Mutation Impact on Polypeptide Length
The question describes a situation where a gene mutation causes the resulting polypeptide to be shorter, specifically lacking the final three amino acids. This implies that the process of protein synthesis (translation) was stopped prematurely.
Understanding Mutation Types
Let's examine the options:
- Missense Mutation: Changes a codon to one that specifies a different amino acid. This alters the protein sequence but doesn't necessarily truncate it.
- Silent/Synonymous Mutation: Changes a codon to another codon that codes for the same amino acid. This does not alter the amino acid sequence.
- Nonsense Mutation: Introduces a premature stop codon (e.g., UAA, UAG, UGA) into the mRNA sequence. This causes translation to terminate early, resulting in a truncated (shorter) polypeptide chain.
Identifying the Correct Mutation
The observed result is a polypeptide that "lacks the last three amino acids". This indicates premature termination of translation.
A nonsense mutation directly causes premature termination by creating a stop codon where one shouldn't be. This is the most direct explanation for a polypeptide being shorter than expected due to the absence of amino acids from the end of the chain.
Therefore, the mutation described is a nonsense mutation.
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 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?
- 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
