Feathers and silk commonly contain . . . . . . .

What does the given figure represent the formation of?

The spiral configuration of $\alpha$-helix and $\beta$-pleated sheet of polypeptide chains are held together by . . . . . . bonds to form the secondary structure of protein.

Proteins consist of which of the following elements?

How many types of amino acids are there in living organisms?

What type of '$R$' group is present in Histidine?