Question

Identify the correct statements.

1. A-bands are dark and contain myosin.
2. I-bands are light and contain actin.
3. During the action, A band contracts.
4. The part between two Z-lines is sarcomere.
5. The central part of thin filament, not overlapped by thick filament is the H-zone.

• A. 1, 2 and 3 are correct
• B. 1, 3 and 5 are correct
• C. 1, 2 and 4 are correct
• D. 1 and 2 are correct
• E. 1, 2, 3 and 5 are correct

Answer 1

Answer: (C)

1, 2 and 4 are correct

• The ‘A’ and ‘I’ bands are arranged alternately throughout the length of the myofibrils. The light bands contain actin(thin filament) and are called I-band or Isotropic band, whereas the dark band called ‘A’ or Anisotropic band contains myosin(thick filament). In the centre of each ‘I’ band is an elastic fibre called ‘Z’ line which bisects it. The thin filaments are firmly attached to the ‘Z’ line. The portion of the myofibril between two successive ‘Z’ lines is considered as the functional unit of contraction and is called a sarcomere
  
• Utilizing the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge.
• This pulls the attached actin filaments towards the centre of ‘A’ band.
  
• The ‘Z’ line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e., contraction. Hence the ‘I’ bands get reduced, whereas the ‘A’ bands retain the length.
• The central part of thick filament, not overlapped by thick filament is the H-zone.
  
• So, the correct answer is '1,2 and 4