Question

Given below are two statements, one is labelled as Assertion (A) and other labelled as Reason (R).
Assertion (A): In eukaryotic cells, the newly synthesized polypeptide chain is often modified after translation.
Reason (R): Post-translational modifications such as phosphorylation, glycosylation, and cleavage may occur to the polypeptide chain to attain its functional form.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A).
• B. Both (A) and (R) are correct but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Remember that "translation" only produces a linear chain of amino acids.
Post-translational modifications (PTMs) like phosphorylation and glycosylation are required to convert this raw chain into a functional, three-dimensional protein.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Translation is the process where ribosomes synthesize polypeptide chains based on mRNA sequences.
However, most nascent polypeptide chains are not immediately functional after leaving the ribosome and must undergo post-translational modifications (PTMs) to fold correctly and gain biological activity.

Step 2: Detailed Explanation:
Let us analyze the biological validity and connection between the two statements:
- Assertion (A): In eukaryotic cells, newly synthesized proteins must go through modification steps in organelles like the endoplasmic reticulum and Golgi apparatus before they can carry out their cellular functions.
Thus, Assertion (A) is correct.
- Reason (R): Post-translational modifications are chemical alterations that expand the functional diversity of the proteome.
Common modifications include:
1. Phosphorylation: The addition of a phosphate group, which acts as a molecular switch to activate or deactivate enzymes.
2. Glycosylation: The attachment of carbohydrate chains, which is essential for protein stability, cell-to-cell recognition, and proper folding.
3. Proteolytic cleavage: Removing specific peptide segments (such as signal peptides or pro-peptides) to convert an inactive precursor (proprotein) into an active enzyme (e.g., proinsulin to insulin).
These modifications are essential for the polypeptide chain to assume its active, functional form.
Thus, Reason (R) is correct.
Since these specific biochemical modifications are the primary post-translational changes referred to in the Assertion, the Reason directly explains why and how newly synthesized polypeptide chains are modified.
Therefore, both statements are correct, and (R) is the correct explanation of (A).

Step 3: Final Answer:
Both (A) and (R) are correct and (R) is the correct explanation of (A).