Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: In multicellular organisms, cells of different lineages have different gene expression profiles.
Reason [r]: Alternative splicing is the only mechanism to generate protein diversity.
Both [a] and [r] are false
Both [a] and [r] are true and [r] is the correct reason for [a]
Both [a] and [r] are true but [r] is not the correct reason for [a]
[a] is true but [r] is false
ANSWER
The correct answer is (D): Assertion [a] is true but Reason [r] is false.
Official answer: DSource: Official answer key, GATE 2023 BT (IIT Kanpur)
Assertion [a]: In multicellular organisms, all cells share the same genome, but differentiated lineages (for example neurons versus hepatocytes) express distinct subsets of genes. This differential gene expression — driven by lineage-specific transcription factors, chromatin accessibility and epigenetic marks — is the well-established molecular basis of cell differentiation. Hence [a] is TRUE.
Reason [r]: The statement claims alternative splicing is 'the only mechanism' to generate protein diversity. Alternative splicing (producing multiple mRNA/protein isoforms from one pre-mRNA) is indeed a real and important source of protein diversity, but the claim of exclusivity must be checked against other known mechanisms.
Multiple other well-established mechanisms independently generate protein diversity: alternative promoter usage / alternative transcription start sites, alternative polyadenylation, RNA editing, and post-translational modifications (phosphorylation, glycosylation, proteolytic cleavage, etc.).
Because at least one other genuine mechanism exists (in fact several), the exclusivity claim 'the only mechanism' in [r] is factually incorrect. Hence [r] is FALSE.
With [a] true and [r] false, the correct match is option (D): '[a] is true but [r] is false'.
Elimination check: (A) requires both false — contradicted since [a] is true. (B) and (C) both require [r] to be true — contradicted since [r] is false. Only (D) is consistent with the analysis.
KEY IDEA
This is an Assertion-Reason question requiring two independent judgments: (1) is [a] true or false on its own biological merits, and (2) is [r] true or false on its own merits — only if both are true do you then ask whether [r] correctly explains [a]. Differential/lineage-specific gene expression (via differential transcription factor activity, chromatin state, epigenetic marks, etc.) is the well-established basis of cellular differentiation in multicellular organisms, making [a] true. Protein diversity, however, arises from multiple independent mechanisms — alternative splicing, alternative promoter usage/transcription start sites, alternative polyadenylation, RNA editing, and post-translational modifications (phosphorylation, glycosylation, proteolytic cleavage) — so the absolute claim that alternative splicing is "the only" mechanism is false, regardless of alternative splicing's real importance.
WATCH OUT FOR
Treating alternative splicing as if it were "a" or "the main" source of protein diversity and conflating that with "the only" mechanism — since alternative splicing genuinely does create isoform diversity, students may misread the exaggerated exclusivity claim in [r] as true and pick (B) or (C) instead of recognizing that "only" makes the reason factually false.
From the GATE 2023 BT paper. Every question of this paper · Download the full paper · All 21 Gene expression questions, with notes
