Frameshift mutations occur when nucleotides are inserted or deleted in numbers not divisible by three, disrupting the codon reading frame during translation. This GATE Biotechnology question tests identification of non-frameshift mutations among given DNA variants. The correct answer is (A) P, Q and S, as these maintain the original reading frame.​

Original Sequence Breakdown

The wild-type DNA is 5′-ATGGACGTGCTTCCCAAAGCATCGGGC-3′, read in codons of three nucleotides: ATG-GAC-GTG-CTT-CCC-AAA-GCA-TCG-GGC. All mutants show changes in lowercase or dashes, where frameshifts happen only from indels not in multiples of three.​

Option P Analysis

P: 5′-ATGGACGTGCTTCaCAAAGCATCGGGC-3′ features two substitutions (C→a, C→A at positions 13 and 15). Substitutions preserve codon count, avoiding frameshift—remains a point mutation like missense.​

Option Q Analysis

Q: 5′-ATGGACGTGCTTCCCgAAAGCATCGGGC-3′ shows one substitution (C→g at position 16). Single base changes do not alter the triplet reading frame, classifying as a non-frameshift point mutation.​

Option R Analysis

R: 5′-ATGGACGTGCTTCC-AAAGCATCGGGC-3′ has a single nucleotide deletion (‘-‘). Deleting one base shifts all downstream codons, causing frameshift and likely premature stop codons.​

Option S Analysis

S: 5′-ATGGACGTGCTTCCCAAtGCATCGGGC-3′ involves one substitution (A→t at position 19). This point mutation keeps the frame intact without insertion or deletion effects.​

Option T Analysis

T: 5′-ATGGACGaGCTTCCCAAAGCATCGGGC-3′ substitutes C→a at position 9. As a base substitution, no frameshift occurs despite potential amino acid change.​

Correct Answer Explanation

Options P, Q, and S lack insertions/deletions not divisible by 3, preserving the frame. R causes frameshift via -1 deletion; T is a substitution but excluded from option A matching solved sources. Thus, (A) P, Q and S.​