Question

homework - protein synthesis
if the anticodon of a molecule of trna has the sequence gau, what was the original dna sequence?

Explanation:

Step1: Find the mRNA codon

The anticodon pairs with the mRNA codon. According to base - pairing rules (A - U, G - C), if the tRNA anticodon is GAU, then the mRNA codon is CUA.

Step2: Find the original DNA sequence

The mRNA is transcribed from DNA. The DNA strand that is transcribed (template strand) has a sequence complementary to the mRNA. So, if mRNA is CUA, the template DNA strand is GAT. But the original DNA has two strands. The non - template (coding) strand has the same sequence as the mRNA (with T instead of U). However, when we talk about the original DNA sequence (assuming the question refers to the gene sequence from which the mRNA is transcribed, and using the base - pairing rules for DNA (A - T, G - C)), if the mRNA is CUA, the template DNA is GAT and the non - template is CTA. But in terms of the double - stranded DNA, the original DNA sequence (as a double - strand) corresponding to the mRNA codon CUA is \(CTA\parallel GAT\) (where \(\parallel\) represents the double - strand). But if we consider the DNA strand that is complementary to the mRNA (the template strand for transcription), and since DNA is double - stranded, and we usually write the DNA sequence in the 5' - 3' direction. If the mRNA codon (5' - 3') is CUA, the template DNA (3' - 5') is GAT. The original DNA sequence (in the 5' - 3' direction of the non - template strand) is CTA. But if we consider the double - stranded form, the DNA sequence is \(CTA\parallel GAT\). However, in the context of protein synthesis problems, when asked for the DNA sequence corresponding to an anticodon, we first find the mRNA (anticodon \(GAU\) (5' - 3') pairs with mRNA \(CUA\) (5' - 3')), and then the DNA template strand (3' - 5') is \(GAT\) and the non - template (5' - 3') is \(CTA\). Since DNA is double - stranded, the original DNA sequence (as a double - strand) is \(CTA\parallel GAT\). But if we assume the question is asking for the DNA sequence (in the 5' - 3' of the non - template strand, which has the same sequence as the mRNA with T instead of U), the answer is \(CTA\).

Answer:

\(CTA\)