Cell Biology Exam 3 Study Material - Part 3: Cytoskeletal Systems and Cellular Movement

What is the cytoskeleton?

a complex network of interconnected filaments and tubules that extends throughout the cytosol from the nucleus to the inner surface of the plasma membrane

What is the purpose of the cytoskeleton?

provides an architectural framework for cellular function, cell movement, cell division, organelle/vesicle, and protein/RNA movement

What 3 things are classified as cellular movement?

1. movement of a cell through its environment

2. movement of components within the cell

3. the shortening of the cell itself (contracting muscles)

What are the 3 components of the cytoskeleton?

microtubules, microfilaments, and intermediate filaments

What is the diameter of microtubules?

25 nm

What is the diameter of microfilaments?

7 nm

What is the diameter of intermediate filaments?

8-12 nm

What are the two types of microtubules?

axonemal and cytosolic

How are axonemal microtubules arranged?

highly organized and stable

Where are axonemal microtubules found?

specific subcellular structures (cilia, flagella)

What is the function of axonemal microtubules?

associated with cellular movement (cilia, flagella)

How are cytosolic microtubules arranged?

loosely organized and dynamic network

Where are cytosolic microtubules found?

cytosol

What are 3 functions of cytosolic microtubules?

1. movement of chromosomes during mitosis and meiosis

2. movement of proteins/RNAs and vesicles/organelles

3. organization and maintenance of cell shape

What is the structure of microtubules?

a hollow cylinder enclosing a lumen

What are the building blocks of microtubules?

tubulin heterodimers (alpha/beta tubulin dimer)

What is a protofilament?

a linear polymer of alpha/beta- tubulin dimers

How many protofilaments does the wall of the cylinder of microtubules contain?

13 protofilaments

Do all the alpha/beta tubulin dimers in the protofilaments have the same orientation?

yes

Do microtubules have polarity?

yes; there is a positive and negative end

What are the 5 steps for microtubule assembly?

1. nucleation 2. elongation 3. protofilaments association 4. closing of tube 5. elongation continues

What happens in nucleation of microtubule formation?

several dimers aggregate into clusters called oligomer

What happens in elongation of microtubule formation?

oligomer grows by addition of subunits at either end, forming protofilament

What happens during protofilament association of microtubule formation?

protofilaments associate with each other side-by-side to form a sheet

What happens during closing of tube in microtubule formation?

sheets of 13 or more protofilaments close into a tube, forming a microtubule

What happens during the second elongation of microtubule formation?

elongation continues by addition of dimmer at one or both ends

What are the three phases of microtubule assembly in vitro?

1. Iag phase

2. elongation phase

3. plateau phase

What does microtubule assembly require?

GTP and Mg2+

What happens during the Iag phase?

microtubule formation is slow at first because the process of nucleation is slow

Is the elongation phase fast or slow?

fast

What happens during the plateau phase?

when the mass of microtubules reaches a point where the amount of free tubulin is diminished, the assembly is balanced by disassembly

What is critical concentration in terms of the phases of microtubule assembly?

it is the tubulin concentration at which MT assembly is exactly balanced by disassembly

What cellular structure do microtubules originate from?

the microtubule-organizing center (MTOC)

What is the purpose of the MTOC?

initiates microtubule assembly and anchors one end of these microtubules

Where are negative ends of microtubules?

anchored in the MTOC

Where are the positive ends of microtubules?

pointed outward toward cell membrane

What is a type of MTOC that many cells have during interphase?

the centrosome

Where is the centrosome positioned?

near the nucleus

What is the structure of the centrosome?

two centrioles and associated pericentriolar material

How are the centriole walls formed?

by nine sets of triplet microtubules

What is the purpose of centrioles?

to recruit pericentriolar material to the centrosome

How do microtubules move proteins/RNA and organelles/vesicles?

by providing a rigid set of tracks for the transport of them

What proteins are used for the mechanical work of the movement across microtubule tracks?

microtubule-associated motor proteins

How do microtubule associated motor proteins work?

they attach to proteins/RNA or vesicles and walk along the MT

What are two major families of microtubule motors?

kinesins and dyneins

Can microtubule aided movement move things in and out of the cell?

yes

How do kinesins and dyneins "walk" along the microtubules?

they couple ATP hydrolysis to conformational changes

Where do kinesins move proteins/RNAS and vesicles/organelles?

towards the positive end of the MTs

Where do dyneins move proteins/RNAs and vesicles/organelles?

towards the negative end of MTs

What is the structure of kinesins?

two globular heads, a coiled helical region, and a light-chain region

What is the function of the globular head of kinesins?

involved in hydrolyzing ATP and attaching to MTs

What is the function of the light-chain region of kinesins?

involved in attaching the kinesin to the cargo (RNA, protein, etc)

How does the kinesin move?

the heads alternate making attachments to a new beta-tubulin subunit (walking)

What is the important function of kinesins?

moving and localizing substances within cells

What is the purpose of microfilaments?

involved in contractile fibrils of muscle cells, cell movement, cytokinesis, and development of cell shape

What is the cortex?

tissue just beneath the plasma membrane that confers structural rigidity on cell surface and facilitates shape changes and cell movement

What is the structure of microfilaments?

two intertwined strands of F-actin

What is F-actin?

filamentous actin; protein building blocks of MFs

What is G-actin?

free actins - proteins

How are microfilaments assembled?

G-actin is added to the growing strands and followed by hydrolysis of the ATP molecule to ADP

Do microfilaments have polarity?

yes; the positive end has ATP-actin and the negative end has the ADP-actin

Does the positive or negative end of microfilaments grow faster?

the positive end grows faster

How do microfilaments move cells?

the leading edge extends through polymerization of actin and the lagging strand contracts

What are two important properties of intermediate filaments compared to the other parts of the cytoskeleton?

they are the most stable and least soluble

What is the function of intermediate filaments?

they play a structure and tension-bearing role in the cell (scaffold of cell)

Do intermediate filaments have polarity?

no

Are intermediate filaments found in unicellular organisms?

no, just multicellular

How are intermediate filaments assembled?

two IF-forming molecules form a dimer, two dimers form a tetramer, tetramers align to form protofilament, and 8 protofilaments assemble to form intermediate filaments

Are the parts of the cytoskeleton structurally and functionally interconnected?

yes

What kinds of proteins link the constituents of the cytoskeleton together?

spectraplakins proteins

Can microtubules resist bending?

yes

Do microfilaments contract to generate tension?

yes

Are intermediate filaments elastic and can withstand tensile forces?

yes