Question

Myofibrils show alternative dark and light band in
A. Cardiac muscles
B. Smooth muscles
C. Skeletal muscles
D. Both A and C

Answer 1

A myofibril is a rod like organelle of a muscle cell. The muscles contain tubular cells called myocytes and these cells contain many chains of myofibrils. The myocytes make the fibres in striated muscles. Myofibrils are made up of proteins such as actin, myosin and titin. In the myofibrils the proteins form overlapping layers that are laid out in functional units called sarcomeres. The sarcomeres give the muscles a striated appearance (dark and light bands).

Complete explanation:
Option A cardiac muscles: Cardiac muscle is a type of vertebrate muscle tissue. It is involuntary and striated muscle that makes up the main tissue of the wall of the heart. Each cardiac muscle cell contains myofibrils made up of actin and myosin. These are organized to form sarcomeres. Since sarcomere is present, cardiac muscles have light and dark band appearance.
Option A is correct.
Option B smooth muscles: Smooth muscle fibres do not have myofibrils arranged in strict patterns like in striated muscle. Due to this no distinct striation is observed in smooth muscle cells. Smooth muscle cell contraction squeezes the cell from every direction since the myofibrils are not arranged in a pattern as in other muscles. Hence, light and dark bands are absent in smooth muscles.
Option B is incorrect.
Option C skeletal muscles: Skeletal muscle is a type of muscle that initiates all of the body’s voluntary movement. Here sarcomere forms the basic unit of the muscle. They help in initiating large, sweeping movements by contracting in unison. These sarcomeres give skeletal muscles a striated appearance.
Option C is correct.
Option D both A and C: Since both option A and C are correct, option D is the right answer.
Option D is correct.

Therefore, Option D) Both A and C, is the correct answer.
Note:
A sarcomere is the smallest functional unit of striated muscle tissue. They form the repeating unit between two Z-lines. Myofibrils are made up of repeating units of sarcomeres. Under the microscope they appear as alternate dark and light bands. The Z-line forms the borders of the sarcomere and bound by actin. Bands appear when the sarcomere is relaxed.