Using the standard genetic code on slide 10 (left table) of week 3 lecture on Proteomics, identify the amino acids X and Y in the provided supplementary figure (Question 1).

Question

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CAU/CAC encode histidine (His), while GUU/GUC/GUA/GUG encode valine (Val). Using the standard genetic code table (mRNA codon → amino acid) is the key to identifying X and Y.

1) Relevant knowledge points and concepts
- Translation reads mRNA in codons (triplets) that map to specific amino acids by the standard genetic code.
- Histidine is encoded by the codons CAU and CAC.
- Valine is encoded by the codons GUU, GUC, GUA and GUG.
- Leucine has distinct codons (UUA, UUG, CUU/CUC/CUA/CUG); aspartic acid is encoded by GAU and GAC.

2) Step-by-step reasoning
- Identify the mRNA codons at the X and Y positions from the supplementary figure (the authoritative answer indicates these codons correspond to His and Val).
- For X: if the triplet is CAU or CAC (first base C, second A, third U or C), lookup in the code table gives histidine.
- For Y: if the triplet is GUA (or any GUx where x = U/C/A/G), lookup gives valine.

3) Intermediate steps / mapping examples
- Example mapping for histidine: mRNA CAU → C (1st) + A (2nd) + U (3rd) → His. Similarly CAC → His.
- Example mapping for valine: mRNA GUA → G (1st) + U (2nd) + A (3rd) → Val. GUU/GUC/GUG similarly map to Val.

4) Why the correct answer is correct
- The standard genetic code directly assigns CAU/CAC to histidine and GUU/GUC/GUA/GUG to valine. If the figure’s X triplet is one of the CA* histidine codons and Y is one of the GU* valine codons, then X = His and Y = Val.

5) Why other options are incorrect
- Option A (His and Leu): Leucine codons are UUA/UUG/CU* and do not match GU* codons for Val.
- Option C (Leu and Asp): Aspartic acid is GAU/GAC, not GU* (Val) or CA* (His).
- Option D (Val and His): reverses the identities; contradicts the codon positions implied by the figure and the provided answer.
- Option E (Asp and Leu): neither Asp nor Leu corresponds to the CA* and GU* codons.

6) Conclusion
Because the codon-to-amino-acid mapping assigns CAU/CAC → His and GUU/GUC/GUA/GUG → Val, the correct identification is X: His (Histidine) and Y: Val (Valine), matching the provided answer.

Using the standard genetic code on slide 10 (left table) of week 3 lecture on Proteomics, identify the amino acids X and Y in the provided supplementary figure (Question 1).单项选择题

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