Cytoskeletons

Question 1

Cytoskeleton Summary

Answer 1

• Keeps the shape of the cell.
• Allows cells to move
• Mediates intracellular transport
• Facilitates cell division

Question 2

Cytoskeleton Components

Answer 2

• Actin filaments - cell movement, provides force
• Microtubules - cell organization, cell movement, transport network.
• Intermediate filaments (IF) - strength, protection

Question 3

General Properties of the Cytoskeleton Components

Answer 3

• Microtubules (~25nm) - long tubes made of tubulin proteins
• Actin filaments (~7nm) - two-strand polymers of actin protein in a helical arrangement
• Intermediate filaments (~8-12nm) - think rope-like fibres made of heterogenous fibrous proteins

Question 3

Actin Cytoskeleton - G-actin

Answer 3

G (globular) are monomers of F-actin - asymmetrical in this form

• bound to protein when polymerising
  => causes + & - interaction is weaker ones at the beginning / was bound first ‘fall off
  => causes depolymerisation - is regulated in the cell for when it’s needed

Question 4

Actin Cytoskeleton - F-actin

Answer 4

F( filamentous) are polymers of G-actin
- Has a plus and minus end (not to do with charge)
- Minus end joins at plus and minus due to staggering is becomes a helix

Question 5

How Actin filaments work

Answer 5

• Allows cells to move + change shape
• Contractile force pushes membrane out to maintain shape
  => requires depolymerisation

Question 6

Microtubules Composition

Answer 6

• Made of alpha-beta-tubulin dimers organised in a tube structure with groups built end to end
• Beta hydrolyses GTP to GDP while alpha can’t

Question 7

How GTP hydrolysis regulates stability of microtubules

Answer 7

Growth + disassembly of MTs
- GTP hydrolysis changes subunit conformation + weeakens bond in polymer
- Protofilament from straight to curved
- Depolymerises as GDP is less stable
- Filament is more flexible + falls apart

Question 7

Cytoskeleton + Intracellular transport

Answer 7

• Motor proteins use microtubule network to move vesicles and organelles
• Diff ones travel in diff directions by recognising the polarity of microtubules

Question 8

Intermediate filament proteins structures

Answer 8

• Head and tail domain with long helical domain
• Form dimers which
  associate with one
  another in antiparallel
  fashion

Question 9

Intermediate filaments not in plants or fungi

Answer 9

Also not in all eukaryotes
=> as those cells without have a cell wall to provide structure and protection of the cell

Question 10

Cell division - A cellular process

Answer 10

• Nuclear Lamins (IFs) break down.
• Microtubules separate chromatids
  (organization and transport).
• Cytokinesis: Actin drives cell division
  (movement of membrane).
• Other proteins help e.g. motor proteins

Question 11

Bacteria have a cytoskeleton

Answer 11

• FtsZ – microtubule-like
  structure.
• MreB – actin-like structure.
• Crescentin and others related
  proteins – Intermediate
  filament-like structure