Q.43 Given below are two statements : One is labelled as Assertion (A) and the other is
labelled as Reason (R).
Assertion (A) :Specific amino acids may be replaced individually by site directed
mutagenesis. This powerful approach to studying protein structure
and function changes the amino acid sequence of a protein by
altering the DNA sequence of cloned gene.
Reasons (R) :When suitably located restriction site A are not present in the
sequence to be altered, an approach called oligonucleotide-
directed mutagenesis can create a specific DNA sequence change.
In the light of the above statements, choose the most appropriate answer from the
options given below :
(1)Both (A) and (R) are correct and (R) is the correct explanation of (A)
(2)Both (A) and (R) are correct but (R) is NOT the correct explanation of (A)
(3)(A) is correct but (R) is not correct
(4)(A) is not correct but (R) is correct
Both (A) and (R) are correct and (R) is the correct explanation of (A), making option (1) the right answer.
Question Breakdown
This assertion-reason tests understanding of site-directed mutagenesis (SDM) techniques in molecular biology. Assertion (A) describes the general principle of SDM for protein engineering, while Reason (R) explains a specific method (oligonucleotide-directed mutagenesis) used when traditional restriction enzyme-based approaches fail.
Option Analysis
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(1) Both correct, (R) explains (A): Correct; (R) details how oligonucleotide primers create precise DNA changes when restriction sites are absent, directly enabling the amino acid replacements described in (A).
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(2) Both correct but (R) doesn’t explain (A): Incorrect; (R) specifically addresses the technical challenge in (A)’s method—lack of suitable restriction sites—making it explanatory.
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(3) (A) correct, (R) incorrect: Incorrect; both statements accurately describe established SDM protocols.
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(4) (A) incorrect, (R) correct: Incorrect; (A) correctly states SDM’s purpose and mechanism for protein structure-function studies.
Site directed mutagenesis assertion reason questions test molecular biology technique understanding critical for GATE Life Sciences. SDM replaces specific amino acids by altering cloned gene DNA, using oligonucleotide-directed mutagenesis when restriction sites are absent.
SDM Techniques
Cassette Mutagenesis: Requires restriction sites flanking target → replace fragment.
Oligonucleotide-Directed: PCR primers with mismatch create point mutations—no restriction sites needed.
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Process: M13 phage/single-stranded template + mutagenic primer → extend → ligate → transform.
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Modern: QuikChange PCR (Pfu polymerase, DpnI digests parental DNA).
Assertion-Reason Logic
| Statement | Content | Relation |
|---|---|---|
| A | Amino acid replacement via DNA change | General principle |
| R | Oligo mutagenesis when no restriction sites | Specific method enabling A |
Exam Strategy
GATE ARQs demand logical connection analysis. Here: (R) provides technical solution to (A)’s implied challenge. Trap: Option (2) ignores method specificity. Master: SDM = protein structure-function tool via precise codon changes.
