The RNA sequence below depicts the part of a 330 nucleotides long mRNA and it encodes the C-terminal portion of a protein. 5'-... ... ... AAC ACC ACG ACC CAU GUG GCG AGA CGG UAG- 3' A mutation was identified in this RNA denoted as 322A$\to$U. This nucleotide change is represented by which of the following class(es) of mutation?
Mutation Identification: 322A→U
The question identifies a mutation at nucleotide position 322 in a 330-nucleotide mRNA sequence, changing Adenine (A) to Uracil (U). We need to determine the type(s) of mutation this represents.
Determining the Codon Change
- Identify Codon Location: The mRNA sequence provided is 5'-... AAC ACC ACG ACC CAU GUG GCG AGA CGG UAG -3'. The total length is 330 nucleotides. The sequence ends with the stop codon 'UAG' (nucleotides 328-330). Counting backwards, the codon 'AGA' occupies positions 322-324.
- Analyze the Mutation: The mutation is specified as $322A→U$. This means the first nucleotide of the 'AGA' codon (position 322) changes from 'A' to 'U'.
- Original and Mutated Codons:
- Original Codon (positions 322-324): $AGA$
- Mutated Codon (positions 322-324): $UGA$
Classifying the Mutation Type
- Classification based on Effect (Nonsense):
- The original codon, $AGA$, codes for the amino acid Arginine (Arg).
- The mutated codon, $UGA$, is a stop codon, signaling the termination of translation.
- A mutation that changes an amino acid-coding codon into a stop codon is defined as a nonsense mutation.
- Classification based on Chemical Change (Transversion):
- The specific nucleotide change is Adenine ($A$) to Uracil ($U$).
- Adenine ($A$) is a purine base.
- Uracil ($U$) is a pyrimidine base.
- A mutation swapping a purine for a pyrimidine (or vice versa) is called a transversion mutation. (A purine-to-purine or pyrimidine-to-pyrimidine change is a transition).
Therefore, the mutation $322A→U$ is classified as both a nonsense mutation and a transversion 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 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.
