A DNA sequence, 5'-ATGGACGTGCTTCCCAAAGCATCGGGC-3', is mutated to obtain P. 5'-ATGGACGTGCTTCaCAAAGCATCGGGC-3′ Q. 5′-ATGGACGTGCTTCCCgAAAGCATCGGGC-3′ R. 5'-ATGGACGTGCTTCC-AAAGCATCGGGC-3′ S. 5'-ATGGACGTGCTTCCCAAtGCATCGGGC-3′ T. 5'-ATGGACGaGCTTCCCAAAGCATCGGGC-3′ [Point mutations are shown in the lower case or ‘–' within the sequences] Which of the above mutant sequences DO NOT have frame-shift?
Understanding DNA Mutations: Frame-Shift vs. Point
Mutations are changes in the DNA sequence. We need to identify which sequences exhibit frame-shift mutations. A frame-shift mutation occurs when insertions or deletions (indels) that are not a multiple of three bases are added to the sequence. This shifts the reading frame for translation. Point mutations (substitutions) change a single base but do not alter the reading frame.
Analyzing Mutant Sequences
Original DNA Sequence: $5'-ATGGACGTGCTTCCCAAAGCATCGGGC-3'$
- Sequence P: $5'-ATGGACGTGCTTCaCAAAGCATCGGGC-3'$
- Mutation: Substitution of 'c' with 'a' at position 13.
- Type: Point mutation (Substitution).
- Frame-shift: No.
- Sequence Q: $5′-ATGGACGTGCTTC CCgAAAGCATCGGGC-3′$
- Mutation: Insertion of 'g' between position 16 ('C') and 17 ('A').
- Type: Insertion (1 bp).
- Frame-shift: Yes, because the insertion is not a multiple of 3.
- Sequence R: $5'-ATGGACGTGCTTCC-AAAGCATCGGGC-3′$
- Mutation: Deletion of 'C' at position 15.
- Type: Deletion (1 bp).
- Frame-shift: Yes, because the deletion is not a multiple of 3.
- Sequence S: $5'-ATGGACGTGCTTCCCAAaTGCATCGGGC-3′$
- Mutation: Substitution of 'g' with 'a' at position 19.
- Type: Point mutation (Substitution).
- Frame-shift: No.
- Sequence T: $5'-ATGGACGaGCTTCCCAAAGCATCGGGC-3′$
- Mutation: Substitution of 'C' with 'G' at position 7.
- Type: Point mutation (Substitution).
- Frame-shift: No.
Identifying Non-Frame-Shift Mutations
Based on the analysis:
- Sequences P, S, and T contain point mutations (substitutions). These do not change the number of bases by a multiple of three, hence they do not cause a frame-shift.
- Sequences Q and R contain single base insertions/deletions, respectively. These disrupt the reading frame, causing frame-shift mutations.
Therefore, the mutant sequences that DO NOT have frame-shift mutations are P, S, and T.
Important Questions from Mutations and Mutagenesis
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
Assertion [a]: Chromosome mutations can change the structure of chromosomes.
Reason [r]: All chromosome mutations arise due to nondisjunction of chromosomes during mitosis or meiosis.- A mutation in a gene that codes for a polypeptide results in a variant polypeptide that lacks the last three amino acids. What type of mutation is this?
- A truncated polypeptide is synthesized due to a nonsense mutation. Where would you introduce another mutation to obtain a full-length polypeptide?
Determine the correctness or otherwise of the following Assertion (a) and the Reason (r).
Assertion : N-methyl-N'-nitro-N-nitrosoguanidine (NTG) is an effective chemical mutagen.
Reason: Mutations induced by NTG mainly are the GC $\rightarrow$ AT transitions.
- Lysine is being produced in a lab-scale reactor by a threonine auxotroph. After 2 weeks of operation it was observed that the concentration of lysine in the reactor was gradually decreasing. Microbiological assays of reactor samples showed absence of contamination and recorded data showed no change in the operating conditions. The most probable reason for decrease in lysine concentration may be attributed to
