Microtubules

Question 1

What is the function of the cytoskeleton?

Answer 1

Provides structure to the cell and play a role in cell movement.
- high level of internal organization on cells & enables them to assume & maintain complex shapes
- Cell movement and cell division, in eukaryotes it actively moves membrane-bounded organelles within the cytosol, cell signaling and cell-cell adhesion

Question 2

Define Modularity

Answer 2

A small number of cytoskeletal elements are deployed in different locations and arranged in different ways to meet the needs of a particular cellular structure

Question 3

What are the major structures of the cytoskeleton?

Answer 3

Microfilaments, intermediate filaments, and microtubules

Question 4

Microtubules

Answer 4

Polymer of the protein tubulin, with a diameter of about 25 nm, that is an integral part of the cytoskeleton and that contributes to the support, shape, and motility of eukaryotic cells; also found in eukaryotic cilia and flagella

Question 5

What are microtubules made of

Answer 5

Tubulin

Question 6

Microtubules are the _____ structural elements of the cytoskeleton

Answer 6

Largest

Question 7

Cytoplasmic microtubules

Answer 7

Loosely organized, dynamic network that pervade the cytosol and are responsible for a variety of functions:
- maintaining axons
- formation of mitotic and meiotic spindles
- maintains and alters cell shape
- disposition and movement of vesicles

Question 8

Axonemal microtubules

Answer 8

Present in highly ordered bundles in the axonemes (the central shaft of a cilium or flagellum) of eukaryotic cilia and flagella and specialized for cell movement

Question 9

What are protofilaments?

Answer 9

Linear polymer of tubulin subunits

Question 10

Microtubules (MTs) are polarized structures with a ___ end and a ___ end

Answer 10

plus and minus

Question 11

What is Nucleation?

Answer 11

Act of providing a small aggregate of molecules from which a polymer can grow. In other words, it is the initiation process of assembling subunits

Question 12

What is elongation?

Answer 12

Growth of microtubules by addition of tubulin heterodimers to either end

Question 13

What is the kinetic assembly of microtubules in vitro?

Answer 13

Microtubule formation is initially slow, a period referred to as the lag phase. This period reflects the relatively slow process of mT nucleation. The elongation phase–the addition of tubulin dimers– is relatively fast compared with nucleation. Eventually, the mass of MTs increases to a point where the [ ] of free tubulin becomes limiting. This leads to the plateau phase, where MT assembly is balanced by disassembly.

Question 14

What is critical concentration?

Answer 14

The tubulin heterodimer concentration at which MT assembly is exactly balanced with disassembly

Question 15

Addition of tubulin occurs at which end?

Answer 15

Plus end

Question 16

When does treadmilling occur?

Answer 16

Arises when a given tubulin molecule incorporated at the plus end is displaced progressively along the MT and eventually lost by depolymerization at the opposite end

Question 17

What is the dynamic instability model?

Answer 17

Model for microtubule behavior that presumes two populations of MTs, one growing in length by continued polymerization at their plus ends and the other shrinking in length by depolymerization

Question 18

Microtubules commonly originate from a structure in the cell called a microtubule-organizing center (MTOC). What is MTOC?

Answer 18

Structure that initiates assembly of microtubules, the primary example being the centrosome

Question 19

What is a centrosome?

Answer 19

Small zone of granular material surrounding two centrioles located adjacent to the nucleus of animal cells; functions as a cell’s main microtubule-organizing center

Question 20

What is the role of MTOCs?

Answer 20

Their ability to nucleate and anchor MTs

Question 21

What are some microtubule-stabilizing/bundling proteins?

Answer 21

Microtubule-associated proteins (MAPs), tau, and MAP2

Question 22

What is MAPs?

Answer 22

Bind at regular intervals along the walls of MTs, allowing interaction with other filaments and cellular structures

Question 23

What is tau?

Answer 23

Causes MTs to form tight bundles in axons

Question 24

What is MAP2?

Answer 24

Is present in dendrites and causes the formation of looser bundles of MTs

Question 25

What are +- TIP (+- end tubulin interacting) proteins?

Answer 25

They stabilize MTs by capturing and protecting the growing plus ends. They also decrease the likelihood that MTs will undergo catastrophic subunit loss

Question 26

What are some microtubule-destabilizing/serving proteins?

Answer 26

Some of these proteins include: Stathmin/Op 18, Catastophins and ketanin

Question 27

What does Stathmin/Op 18 do?

Answer 27

It binds to tubulin heterodimers and prevents them from polymerizing

Question 28

What does Catastrophins do?

Answer 28

Act at the ends of MTs and promote peeling of subunits from the ends

Question 29

What does Katanin do?

Answer 29

Sever MTs

Question 30

What is “The Mesh”?

Answer 30

A network of microtubule connectors that stabilizes individual kinetochore fibers of the mitotic spindle where the chromosome attaches. The mesh holds and stabilizes MTs and holds two MT together at a time.