The alpha-helix and beta-sheet are found at which level of protein organization?
A. Primary structure
B. Secondary structure
C. Tertiary structure
D. Quaternary structure

Hint: The proteins are composed of carbons, hydrogen, oxygen, nitrogen, and sulphur. The protein molecules are very large and highly complex and have very high molecular weights.
Primary has four structural levels: primary, secondary, tertiary, and quaternary structure.

Complete answer: A polypeptide chain is often coiled into a regular, spiral, called the alpha helix, to have a 3-dimensional form.
1. The amino acids are so placed that their side chains extend outward from the spiral.
2. The helical structure is maintained by a series of regularly spaced intramolecular hydrogen bonds formed between the carboxyl (-CO) and amino (-NH) group of amino acid units in the successive turns of the spiral. Tha alpha coil may at places be much less regular, forming random coils.
3. Two or more polypeptide chains may join together by intermolecular hydrogen bonds and may bend into parallel folds to form a β- pleated sheer.
4. The alpha helix, random coil, and β pleated conformations are termed secondary structure of proteins.

So, the correct answer is “Option B”.

Additional Information: Primary structure: The linear sequence of amino acid units in a polypeptide chain is called primary structure.
Tertiary structure: The helical polypeptide molecule may fold on itself and assume a complex but specific form-spherical, rod-like, or any form in between these.
These geometrical shapes are known as tertiary structure
Quaternary structure: Certain proteins consist of a bundle of two or more polypeptide chains, each having appropriate primary, secondary, and tertiary structures.

Note: Keratin of hair and myosin of muscles have helical structure, and fibroin, the protein in silk fibers produced by insects and spiders, has a pleated structure.
The protein lysozyme occurs in both helical and pleated forms.
The position of certain amino acids is crucial for the form and function of the proteins.
A change of just one amino acid alters the normal haemoglobin to the haemoglobin of sickle cell aneamia