Which of the following statements about the molecular...

Which of the following statements about the molecular arrangement of actin and myosin in myofibrils islare incorrect? (i) Each actin (thin filament) is made of $2 \mathrm{~F}$ (filamentous) actins. (ii) F-actin is the polymer of G (globular) actin. (iii) 2 F-actins are twisted into a helix. (iv) Two strands of tropomyosin (protein) lie in the grooves of F-actin. (v) Troponin molecules (complex proteins) are distributed at regular intervals on the tropomyosin. (vi) Troponin forms the head of the myosin molecule. (vii) The myosin is a polymerised protein. (a) (i), (iii) and (vii) (b) (ii), (iv) and (v) (c) Only (vi) (d) Only (iii)

Which of the following statements about the molecular arrangement of actin and myosin in myofibrils islare incorrect?
(i) Each actin (thin filament) is made of $2 \mathrm{~F}$ (filamentous) actins.
(ii) F-actin is the polymer of G (globular) actin.
(iii) 2 F-actins are twisted into a helix.
(iv) Two strands of tropomyosin (protein) lie in the grooves of F-actin.
(v) Troponin molecules (complex proteins) are distributed at regular intervals on the tropomyosin.
(vi) Troponin forms the head of the myosin molecule.
(vii) The myosin is a polymerised protein.
(a) (i), (iii) and (vii)
(b) (ii), (iv) and (v)
(c) Only (vi)
(d) Only (iii)

Key Concepts

Helical Organization of Filaments in Myofibrils

Both actin and myosin filaments exhibit helical characteristics; F-actin is inherently helical, which is key for its interaction with regulatory proteins, while the organization of myosin into thick filaments enables effective cross-bridge cyclings for muscle contraction. The precise spatial arrangement of these filaments within myofibrils is essential for the coordinated action of muscles.

Regulation of Thin Filaments by Tropomyosin and Troponin

Tropomyosin is a long, helical protein that runs along the grooves of F-actin, providing a binding site for troponin complexes. Troponin acts as a regulatory complex that, upon binding calcium, induces conformational changes in tropomyosin to expose or block myosin binding sites on actin, thereby controlling muscle contraction.

Myosin Molecular Structure

Myosin is a motor protein composed of distinct domains including a head region that binds actin and hydrolyzes ATP, a neck region, and a tail domain. It assembles into thick filaments that interact with actin filaments in muscle fibers, facilitating contraction. Importantly, regulatory proteins such as troponin are not a part of myosin's structure.

Actin Polymerization

Actin monomers, known as G-actin, polymerize to form long, filamentous structures called F-actin. This process is fundamental for the integrity and dynamics of the cytoskeleton and plays a crucial role in muscle contraction by forming the thin filaments of myofibrils.

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