Locomotion and MovementMedium

Question

Fine structure of striated muscle fibres has been elucidated with the help of electron microscope. Study the following statements and choose the incorrect ones:

Options

A.The dark and light bands of myofibrils have been respectively, named as 'A' [Anisotropic under polarized light] and 'I' [Isotropic under polarized light] bands.
B.The 'A' bands contain about $120\text{Å}$ thick and $18\text{Å}$ long actin filaments.
C.Each 'I' band is divided into two equal halves by a thin, fibrous and transverse zig-zag partition called 'Z' band.
D.The major middle lighter region of 'A' band is called 'M zone

Solution

Sol. Key concept: The A band, I band, H zone and M line refer to different sarcomere regions.

a) Correct: A bands are dark and anisotropic under polarized light; I bands are lighter and isotropic. A sarcomere extends between successive Z discs.

b) Incorrect: thick filaments are myosin, while thin filaments are actin. The A band spans the thick myosin filaments, typically about 1.5–1.6 µm long; thin actin filaments are about 1 µm long and approximately 6–8 nm in diameter. The filament identity/dimensions in the printed statement are unsuitable.

c) Correct: a Z disc passes through the middle of an I band and anchors thin filaments, dividing that I band between adjacent sarcomeres.

d) Incorrect: the central lighter part of the A band is the H zone. The M line lies at its centre and helps organize thick filaments. Hence the required incorrect statements are b and d.

Correction / clarification: The earlier solution could be read as saying that the A band contains no actin. In fact, actin overlaps myosin in the outer parts of the A band; only the H zone lacks thin-filament overlap in the resting arrangement. [R6]

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