediate filaments: keratin filaments | class of intermediate filament abundant in epithelial cells, where it provides tensile strength. (main structural component of hair, feathers, and claws) |
| Intermediate filaments: the rare human genetic disease, ____________ ___________ ___________, in which mutations in the keratin genes interfere with the formation of keratin filaments in the epidermis resulting in highly vulnerable skin | epidermolysis bullosa simplex |
| many of the intermediate filaments are further stabilized and reinforced by accessory proteins, such as ___________, that cross-link the filaments into bundles and link them to microtubules, to actin filaments,and to adhesive structures in the desmosomes. | plectin |
| Intermediate filaments: mutations in the gene for plectin cause what? | disease that combines disruption of skin keratin, of intermediate filaments in muscle, and of neurofilaments: ( epidermolysis bullosa simplex, muscular dystrophy, and neurodegeneration)_ |
| intermediate filaments form a tough nuclear lamina, constructed from intermediate filament proteins called ___________. | lamins |
| Intermediate filaments: the nuclear lamina disassembles and reforms at each cell division. What is the disassembly and reassembly of the nuclear lamina controlled by? | phosphorylation and dephosphorylation of the lamins |
| Intermediate filaments: When the lamins are phosphorylated by protein kinases, the consequent conformational change does what to the binding between the lamin tetramers? | weakens the binding and causes the filaments to fall apart |
| Intermediate filaments: Whem the lamins are dephosphorylated by protein phosphatases , what happens? | lamins reassemble |
| Intermediate filaments: Defects in a particular nuclear lamin are associated with certain types of _________, rare disorders that cause individuals to age prematurely | progeria |
| Microtubule | long, stiff, cylindrical structure composed of protein tubulin and used in eukaryotic cells for organization and transportation of the cytoplasm |
| Microtubules: built from subunits, molecules of __________, each of which is itself a dimer composed of two very similar globular proteins called alpha-____________ and beta-____________, bound tightly together | tubulin |
| Microtubules: tubulin dimers stack together to form the wall of the hollow cylindrical microtubule. Its made of 13 parallele ________________, each a linear chain of tubulin dimers with alpha- and beta- tubulin alternating along its length | protofilaments |
| Microtubules: Each protofilament has structural ____________, with alpha-tubulin exposed at one end and beta-tubulin at the other, giving structural ___________ to the entire microtubule | polarity |
| Microtubules: polarity | inherent asymmetry that allows one end of an object to be distinguished from another |
| Microtubules: One end of the microtubule, thought to be the beta-tubulin end, is called the ______ end, and the other, the alpha-tubulin end, its ______ end | plus, minus |
| Microtubules: Tubulin dimers will add to either end of a growing microtubule, however, they add more rapidly to the _______ end than to the _______ end | plus, minus |
| Microtubules: It is the _____________ of the microtubule that makes it crucial for the assembly of microtubules and their role once they are formed | polarity |
| Microtubules: the ______________ is the major microtubule-organizing center in animal cells | centrosome |
| Microtubules: centrosome | microtubule-organizing center that sits near the nucleus in an animal cell |
| Microtubules: The centrosome consists of a pair of ___________, surrounded by a matrix of proteins | centrioles |
| Microtubules: centrioles | cylindrical array of microtubules usually found in pairs at the center of a centrosome in animal cells |
| Microtubules: the centrosome matrix includes hundreds of ring-shaped structures formed from a special type of tubulin called _______-tubulin | gamma-tubulin |
| Microtubules: Each gamma-tubulin ring complex serves as the starting point, or _______________ _______ for the growth of one microtubule | nucleation site |
| Microtubules: The alpha-beta-tubulin dimers add to each gamma-tubulin ring complex in a specific orientation, with what result? | minus end of each microtubule is embedded in the centrosome, and growth occurs only at the plus end that extends into the cytoplasm |
| Microtubules: What do centrioles do? | act as organizing centers for the microtubules in cilia and flagella (where they are called basal bodies) |
| Microtubules: growing microtubules display dynamic ______________ | instability |
| Microtubules: dynamic instability | the rapid switching between growth and shrinkage shown by microtubules |
