One question on cytoskeleton

Question 1

a. Cytoskeleton:

Answer 1

network of interconnecting fibers and tubules which is compromised for 3 different types of molecules
i. Microtubules, microfilaments (actin filaments), and intermediate filaments

Question 2

b. Roles:

Answer 2

determining the shape and size of the cell, movement of organelles within the cell, cell movement, interactions with other cells or with the ECM

Question 3

c. Cytoskeletal elements:

Answer 3

i. Microtubules, ii. Actin filaments, iii. Intermediate filaments:

Question 4

i. Microtubules

Answer 4

found in cytoplasm, cilia, flagella and mitotic spindles

1. Structure: heterodimer formed of one molecule each of an alpha and beta tubulin; heterodimer polymerize to form tubular structure  protofilament arranged in spiral organization with each turn formed of 13 heterodimers
2. Microtubules typically grow from a region near the nucleus called the microtubule organizing centers (MTOC)
   a. Centrosomes  mitotic spindles
   b. Basal bodies  from which grow cilia or flagella
3. Gamma tubulin molecules act as nucleation sites for the growth of microtubules
4. Growing end (+), end attached to MTOC (-)
5. Dynamic instability: microtubules can switch rapidly from polymerization to depolymerization and vice versa
   a. Regulated by: Ca 2+ concentration, Mg2+ concentration, GTP- assemble, GDP- disassemble, MAPs (microtubule-associated proteins)
6. Action of colchicine: anti-mitotic agent; binds to tubulin, prevents its addition to the (+) end, depolarization occurs at the (-) end
7. Microtubule- associated motor proteins: kinesins (organelles from MTOC toward (+) end of microtubule) and dyneins (organelles toward (-) of microtubule)

Question 5

ii. Actin filaments

Answer 5

made of globular subunits (G-actin: actin monomers NOT incorporated into the filament), which when connected together -> F-actin (incorporated into a filament)
1. Location and function:
a. Muscle cells as part of sarcomere
b. Network of fibers just inside the plasmalemma, which contributes to cell shape and locomotion
c. Cytoplasmic streaming (movement) of organelles
d. Form stress fibers in which cells “crawl”, aid in cell locomotion
2. Non cell muscles- actin filaments can rapidly polymerize and depolymerize under the control of actin-binding proteins, and changes in Ca2+ and cAMP, severing proteins (chop off portion of actin), bundling proteins

Question 6

iii. Intermediate filaments

Answer 6

more stable and more variable than microtubules or actin filaments; formed of coiled tetramers (anti-parallel)
1. Keratins: in epithelial cells, abundant in surface layer of epidermis
2. Vimentin and vimentin-like proteins: found in mesenchymal-derived tissues
3. Neurofilaments: rapid transport between dendrites; found only in neurons
4. Lamins: form structural framework for nuclei, only found in cell nuclei
5. Beaded filaments: lens specific, phakinin and filensin