Section 2 Lecture 2: Cytoskeletal Networks

Question 1

The cytoskelaton

Answer 1

system of filaments that allow cells to move around, grow, and interact with their environment. Organization confers polarity

Question 2

Actin filaments

Answer 2

Determine the shape of the cell’s suface and are necessary for whole-cell locomotion. Also drive pinching of cell in two. Dispersed throughout the cell but concentrated in the cortex

Question 3

Microtubules

Answer 3

determine the positions of organelles, direct intracellular transport, and form the mitotic spindle that segregates chromosomes in cell division. Long, hollow cilynders made of tubulin protwin. More rigid than actin. Minus end attached to centrosome so that plus end can grow.

Question 4

Intermediate filaments

Answer 4

Rope-like fibres made of intermediate filament proteins. Form junctions in epithelial tissue, the nuclear envelope, and hair and nails.

Question 5

Accessory proteins

Answer 5

Regulate and link the filaments to other cell components and to each other. Essential fir controlled assembly of filaments to an area. Includes motor proteins

Question 6

Motor proteins

Answer 6

Molecular machines that convert the energy of ATP hydrolysis into mechanical force that can move filaments or organelles along filaments

Question 7

Cell cortex

Answer 7

Layer pof actin filaments below the plasma membrane of animal cells that provides strength and shape to the lipid bilayer

Question 8

Filopodia, lamellipodia, pseudopodia

Answer 8

Actin filaments projecting from the cell surface that allows the cell to explore territory and move around

Question 9

Contractile ring

Answer 9

Formed by a belt of actin and myosin that constricts to pinch in tow for cytokinesis

Question 10

Neutrophils

Answer 10

White blood cell that chases and engulfs bacterial and fungal cells by extending a protrusive structure filled with newly polymerized actin that can be reorganized to change direction

Question 11

Actin Subunit

Answer 11

Compact and globular subunit that binds actin filaments. Asymmetrical and bind head to tail. Provides structural polarity and contains wither an ATP or ADP bound in a deep cleft in the centre of the molecule

Question 12

Tubulin subunit

Answer 12

Compact and globular subunit that makes up microtubules. Held together by non-covalent interactions that are easily formed and broken. alpha and beta tubulin assemble head-to-tail. Alpha, the minus end, is permenantly GTP-bound whereas the beta, plus, end can preform GTP hydrolysis and have a GTP or GDP-bound state

Question 13

Protofilaments

Answer 13

linear strings of subunits joined end to end that associate with one another laterally to form a hollow cylinider. 13 protofilaments make up microtubules.

Question 14

Brownian motion

Answer 14

drives the diffusion of all the molecules inside cells

Question 15

F-actin

Answer 15

filamentous actin, assembly of actin subunits head-to-tail to form a tight right-handed helix structure about 8nm wide and polar, with a slow-growing minus, pointed, end and a fast-growing, plus, barbed end

Question 16

Persistance length

Answer 16

The average length than microtubules/actin stay straight. The minimum filament length at which random thermal fluctuations are likely to cause it to bend. Reflects the stiffness of a filament

Question 17

Initial oligomer

Answer 17

The unstable, initial association of actin subunits into a nucleus

Question 18

Filament nucleation

Answer 18

The process of stabalizing the initial oligomer by rapidly elongating with the rapid addition of more subunits. The rate-limiting step.

Question 19

Lag phase

Answer 19

phase when polymerization is initiated and no filaments are observed. Time taken for nucleation to begin. Lag phase is eliminated if fragments of filaments (seeds) are added in solution.

Question 20

Critical concentration

Answer 20

Cc= Kon/Koff = Kd. The concentration of free subunits left in solution at the steady state. C>Gc= growth,C<Cc= shrink, C=Cc, equillibrium

Question 21

Steady state

Answer 21

When the rate of addition and rate of dissociation are exactly balanced so that the filament length does not change

Question 22

Growth phase

Answer 22

occurs as monomers add to the exposed ends of the growing filament, causing filament elongation

Question 23

nucleotide hydrolysis

Answer 23

soon after actin/tubilin subunits form a polymer, the tightly-bound ATp (for actin)/ GTP( for tubulin) is hydrolyzed to an ADP/GDP. Hydrolysis reduces the binding affinity of the subunit

Question 24

GTP/ATP cap

Answer 24

a cap of subunits occurs when the rate of addition of subunits to a growing actin filament/microtubule is faster than the rate at which to ATP/GTP is hydrolyzed

Question 25

Treadmilling

Answer 25

Occurs in actin filaments due to the greater Cc at the minus end and lower Cc at the plus end. Polymerization proceeds until it is above that of plus and below that of minus, resulting in net assembly at the plus end and net disassembly at the minus end, resulting in movement of the polymer towards he plus end at a constant length

Question 26

Dynamic instability

Answer 26

Occurs when microtubules alternate betweeen a period of growth followed by a period of rapid instability. This occurs due to beta tubulin alrenating between its GTP and GDP bound states. Depolymerization happens 100x faster at a GDP-cap than at a GTP-cap. If a GTP-cap is lost, rapid depolymerization follows