cytoskeleton

Question 1

what is an organelle

Answer 1

any membrane limited structure found in the cytoplasm of cell.

Question 2

what is a biomembrane

Answer 2

permeability barrier, surrounding
cells and organelles that consists of a phosphor-lipid bilayer, associated membrane proteins and cholesterol and glycolipids

Question 3

cytoplasm definition

Answer 3

viscous contents of a cell contained within the plasma membrane but outside nucleus

Question 4

cytoskeleton definition

Answer 4

System of protein filaments in the cytoplasm of a eukaryotic cell that gives the cell shape and the capacity for directed movement

Question 5

why does a cell need a cytoskeleton

Answer 5

• organise in space and interact mechanically with environment.
• to be correctly shaped, physically and properly structured internally.
• to change shape
• to move from place to place.
• to rearrange internal components as grow,
  divide and adapt to changes.
• spatial and mechanical functions highly developed

Question 6

functions of the cytoskeleton

Answer 6

I. Pulls chromosome apart at mitosis and splits the dividing cell in two.

II. Drives and guides intracellular traffic of organelles, ferrying materials
from one part of the cell to another.

III. Supports the plasma membrane and mechanical linkages that bear
the stresses and strains

IV. Enables some cells, such as sperm to swim,
and others, such as fibroblasts and white
blood cells to crawl.

V. Provides machinery in muscle cell contraction
and in the neuron to extend an axon and
dendrites.

VI. Summary- Varied functions of cytoskeleton
centre on behaviour of 3 families of protein
molecules

Question 7

what are the 3 types of cytoskeleton filaments and what are their roles

Answer 7

1) actin filaments (microfilaments)- maintenance of cell shape and controls cell attachment and motility.

2) intermediate fibres - maintaining cell
shape.

3) microtubules- movement of cell organelles (e.g.
chromosomes during mitosis) and participate with microfilaments in the control of cell movement.

Question 8

what are cytoskeleton systems

Answer 8

dynamic and adaptable individual macromolecular components that make up these structures that are in a constant state of flux

Question 9

describe on rates and off rates are in cytoskeleton chemistry

Answer 9

a linear polymer of protein assembles by polymerisation and disassembles by depolymerisation by addition and removal of subunits at the end of the polymer

the rate of addition is given by rate constant k¬on or k¬off

Question 10

describe the process of nucleation

Answer 10

helical polymer is stabilized by multiple contacts between adjacent subunits.

monomer -dimer - trimer

further monomer addition can take place and this then acts as a nucleus for polymerisation

this is a slow process therefor this explains the lag phase during polymerisation

Question 11

describe the graph of the time course of polymerisation

Answer 11

lag phase - growth phase - equilibrium phase

lag phase - time taken for nucleation
growth phase - when monomers add to exposed ends of growing filament, causing filament elongation
equilibrium phase - when growth of polymer due to monomer addition is balanced by shrinkage of polymer due to disassembly back to monomers.

Question 12

what is the plus end and what is the minus end

Answer 12

plus end - fastest growing end
minus - slowest growing end

Question 13

why is there a difference in rates of growth of either end

Answer 13

conformational change of each subunit

Question 14

how can you abolish or reduce the lag phase

Answer 14

if premade nuclei are added

Question 15

what is the structure of actin filaments

Answer 15

2 stranded helical polymer of protein actin
- flexible
- diametre approx 7nm
- organised into linear bundles, 2D networks and 3D semi solid gels
- actin binds to protiens
- filaments disepersed through cell,

Question 16

where are filaments most highly concentrated

Answer 16

cortex, just beneath the plasma membrane

Question 17

are filaments polar

Answer 17

yes

Question 18

how is the cytoskeleton regulated

Answer 18

cells regulate length, stability, number and geometry

cells regulate attachment to one another and other components in cell

Question 19

how are actin filaments organised

Answer 19

actin bundles - closely packed parallel arrays (at least 2 domains that bind actin)
actin networks - loosely cross linked in arrays (large flexible protiens)

Question 20

what are the actin binding proteins

Answer 20

• Fimbrin- 68kDa- small cross-linker- 2 actin binding
  domains close together in a single polypeptide
  chain.
• α-actinin-102kDa two actin binding domains
  further apart

Question 21

where is α-actinin and fimbrin found

Answer 21

α-actinin - concentrated in stress fibres, responsible for loose
cross-linking. This allows Myosin II to bind and allows cells to
contract

fimbrin - filopodia which are non-contractile bundles- no
myosin II binding.

Question 22

what is the structure and function of intermediate filaments

Answer 22

ropelike fibres
diameter - 10nm
meshwork in nucleus
extend across the cytoplasm giving mechanical strength

Question 23

describe the structure of tubulin filaments

Answer 23

microtubules - long hollow cylinders
- diameter 25nm
- ridged
- long and straight
- Attached to a single microtubule organizing centre (MTOC)
called a centrosome-slow end binds to γ tubulin.
- Centrosome linked to centrioles!