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chapter 17 (one long, pointless lecture)
Terms in this set (65)
What are the two "points" of the cytoskeleton?
it gives cell its shape and allows cell to organize its internal components
Where are the microtubules in reference to the cytoskeleton?
Where are the actin in reference to the cytoskeleton?
on the periphery of the cell
How does the intermediate filaments strengthen the epithelium?
it forms a continuous mechanical link from cell to cell because it is coupled between cells by desmosome junctions
What are intermediate filaments?
ropes made of long, twisted strands of protein
What do IF do?
provide support from mechanical stress throughout cells
Why are they called "intermediate"?
in reference to their size (10 nm diameter)
actin < IF < myosin
How is the eight tetramer filament made?
1. long alpha helices w/ globular N term head and C term tail
2. dimerize to coiled coils
3. dimers overlap to form staggered tetramer
4. end to end packing of two tetramers
5. bundle into filaments twisted together
What is the function of IF?
tensile strength: resistance to stretching
How does the structure of IF lead to the function of these IF?
when the epithelial cells are stretched by external forces, the network of IF and desmosomal junctions extends through sheets to develop tension and decrease stretching
What does plectin do?
protein networks that link IF to other IF, microtubules, or actin filaments
What is unique about IF that is not applicable in other elements in cells?
they are a stable part of the cytoskeleton
In the nuclear cortex, what do the lamin Ifs form?
a regular lattice inside the nuclear envelope
What is unique about these lamin Ifs?
unlike cytosolic IFs which are very stable, nuclear cortex disassembles and reforms every mitosis, triggered by phosphorylation of lamins
What is the largest cytoskeletal element?
microtubules (25 nm in diameter)
What are the two functions of microtubules?
cell highways for transport and positioning of structures:
1. major organizer or organelles
2. conduct traffic in animal cells
Why are microtubules crucial for directing traffic?
during mitosis, MT are totally reorganized, so it is necessary that they be set in place
What do microtubules form?
motile structures, such as cilia and flagella
What makes microtubules different from IF?
microtubules grow out of an organizing structure, like the centrosome, spindle pole, and basal body of cilia
What are microtubules composed of?
heterodimers (alpha and beta tubulins) of globular proteins
How are the microtubules components combined to make their structures?
they alpha line up on top of each other to form linear protofilaments of dimers, which organize (13 of them) into a hollow tubule to make a microtubule
How is there polarity preserved in microtubules?
alpha tubulin at "minus" end, beta tubulin at "plus" end
What is dynamic instability in MT?
-when MT grow at the same time as other shrink
-occurs independently of neighbors
-"shrink partially and then no less suddenly, start growing again or it may disappear or be replaced"
What allows MT to have dynamic instability?
the capacity of tubulin molecules to hydrolyze GTP
What does the inherent GTPase of tubulin do?
hydrolyzes GTP to GDP
What occurs when it is GDP tubulin?
there is a weaker association, so that it is more likely to disassemble
GTP-tubulin dimers are added at what end?
What occurs if GTP-tubulin is not added?
assembly stops and disassembly starts
What occurs at a growing microtubule?
Tubulin dimers with GTP bind more tightly to one another. So, they add to end of microtubule, and this proceeds faster than GTP hydrolysis and continues to grow.
What is a GTP cap?
when there is enough tubulins so that they don't fall off
What occurs at a shrinking microtubule?
Subunits in GTP cap hydrolyze GTP --> GDP. GDP tubulins peel away from MT wall, and GDP tubulin is released to the cytosol.
What are capping proteins?
stabilizing proteins that interact with MTS and bind the plus end to prevent disassembly
How do you establish cell polarity?
selectively stabilize the MTs in one direction; localize to one part of the cell
What is a characteristic of many fully differentiated cells that don't divide?
a constant cytoskeleton
What do dividing cells need to do to undergo mitosis and become motile?
they need to disassemble all or most of their MTs and actin filaments
What drugs can stop mitosis and motility?
drugs that either stabilize or prevent formation of MTs and actin filaments, such as anti-cancer
How are microtubules evident in neurons?
they are evident because you need to transport vesicles and organelles from the cell body to its extremities (axons and dendrites)
What type of motion is involved by microtubules?
saltatory, one-directional, but not constant motion
-runs for a while, then stops
What does the transport and activity involving MTs require?
molecular motors that hydrolyze ATP
What are the two families of motors?
dyneins and kinesins
Where do most dyneins move towards?
the minus end (toward centrosome and nucleus)
Where do most kinesins move towards?
the plus end (toward cell periphery)
What does the disruption of MTs cause?
loss of organization
-ER ain't near far from the nucleus anymore
-Golgi ain't near centrosome/nucleus anymore
What powers transport of kinesin along a microtubule?
kinesin ATPase, with a stepping motion, from one tubulin molecule to the next involving ADP/ATP interaction
Cilia and flagella are motile organelles constructed by:
a special arrangement of MTs
What does cilia move?
external fluids past cells using a power/recovery stroke
-such as moving mucus out respiratory tract to avoid cluttering of lungs
What does flagella move?
What causes motion of one MT against another?
What is the effect of cross linking proteins (sheet, spoke, nexin) inside cilia and flagella?
the linking proteins are flexible, and don't let the microtubules slide. instead, the dyne in causes microtubules to bend.
What are the four multiple roles of actin filaments?
1. skeleton of non motile structures (microvilli)
2. contraction (cell fibers and muscle)
3. motility (podia)
4. finishing mitosis (contractile ring divides cells)
What is the structure of actin (thin) filaments?
5-8 nm wide, helical arrangement of globular actin monomers all facing the same way --> gives polarity and possibility of directional motion
How are actin filaments assembled?
actin binds ATP, assembles into filaments (F-actin) in ATP-bound form - adds to plus end
What does ATP hydrolysis after the assembly of actin filaments do?
it decreases the stability of actin polymers because it less tightly binds the actin...with the minus end more likely to lose this ADP-actin
How is cell motility (aka crawling) acquired?
by extending podia (filopodia and lamellopodia) that have high concentrations of actin
What does the leading edge of lamellopodium contain?
a web of branched actin which grow in direction of cell motion and push it forward
How does filipodia encourage straight growth of loosely bundled actin filaments?
using form in, which binds to the plus end and holds onto the actin subunits, pulling in new actin monomer recruits
What type of myosin is in most cells?
How does myosin I move actin or vesicles on actin?
by ATP hydrolysis
-head group always walks towards positive end of actin filament it contracts
What can bipolar myosin II do to two oppositely oriented actin filaments?
bind them and pull them together
How does a sarcomere contract?
myosin move on actin filaments by ATP hydrolysis, pulling them together
How much do these myosin and actin filaments move?
they move a short distance, but happens simultaneously in each sarcomere for a macroscopic effect
What triggers coordinated contraction in striated muscle?
the rapid release of Calcium
What causes the cell to open its calcium channels?
depolarization of PM travels to interior of cell via T-tubules --> releases some calcium --> sarcoplasmic reticulum senses rise in calcium and opens its calcium channels
What is the effect of the rise in calcium in sarcomere?
troponin binds to calcium --> moves tropomyosin out of the way --> myosin binds to actin via ATP binding
What does tropomyosin do?
it blocks actin-myosin interaction
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