| Microtubules: Why is dynamic instability of microtubules beneficial? | it allows them to undergo rapid remodeling and is crucial for their function |
| Microtubules: shrinkage is promoted by what? | the hydrolysis of GTP that is tightly bound to tubulin dimers (specifically beta-tubulin), reducing the affinity of the dimers for their neighbors and thereby promoting microtubule disassembly |
| Microtubules: microbtubules can be stabilized by localized proteins. How? | localized proteins capture the plus ends, helping to position the microtubules and harness them for specific functions |
| Microtubules: microtubule dynamics can be modified by _______ | drugs |
| Microtubules: Drugs that prevent the ____________________ or __________________ of tubulin dimers can have a rapid and profound effect on the organization of microtubules, and thereby on the behavior of the cell | polymerization, depolymerization |
| Microtubules: colchine | binds tightly to free tubulin dimers and prevents their polymerization into microtubules |
| Microtubules: taxol | binds tightly to microtubules and prevents them from losing subunits, meaning microtubules can grow, but not shrink |
| Microtubules: both of these drugs, colchine and taxol, have what effect? | arresting dividing cells in mitosis |
| Microtubules: microtubules guide the transport of ___________, _____________, and _____________ in both directions along a nerve cell axon | organelles, vesicles, and macromolecules |
| Microtubules: What provides directionality during movement from cell body to axon terminal? | polarity |
| Microtubules: in the nerve cell, all microtubules in the axon point in the same direction with their ______ ends towards axon terminals, | plus |
| Microtubules: microtubules don't act alone, their activity depends on a large variety of accessory proteins that bind to them. Some of these are called ______________-______________ proteins | microtubule-associated proteins |
| Microtubules: microtubule-associated proteins are accessory proteins that binds to microtubules and can do what? | can stabilize microtubule filaments, link them to other cell structures, or transport various components along their length |
| Microtubules: ___________ and ___________ are microtubule-associated motor proteins that used ATP hydrolysis to move unidirectionally along microtubules and carry specific organelles, vesicles, and other types of cargo to particular locations in the cell | kinesins and dyneins |
| Microtubules: motor proteins | protein that uses energy derived from ATP hydrolysis to propel itself along a protein filament or polymeric molecule |
| Microtubules: __________ generally move toward the plus end of a microtubule (outward from the cell body) while __________ move toward the minus end (toward the cell body) | kinesins, dyneins |
| Microtubules: kinesin and dynein are dimers that have two globular ATP-binding heads and a single tail. The ________ of the motor protein generally binds to vesicle or organelle and thereby determines the type of cargo that motor protein can transport | tail |
| Microtubules: microtubules and motor proteins position ____________ in the cytoplasm | organelles |
| Microtubules: Eukaryotic _________ and ____________ contain a bundle of stable microtubules. Their rhythmic beating is caused by bending of the microtubules, drive by the ________ _________ motor protein | cilia, flagella, ciliary dynein motor protein |
| Actin filaments | thin, flexible protein filament made from a chain of globular actin molecules, essential for cell movement and for the contraction of muscle cells |
| Actin filaments: helical polymers of globular _______ monomers, are present in all eukaryotic cells and essential for many of the cell's movements, especially those involving the cell surface | actin |
| Actin filaments: they are more _________ than microtubules and are generally found in bundles or networks | flexible |
| Actin filaments: actin-dependent movements usually require actin's association with a motor protein called __________ | myosin |
| Actin filaments: Actin and Tubulin _______________ by similar mechanisms | polymerize |
| Actin filaments: like microtubules, actin filaments are ____________, with a fast-growing plus end and a slow-growing minus end | polarized |
| Actin filaments: Their assembly and disassembly are controlled by what? | the hydrolysis of ATP tightly bound to each actin monomer, and by various actin-binding proteins |
| Actin filaments: As with the hydrolysis of GTP to GDP in a microtubule, hydrolysis of ATP to ADP in an actin filament _____________ the strength of binding between the monomers, thereby ____________ the stability of the polymer | reduces, decreasing |
| Actin filaments: In both cases, nucleotide hydrolysis promotes ___________________, helping the cell to disassemble its microtubules and actin filaments after they have formed | depolymerization |
