Describe the molecules involved and the role that each plays in the process of microtubule assembly. 

•  Non-branching rigid hollow tubes of protein (20-25)

•  Walls contain dimers of α- and β-tubulin – arranged in a circle, one turn of which is composed of 13 of them
  (+) end is β-tubulin end (polymerization occurs)
  (–) end is α-tubulin end

•  dynamic instability: Rapidly assemble/disassemble  
Most grow from microtubule organizing center (MTOC; centrosome) near nucleus toward the cell periphery

•  The protofilament molecules are assembled in the MTOC with the help of GTP–tubulin has GTP-ase activity

•  γ-tubulin–serves as nucleation site (γ-tubulin rings found at MTOC)
•  Microtubule-associated proteins (MAPs) regulate microtubule assembly, anchor microtubules to specific organelles, and stabilize microtubules by preventing depolymerization

•  Assemble from neg end (at MTOC) to positive (growing) end

Identify microtubule functions and the cellular structures associated with these functions.

•  Movement of cilia and flagella
•  Attachment of chromosomes to mitotic spindle, movement during cell division
•  Cell elongation and movement
•  Maintenance of cell shape 
•  Intracellular vesicular transport: serve as “railroad tracks” along which organelles move with aid of the aid of special MAPS called molecular motor proteins

Identify and describe the motor proteins associated with microtubules and how each interacts with microtubules to carry out cellular functions.

•  Movement of intracellular organelles is also controlled motor proteins:

• Kinesins–move toward plus end carrying organelles toward periphery 
• Cytoplasmic dyneins–move toward minus end of microtubule, i.e. from periphery toward MTOC;  
• Axonemal dynein–causes movement of cilia and flagella by shifting movement between microtubules

Identify and describe two forms that actin can take in the cytoplasm and what role actin-binding proteins play in the balance of these two forms.

• G-actin (“globular”)–free cytoplasmic actin molecules
• F-actin (“Filamentous”)–polymerized actin filaments

• Actin-binding proteins

• Regulate actin function and polymerization
• Can prevent or enhance polymerization of actin filaments from G-actin
• Can induce branching
  Can break filaments into short pieces

Describe four classes of intermediate filaments and the particular cell types and structures with which they are associated.

1. Keratins (or cytokeratins)

• found in cells of epithelial origin
• hard keratins – found in hair and nails
• attach to keratin filaments in adjacent cells via desmosomes

2. Vimentin, and vimentin-like filaments 

• most abundant intermediate filament
• found in mesoderm-derived cells (CT, muscle, neuroglia) 
• desmin, a vimentin-like filament, is found in muscle
• glial fibrillary acidic protein (GFAP) is found in glial
  cells

3. Neurofilaments

• extend from the cell bodies of neurons to the
  tips of axons and dendrites
• provide structural support

4. Lamins

• formed of 2 proteins: lamin A and lamin B
• associated with the nuclear envelope
• provide structural framework for the nucleus
• phosphorylation of lamins by protein kinases
  causes them to disassemble; dephosphorylation causes them to reassemble

Compare and contrast microtubules, actin filaments, and intermediate filaments with respect to size, structure, function, and energy required for polymerization.

Microtubules: 20-25 nm, non-branching rigid hollow tubes, 13 dimers of α- and β-tubulin, movement (cilia, flagella), GTP–tubulin has GTP-ase activity

Actin filaments: 5-9 nm, branching, solid filaments, absorption (microvilli), have ATP-ase activity

Intermediate filaments: 10-12 nm, a pair of helical monomers twisting together to form coiled-
coil dimers, great tensile strength, ropelike, form networks throughout cytoplasm, structural, do not possess enzymatic activity (stable, no dynamic instability)

Discuss the role of the centriole in the MTOC and its relationship to the centrosome.

Centriole: the focal point around which the MTOC assembles

Centrosome: region of the cell containing the
centrioles and surrounding material (also
called the MTOC)

Function of centrosome:

• Location of formation for most microtubules
• Controls number, polarity, direction, orientation and organization of microtubules formed during interphase 
• MTOC depends on presence of centrioles

Identify two major roles of the centriole and explain their significance for the cell.

• Controls number, polarity, direction, orientation  and organization of microtubules formed during interphase

• Provide basal bodies for assembly of cilia and flagella

• Centrioles replicate forming “procentrioles”
• Procentrioles migrate to cell surface to become basal bodies
• Basal bodies serve as organizing centers for the assembly of microtubules of the cilium or flagellum
  
• The core structure of the cilium (axoneme) is composed of 2 central microtubules surrounded by 9 doublets 
• These doublets are continuous with the A and B microtubules of the basal body, and grow by  addition of α-/β-tubulin dimers to the plus end

Describe the process of centriole replication and relate it to processes occurring within the cell.

• Centriole replication and mitotic spindle formation

• occurs during DNA synthesis (S phase) for cell division
• procentriole–a small granular mass appears at the side of each centriole and enlarges to form a right angle appendage to the adult 
• Microtubules develop in the mass as it grows
• Mitotic spindle formation–after duplication parent/daughter pairs separate and produce astral microtubules that define the poles for the developing mitotic spindle.