Exam 4 Review

Question 1

What are the 3 major components of the cytoskeleton?

Answer 1

Intermediate filaments, microfilaments, microtubles

Question 2

What are the characteristics of Intermediate Filaments?

Answer 2

• strong ropelike structures, gives shape to nucleus of most eukaryotes and cytoplasm of vertebrate cells
• provide tensile strength, allow layers of cells (epithelia) to stretch without rupturing.
• resist shearing

Question 3

What protects skin cells exposed to mechanical stress?

Answer 3

keratins

Question 4

Define a keratin?

Answer 4

• fibrous structural protein
• makes outer layer of skin
• protects skin cells from stress, damage

Question 5

Name the three major filament systems of the cytoskeleton. Briefly describe the role of each. Give an example of each filament system and where it is used in the cell.

Answer 5

• Intermediate filaments: cell shape, rigid rods; ex: neurofilaments, keratin
• Microfilaments: filamentous actin; intracellular transport, cell adhesion, etc.
• Microtubules: spindle, transport

Question 6

Which of the following types of cells would you expect to contain a high density of intermediate filaments in their cytoplasm? Explain your answer.
-Amoeba proteus, Skin Epithelial cell, keratins, E coli, sperm cell, plant cell

Answer 6

a. Amoeba proteus (a free living amoeba) no, motile cellb. Skin epithelial cell yes, keratins, cell structurec. E. coli no, prokaryoted. Sperm cell no, motile celle. Plant cell no IFs in plants except nuclear lamins

Question 7

G-actin units assemble into microfilaments or F-actin. Describe this process.

Answer 7

G-actin with ATP bound adds to the (+) end of the filament, preferentially

Question 8

What does “treadmilling” refer to?

Answer 8

Actin filaments that ‘crawl’ by addition at the (+) end and removal at the (-) end the filaments move L to R, but subunits themselves don’t move

Question 9

Compare and contrast lamellipodia and filopodia.

Answer 9

• Lamellipodia: extension through actin assembly into branching networks
• Filapodia: extension through actin assembly into parallel filaments

Question 10

Actin originates from the ARP 2/3 complex which can form off of existing actin filaments, how does this play a role in lamellipodial extension?

Answer 10

New nucleation site (nucleating seed) along existing filaments extend new filaments that contact membrane, exert force and thereby contribute to lamellipodial extension

Question 11

How are stress fibers formed and what functions do they serve in the cell?

Answer 11

Actin filaments with α-Actinin forms bundles called “stress fibers”Stress fibers 1provide mechanical strength to the cell and 2attach cell to the substratum at focal adhesions

Question 12

A transmembrane protein called ____ links ____ fibers to the extracellular matrix at ____ ____.

Answer 12

integrin, stress, focal adhesions

Question 13

Myosin is an “actin-dependent” ATPase that acts as a molecular motor. What are the steps that myosin cycles through to move along actin.

Answer 13

1. Myosin head tightly bound to actin, “rigor”
2. ATP binds which releases myosin from actin
3. ATP hydrolysis “cocks” myosin; myosin binds actin weakly
4. Pi is released, strengthening the binding of myosin to actin
5. Myosin bound tightly to actin undergoes “power stroke” and ADP is released

Question 14

Compare single-headed myosin and type II myosin.

Answer 14

• Single headed: vesicle transport;

- Type II: slides antiparallel actin filaments as myosin walks toward the (+) ends- cell contraction/cell division

Question 15

Describe the mechanism of movement for prokaryotic flagella and name 2 features that distinguish them from eukaryotic flagella.

Answer 15

Prokaryotic flagella are bidirectional rotary motors (screw propeller movement), made of flagellin, powered by a proton gradient across the plasma membrane; CCW for moving forward, CW for stallingDifferences: proks not made of MTs, no plasma membrane around prok flagella

Question 16

Describe MT assembly and structure?

Answer 16

α,β tubulin dimers assemble to make MT protofilamentsMTs are polar, assembly favored at the + (β) end, disassembly favored at the – (α) end

Question 17

How do dynein motors cause microtubules to slide?

Answer 17

Dynein is a (-) end motor (ATPase) that ‘walks’ on microtubules causing isolated doublets to slide which results in bending

Question 18

How are basal bodies and centrioles similar?

Answer 18

They have the same structure of 9+0 triplets, functionally interchangeable in some cells

Question 19

The combined growing and shrinking of a microtubule is called ____ ____

Answer 19

dynamic instability

Question 20

In a growing microtubule, the rate of ______ exceeds the rate of _______.

Answer 20

elongation (polymerization), shrinking (depolymerization)

Question 21

What is the role of γ-TuRC in microtubule assembly?

Answer 21

γ-TuRC is located in pericentriolar material and nucleates MT assembly and also caps the (-) ends

Question 22

In terms of microtubule dynamics, what does ‘catastrophe’ and ‘rescue’ refer to?

Answer 22

GTP on β subunit of dimer; GTP hydrolysis exceeds addition = catastrophe, addition proceeds faster than GTP hydrolysis = rescue

Question 23

How do microtubules contribute to cell organization and polarity?

Answer 23

Polarized cell distribution (organize Golgi and ER), polarized directional transport (vesicle movement)

Question 24

T/F The minus end of microtubules and microfilaments is so named because the subunits are lost and not
added there

Answer 24

F, subunit removal is preferred, but subunits can add

Question 25

T/F The energy required for tubulin and actin polymerization is provided by hydrolysis of nucleotide triphosphate

Answer 25

F, hydrolysis of nucleotides destabilizes MTs, but not directly providing polymerization energy

Question 26

T/F Microtubules and microfilaments in a typical eukaryotic cell exist in dynamic equilibrium with a pool of subunit proteins

Answer 26

True

Question 27

T/F An algal cell contains neither tubulin nor actin

Answer 27

False

Question 28

T/F All of the protein subunits of intermediate filaments are encoded by genes in the same gene family

Answer 28

True

Question 29

T/F Microtubules within animal cells generally have their minus ends anchored at the centrosome

Answer 29

True

Question 30

T/F As long as actin monomers continue to be added to the plus end of a microfilament, it will continue to elongate

Answer 30

F, also depends on rate of removal from (–) end

Question 31

Dyneins “walk” toward which end of MT? Kinesins “walk” toward which end?

Answer 31

Minus, plus

Question 32

How do these walking proteins play a role in vesicle transport, and cell division?

Answer 32

Motors coat vesicle surface and transport it along microfilaments or microtubules. In animal cells myosin motors help contract the contractile ring.