Thin Filament Structure and Regulation — NEET Biology
📚 Practice ConceptEach actin (thin) filament is made of two 'F' (filamentous) actins helically wound to each other. Each 'F' actin is a polymer of monomeric 'G' (Globular) actins. Two filaments of another protein, tropomyosin also run close to the 'F' actins throughout its length. A complex protein Troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments.
Which of the following statements about muscle structure and protein function are correct? S1: Each 'F' actin filament, a polymer of monomeric 'G' actins, is associated with two helical tropomyosin filaments that run throughout its length. S2: The heavy meromyosin (HMM) component, which includes the globular head and a short arm, projects outwards as the cross arm and has an active ATPase enzyme activity. S3: Troponin is a complex protein distributed at regular intervals on the F-actin filament, masking the myosin binding sites in a resting muscle. S4: The Z-line is an elastic fibre that bisects the 'I' band and serves as the anchoring point for the thick filaments. S5: In a resting state, the 'H' zone represents the central part of the 'A' band where the thick filaments are partially overlapped by the thin filaments.
View solution & NCERT explanation
Correct answer: B — S1 and S2 are correct
Let's evaluate each statement regarding muscle structure and protein function based on NCERT: S1: Each 'F' (filamentous) actin is made of two 'F' actins helically wound to each other, and two filaments of tropomyosin also run close to the 'F' actins throughout its length. This statement is correct. S2: The heavy meromyosin (HMM) component, consisting of the globular head and a short arm, projects outwards as the cross arm. The globular head is indeed an active ATPase enzyme. This statement is correct. S3: Troponin is a complex protein distributed at regular intervals on the *tropomyosin*, not directly on the F-actin filament. It masks the active binding sites for myosin on the actin filaments in the resting state. Therefore, this statement is incorrect. S4: The Z-line is an elastic fibre that bisects the 'I' band, and the *thin* filaments (actin) are firmly attached to the 'Z' line, not the thick filaments. Therefore, this statement is incorrect. S5: In a resting state, the 'H' zone is the central part of the thick filament *not overlapped* by thin filaments. Therefore, this statement is incorrect. Only S1 and S2 are correct.
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