Question

Which of the following statements about the molecular arrangement of actin and myosin in myofibrils is/are incorrect ?
(i) Each actin (thin filament) is made of $2F$ (filamentous) actins.
(ii) $F$-actin is the polymer of $G$ (globular) actin.
(iii) $2F$-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)

Answer 1

Answer: (C)

A complex troponin protein of three globular peptides (Troponin $T$ - Binding to tropomyosin as well as to the other two troponin components; Troponin $I$ - inhibiting the $F$-actin - myosin interaction, also binding to other components of troponin; Troponin $C$ - calcium binding polypeptide) is distributed at regular intervals on the tropomyosin. In the resting stage of muscle fibre, a sub - unit of troponin masks the active sites for myosin on the actin filaments.