Question

which type of dna mutation results in a change in the reading frame of an mrna?
deletion of a single nucleotide
substitution of one codon for another
substitution of one nucleotide with another
duplication of a single codon

Explanation:

• Deletion of a single nucleotide: Since mRNA is read in codons (groups of 3 nucleotides), deleting one nucleotide shifts the reading frame. For example, if the original sequence is \(ABCDEF\) (read as \(ABC\), \(DEF\)), deleting \(B\) makes it \(ACDEF\) (read as \(ACD\), \(EF\) - frame - shifted).
• Substitution of one codon for another: A codon is a group of 3 nucleotides. Substituting one codon (e.g., replacing \(AAA\) with \(GGG\)) does not change the reading frame. The mRNA is still read in groups of 3.
• Substitution of one nucleotide with another: This is a point mutation. For example, in the sequence \(AATG\), changing \(A\) to \(C\) (\(CATG\)) - the reading frame (groups of 3) is still intact (\(CAT\), \(G\) if we assume more nucleotides after).
• Duplication of a single codon: Duplicating a codon (e.g., \(AAA\) becomes \(AAAAAA\)) just adds more codons in - frame. The reading frame (groups of 3) is maintained.

Answer:

deletion of a single nucleotide