| Actin filaments: What happens to the rate of growth of actin filaments with the concentration of actin monomers is very high? | actin filament will grow rapidly, adding monomers at both ends |
| Actin filaments: What happens to the rate of growth of actin filaments at intermediate concentrations of actin monomers? | the treadmilling |
| Actin filaments: treadmilling | simultaneous gain of monomers at the plus end of an actin filament and loss at the minus end |
| Actin filaments: Actin-binding proteins | interacts with actin monomers or filaments to control the assembly, structure, and behavior of actin filaments and networks |
| Actin filaments: Small proteins, such as ___________ or _________, bind to actin monomers in the cytosol, preventing them from adding to the ends of actin filaments | thymosin, profilin |
| Actin filaments: How do proteins such as thymosin and profilin play a crucial role in regulating actin polymerization? | they keep actin monomers in reserve until they are required |
| Actin filaments: When actin filaments are needed, other actin-binding proteins such as _________ and ________-___________ proteins promote actin polymerization | formins, actin-related proteins (ARPs) |
| Actin filaments: varied arrangements and functions of actin filaments in cells stem from the diversity of actin-binding proteins, which can control what? (3) | actin polymerization, cross-link actin filaments to membranes, or move two adjacent filaments relative to each other. |
| Actin filaments: a concentrated network of actin filaments underneath the plasma membrane forms the bulk of the ______ __________, which is responsible for the shape and movement of the cell surface | cell cortex |
| Actin filaments: cell cortex | specialized layer of cytoplasm on the inner face of the plasma membrane. In animal cells it is rich in the actin filaments that govern cell shape and drive cell movement |
| Actin filaments: Many eukaryotic cells move by ____________ over surfaces, rather than by swimming by means of beating cilia or flagella. | crawling |
| Actin filaments: The molecular mechanism of cell crawling have 3 general processes: | (1) cell pushes out protrusions at its "front" or leading edge (2) these protrusions adhere to the surface over which the cell is crawling (3) the rest of the cell drags itself forward by traction on these anchorage points |
| Actin filaments: All three processes involve ________ but in different ways | actin |
| Actin filaments: (1) the pushing forward of the cell surface, is driven by actin ________________ | polymerization |
| Actin filaments: actin polymerization at the plus end protrudes ________________, which contain a dense meshwork of actin filaments | lamellipodia |
| Actin filaments: many cells also extend thin, stiff protrusions called ____________, both at the leading edge and elsewhere on their surface | filopodia |
| Actin filaments: Both filopodia and lamellipodia are _____________, _________ structures that form and retract with great speed | exploratory, motile |
| Actin filaments: Both filopodia and lamellipodia are thought to be generated by what? | the rapid local growth of actin filaments, which assemble close to the plasma membrane and elongate by the addition of actin monomers at their plus ends. |
| Actin filaments: The formation and growth of actin filaments at the leading edge of a cell are assisted by various ______-____________ proteins | actin-binding proteins |
| Actin associates with ____________ to form contractile structurs | myosin |
| all actin-dependent motor proteins belong to the myosin family. they bind to and hydrolyze ____, which provides energy for their movement along actin filaments toward the plus end. | ATP |
| There are various types of myosins in cells. What are the two most abundant? | Myosin-I and Myosin-II |
| In nonmuscle cells, what can myosin-I do? | carry organelles or vesicles along actin-filament tracks |
| In nonmuscle cells, what can myosin-II do? | cause adjacent actin filaments to slide past each other in contractile bundles |
| myosin-I | simplest type of myosin, present in all cells; consists of a single actin-binding head and a tail that can attach to other molecules or organelles |
| What structural component of myosin-I determines what type of cargo the myosin drags along? | the tail |
| The extracellular signal molecules that regulate the actin cytoskeleton activate cell-surface receptor proteins, which activate various intracellular signaling pathways. These pathways often converge on a group on monomeric GTPase proteins called what? | Rho protein family |
| Rho protein family | family of small, monomeric GTPases that controls the organization of the actin cytoskeleton |
