Question

Skeletal muscles appear striated due to presence of two characteristic proteins in alternating dark and light bands. Which of the following is a correct match of the protein with its light refractive property and colour?
A) Protein: Myosin, Colour: Light, Property: Anisotropic
B) Protein: Actin, Colour: Dark, Property: Anisotropic
C) Protein: Myosin, Colour: Light, Property: Isotropic
D) Protein: Actin, Colour: Light, Property: Isotropic

Answer 1

Hint:The Myosin protein is a superfamily of motor proteins which is best known for its roles in muscle contraction and in a wide range of other motility processes in eukaryotes. Whereas, Actin protein is an important contributor to the contractile property of muscle and other cells.

Complete answer
There are light and dark bands in muscles. The isotropic band (I) contains only actin-containing thin filaments. While, the myosin containing bands are called anisotropic bands (A bands). Together the I-bands and A-bands contribute to the striated appearance of skeletal muscle. When Isotropic bands pass through the (I) bands, they indicate the behaviour of polarized light.

1. In Myosin, the thick filaments are set while it is shown as Dark. As we have mentioned above, it is found in A-bands, which is called anisotropic. So, the first option is wrong. Because, it should not be light.
2. For Actin, there we can’t find any dark colour, because all thin filaments are found in I-bands. This is called Isotropic. Thus, the second option is wrong. The colour of filament and band pattern are wrong in this option.
3. In the third option, the Myosin is shown in I-bands but it should be in Dark mode as the thick filaments are found in Myosin. So, the third option is also wrong.
4. The Actin with light thin filaments are found in between the Anisotropic bands, which is called Isotropic Bands. So, the fourth option is correct according to the match as given. Actin with thin filaments shows the light colour and the ‘Isotropic’ refractive property will be appropriate for Actin. So, the fourth option is correct here.

Hence the correct answer is option ‘D’.

Note:We have seen that the Actin and Myosin bands of protein repeat themselves. These bands are present along with Myofibrils. Dark A bands and light I bands repeat along myofibrils and the alignment of Myofibrils in the cell thus causes the entire cell to appear as striated or banded.