Question Detail

Original DNA triplet is ATC. Thymine (T) substitutes for Cytosine (C) in this triplet, so the new DNA triplet is ATT (since the T replaces the C, the sequence becomes A T T). Now, transcription produces mRNA complementary to this DNA template strand. DNA: A T T. mRNA (complementary, with U instead of T): U A A? Wait, A pairs with U, T pairs with A, T pairs with A. So mRNA would be UAA. That's not an option. Perhaps the question means the base sequence on the DNA is ATC, and the mutation substitutes T for C somewhere? Or maybe they mean the original DNA triplet is ATC, and the mutation changes it to ATT. Let's test the options. Option C says UAG on mRNA and AUC on tRNA. For mRNA to be UAG, the DNA template strand must have been ATC (because A->U, T->A, C->G? That gives UAG? No, that's wrong. ATC on DNA template would give UAG on mRNA? Yes: DNA template A gives RNA U, DNA T gives RNA A, DNA C gives RNA G. So U A G. Perfect! So the original DNA triplet was ATC, giving mRNA UAG. A mutation that substitutes T for C would change the DNA triplet from ATC to ATT? That would give mRNA UAA. But the question says 'substitution of thymine for cytosine in the base sequence ATC'. That means in the triplet A-T-C, the C is replaced with T, making it A-T-T. That yields mRNA U-A-A. This isn't an option. Option C has UAG and AUC. UAG mRNA would come from DNA ATC. AUC tRNA would come from DNA TAG. This doesn't fit. Option D: UGG mRNA (DNA template ACC) and ACC tRNA (DNA template TGG). This doesn't fit either. There's a common trick: maybe the original DNA is ATC, and the mutation substitutes T for C in the complementary strand, or maybe they mean the coding strand. This is confusing. Given the options, C is the most likely answer, as UAG is a well-known stop codon. Perhaps the mutation in the DNA (ATC) is on the coding strand. If the coding strand is ATC, the template strand is TAG, giving mRNA AUC. If a T substitutes for C in the coding strand, it becomes ATT, making the template strand TAA, giving mRNA AUU. Not matching. I'm going to go with the pattern of many similar questions where the answer is UAG on mRNA and AUC on tRNA, which corresponds to a DNA triplet of ATC on the template strand.