| Monomeric GTPases behave as molecular switches that control intracellular processes, how? | by cycling between an active GTP-bound state and an inactive GDP-bound state |
| Muscle contraction depends on interacting filaments of ________ and __________ | actin and myosin |
| muscle myosin belong to what subfamily of myosins? | myosin-II |
| myosin-II | type of myosin that exists as a dimer with two actin-binding heads and a coiled-coil tail; can associate to form long myosin filaments |
| Clusters of myosin-II molecules bind to each other through their coiled-coil tails, forming a bipolar __________ _____________ from which the heads project | myosin filament |
| The myosin filament is like a double-headed arrow. Explain | it has two sets of myosin heads pointing in opposite directions, away from the middle. They bind to actin filaments and move them in opposite directions |
| If actin filaments and myosin filaments are organized together in a bundle, the bundle can generate a strong ____________ force | contractile |
| the contractile force is seen most clearly in muscle contraction, but it also occurs in much small ____________ _______________ of actin filaments and myosin-II filaments that assemble transiently in nonmuscle cells | contractile bundles |
| the contractile force is seen most clearly in muscle contraction, but it also occurs in the ____________ ________ that pinches a dividing cell in two by contracting and pulling inward on the plasma membrane | contractile ring |
| The bulk of the cytoplasm is made up of ____________, the contractile elements of the muscle cell | myofibrils |
| myofibril | long, cylindrical structure that constitutes the contractile element of a muscle cell; constructed of arrays of highly organized bundles of actin, myosin, and other accessory proteins |
| a myofibrils consists of a chain of identical tiny contractile units called _________________ | sarcomeres |
| sarcomere | highly organized assembly of actin and myosin filaments (mysoin-II) that serves as the contractile unit of a myofibril in a muscle cell |
| Sarcomeres are highly assemblies of actin and myosin filaments. Explain the structure | myosin (thick) filaments are centrally positioned in each sarcomere while the slender actin filaments (thin) extend inward from each end of the sarcomere, where they are anchored by their + ends to the z disc. - ends overlap ends of myosin filaments |
| How is the contraction of a muscle cell caused by? | simultaneous shotending of all the cell's sarcomeres, which is caused by the actin filaments sliding past the myosin filaments, with not change in the length of either type of filament |
| What is the sliding motion generated by? | myosin heads that project from the sides of the myosin filament and interact with adjacent actin filaments |
| When a muscle is stimulated to contract, what does the myosin head do? | starts walking along actin filament in repeated cycles of attachment and detachment. (using ATP hydrolysis) |
| after contraction is complete, what do the myosin heads do? | they all lose contact with actin filaments, and the muscle relaxes |
| What is muscle contraction triggered by? | sudden rise in cytosolic Ca2+ |
| When an action potential is triggerd in the muscle cell plasma membrane, the excitation spreads into a series of membranous tubes, called ____________ _____________ that extend from the plasma membrane around each myofibril | transverse tubules |
| The electrical signal is then relayed to the _______________ ________________, which surrounds each myofibril like a net stocking | sarcoplasmic reticulum |
| sarcoplasmic reticulum | specialized region of the ER muscle cells that contains a high Ca2+ concentration |
| What does the sarcoplasmic reticulum do in response to incoming electrical excitation? | releases Ca2+ into the cytosol through voltage-gated ion channels |
| In muscle, the rise in cytosolic Ca2+ activates a molecular switch made of specialized accessory proteins closely associated with actin filaments. One of these proteins is tropomyosin: | rigid, rod-shaped molecule that binds in the groove of the actin helix, where it prevents myosin heads from associating with the actin filamet |
| In muscle, the rise in cytosolic Ca2+ activates a molecular switch made of specialized accessory proteins closely associated with actin filaments. One of these proteins is troponin: | protein complex that includes Ca2+ sensitive protein associated with the end of a tropopyosin molecule. When Ca2+ binds to troponin, it changes shape allowing myosin heads to bind to actin filaments, initiating contraction |
| As soon as Ca2+ concentrations returns to resting level, troponin and tropomyosin molecules do what? | move back to their original positions, once again blocking myosin binding to actin filaments, ending contraction |
Created by:
cmccartney2