Biology · Locomotion and Movement · NEET
Actin is the THIN filament. NCERT says actin filaments are thinner than myosin filaments, so actin is the thin filament and myosin is the thick filament. Do not mix these up in the exam. Simple line to remember: actin = thin, myosin = thick.
The light I-band contains only actin (thin filaments). The dark A-band contains myosin (thick filaments), and its two ends also overlap with actin. So actin is present in BOTH the I-band and part of the A-band, but myosin is present ONLY in the A-band. This overlap is why the A-band stays dark.
Each thin (actin) filament is made of two F-actin (filamentous actin) strands wound helically around each other. Each F-actin is a polymer (long chain) of small round units called G-actin (globular actin). So the order is: G-actin joins to form F-actin, and two F-actins form one thin filament. Myosin has no G-actin or F-actin.
Actin (the thin filament) carries the two extra proteins: tropomyosin runs along the F-actin, and troponin sits at regular spots on the tropomyosin. In the resting muscle, a subunit of troponin masks (covers) the myosin binding sites on actin. Myosin does NOT carry troponin or tropomyosin. This is a very common NEET trap.
Meromyosin is the building block of the thick (myosin) filament. Many meromyosin units polymerise to form one myosin filament. Each meromyosin has a globular head with a short arm (the cross-bridge, which has ATPase activity and binds actin) plus a tail. Actin has no meromyosin — it is made of G-actin instead.
Myosin has the ATPase enzyme in its globular head. The myosin head breaks down ATP to release energy for the power stroke that pulls actin inward. Actin itself has no ATPase; it only provides the binding sites for the myosin head. So during contraction, myosin is the active motor and actin is the track it pulls.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Actin is the thin filament made of F-actin (from G-actin) and it forms the light I-band; myosin is the thick filament made of meromyosin and it forms the dark A-band. Myosin also has the head with ATPase that pulls actin during contraction.
Yes. NCERT clearly states actin filaments are thinner than myosin filaments, so myosin is the thick filament and actin is the thin filament.
Both actin and myosin are contractile proteins. Together they cause contraction: the myosin head binds actin, forms a cross-bridge, and pulls the actin filament toward the centre of the A-band, shortening the sarcomere.
The A-band contains the thick myosin filaments, which stay the same length during contraction. Since myosin length does not change, the A-band does not shorten; only the I-band and H-zone reduce as actin slides in. This is why the A-band stays dark and constant.
No. Neither actin nor myosin filaments shorten or change in number during contraction. They only slide past each other (sliding filament theory), which shortens the sarcomere without shortening the filaments themselves.
