Cytoskeleton Structure and Cell Motility (2A) - Bio & Biochem

1. Which motor protein moves toward the plus end of microtubules?

   Kinesin. Kinesins are ATP-dependent motors that transport cargo anterogradely along microtubules toward the cell periphery.

2. Which end of an actin filament typically elongates faster in cells?

   The plus (barbed) end. The plus end has a lower critical concentration for actin addition, promoting faster polymerization.

3. Which cytoskeletal filament type forms the contractile ring in cytokinesis?

   Actin microfilaments with myosin II. Actin-myosin interactions generate the contractile force to divide the cytoplasm in cytokinesis.

4. What is the approximate diameter of a microtubule?

   About $25\ \text{nm}$. Microtubules are the largest cytoskeletal filaments, formed from tubulin dimers, aiding in rigidity and transport.

5. Identify the property of actin filaments that creates a plus and minus end.

   Polarity due to asymmetric actin subunit orientation. Asymmetric orientation of actin subunits imparts structural polarity, influencing assembly and motor protein directionality.

6. Which end of a microtubule typically elongates faster in cells?

   The plus end. The plus end exhibits faster polymerization due to favorable kinetics of tubulin addition.

7. What is the microtubule-organizing center in most animal cells?

   The centrosome. The centrosome serves as the primary site for microtubule nucleation and organization in animal cells.

8. Which motor protein moves toward the minus end of microtubules?

   Dynein. Dyneins facilitate retrograde transport, moving cargo toward the microtubule minus end at the cell center.

9. What nucleotide is hydrolyzed during microtubule polymerization dynamics?

   GTP (tubulin is a GTPase). GTP hydrolysis by tubulin drives dynamic instability, enabling rapid microtubule remodeling.

10. What is treadmilling in actin filaments?

    Plus-end addition with minus-end loss at steady state. Treadmilling maintains constant filament length through net assembly at one end and disassembly at the other.

11. Identify the actin-based cell motility structure that drives membrane protrusion at the leading edge.

    Lamellipodia (branched actin network). Lamellipodia use Arp2/3-mediated actin branching to push the plasma membrane forward during cell migration.

12. Which motor protein moves toward the plus end of actin filaments?

    Myosin (most myosins are plus-end directed). Myosins interact with actin to generate force, with most types walking toward the barbed end.

13. What is the approximate diameter of an intermediate filament?

    About $10\ \text{nm}$. Intermediate filaments have an intermediate size, providing mechanical strength without polarity.

14. Identify the property of microtubules that creates a plus and minus end.

    Polarity from head-to-tail tubulin dimer assembly. Head-to-tail arrangement of tubulin dimers creates inherent polarity, directing growth and motor movement.

15. What is the monomeric subunit of microtubules?

    $\alpha$-tubulin and $\beta$-tubulin heterodimers. Tubulin heterodimers assemble into protofilaments that form the hollow tubular structure of microtubules.

16. What are the three major cytoskeletal filament systems in eukaryotic cells?

    Microfilaments (actin), microtubules, and intermediate filaments. These filaments provide structural support, enable motility, and facilitate intracellular transport in eukaryotic cells.

17. What is the primary cytoskeletal role of intermediate filaments?

    Tensile strength and mechanical stability. Intermediate filaments resist mechanical stress, maintaining cell integrity under tension.

18. What nucleotide is hydrolyzed during actin filament polymerization dynamics?

    ATP (actin is an ATPase). ATP hydrolysis by actin subunits regulates filament dynamics, including treadmilling and critical concentration differences.

19. What is dynamic instability in microtubules?

    Switching between growth and rapid shrinkage (catastrophe/rescue). Dynamic instability allows microtubules to explore cellular space and reorganize rapidly during processes like mitosis.

20. Which cytoskeletal filament type is the main component of the mitotic spindle?

    Microtubules. Microtubules form the spindle apparatus to separate chromosomes during cell division.

21. What is the classic $9+2$ microtubule arrangement found in eukaryotic cilia and flagella?

    Nine outer doublets surrounding two central singlets. The 9+2 axoneme structure enables coordinated bending for motility in cilia and flagella.

22. Which motor protein generates ciliary and flagellar bending by sliding microtubule doublets?

    Axonemal dynein. Axonemal dynein arms produce sliding forces between microtubule doublets, causing ciliary bending.

23. What is the monomeric subunit of actin microfilaments?

    G-actin (globular actin). G-actin monomers polymerize to form filamentous F-actin, the structural basis of microfilaments.

24. What is the approximate diameter of an actin microfilament?

    About $7\ \text{nm}$. Actin microfilaments are the thinnest cytoskeletal elements, composed of polymerized G-actin subunits.

25. Which microtubule structure nucleates microtubule growth at the centrosome?

    $\gamma$-tubulin ring complex ($\gamma$-TuRC). γ-TuRC acts as a template for tubulin dimer addition, initiating microtubule polymerization from the minus end.

Study these flashcards