Q.25 Which one of the following statements is true for actin?
(A) Actin filament is structurally polarized and the two ends are not identical
(B) De novo actin polymerization is a single–step process
(C) The pointed end of the actin filaments is the fast growing end
(D) Actin forms spindle fibers during mitosis

Actin Filaments in Cell Biology: Understanding Structural Polarity and Polymerization

Actin is a key cytoskeletal protein that forms filaments essential for cell shape, motility, and division. The correct answer to the query is option (A): Actin filament is structurally polarized and the two ends are not identical.​

Option Analysis

Option (A): Correct
Actin filaments (F-actin) consist of globular actin (G-actin) monomers oriented unidirectionally, creating inherent polarity. This results in a barbed (plus) end and a pointed (minus) end, which differ in structure and growth rates, as confirmed by electron microscopy with myosin decoration.​

Option (B): Incorrect
De novo actin polymerization is nucleated and multi-step: it begins with dimer/trimer formation (slow nucleation), followed by rapid elongation. ATP-bound G-actin adds preferentially, with hydrolysis regulating dynamics—never a single step.​

Option (C): Incorrect
The pointed end grows slowly, while the barbed (plus) end is the fast-growing end due to lower critical concentration for monomer addition. Treadmilling reinforces this asymmetry.​​

Option (D): Incorrect
Actin contributes to the contractile ring in cytokinesis but does not form spindle fibers, which are microtubules (tubulin-based) for chromosome segregation in mitosis.​

Key Properties of Actin Filaments

Actin filaments drive cellular processes through dynamic instability:

• Polarity enables directionality: Myosin motors “walk” toward the barbed end, powering contraction.​

• Nucleation and treadmilling: Capping proteins (e.g., CapZ at barbed end) regulate growth for lamellipodia formation.​

• Regulation: Profilin/ARP2/3 complexes promote branching; cofilin severs filaments.​