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MCQEasyJEE 2025Proteins (Amino Acids, Structure, Denaturation)

JEE Chemistry 2025 Question with Solution

The alpha-helix and beta-pleated sheet structures of a protein are associated with its:

  • A

    quaternary structure

  • B

    primary structure

  • C

    secondary structure

  • D

    tertiary structure

Answer

Correct answer:C

Step-by-step solution

Standard Method

Given: The question asks which level of protein structure includes alpha-helix and beta-pleated sheet arrangements.

Find: The correct level of protein structure associated with these patterns.

Proteins have different levels of structural organization:

  • Primary structure: linear sequence of amino acids.
  • Secondary structure: regular local folding patterns such as alpha-helix and beta-pleated sheet.
  • Tertiary structure: overall three-dimensional shape of a single polypeptide chain.
  • Quaternary structure: association of more than one polypeptide chain.

The alpha-helix and beta-pleated sheet are part of the secondary structure of proteins and are stabilized by hydrogen bonding between backbone atoms.

Therefore, the correct option is C.

Structure-Level Comparison

Given: Different levels of protein structure are listed in the options.

Find: Which one specifically contains alpha-helix and beta-pleated sheet motifs.

  1. Primary structure refers only to the amino acid sequence.
  2. Secondary structure refers to local folding of the polypeptide backbone.
  3. Tertiary structure refers to the complete three-dimensional folding of one polypeptide chain.
  4. Quaternary structure refers to the arrangement of multiple polypeptide chains.

Since alpha-helix and beta-pleated sheet are the classic examples of local backbone folding, they belong to the secondary structure.

Hence, the correct answer is secondary structure, that is, Option C.

Common mistakes

  • Confusing primary structure with secondary structure. The primary structure is only the linear amino acid sequence, whereas alpha-helix and beta-pleated sheet arise from folding of the backbone into regular patterns.

  • Choosing tertiary structure because it sounds like a higher level of folding. Tertiary structure is the overall three-dimensional shape of a polypeptide, not the specific local motifs of alpha-helix and beta-sheet.

  • Selecting quaternary structure by assuming all complex folding belongs there. Quaternary structure applies only when multiple polypeptide chains associate, so it is not the correct classification here.

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