Using the genetic code provided below, which polypeptide could be made from the following piece of an mRNA: 5’-...UUAUGCGCAAA...-3’ ? (first three amino acids are indicated in the followings)

Question

BlackTom AI · Accepted Answer

AUG codes for methionine, so the biologically relevant reading frame usually begins at an AUG start codon. Translation reads mRNA 5'→3' in consecutive triplets (codons), and the presence of AUG typically defines the beginning of the translated polypeptide.

1) Relevant knowledge points and concepts
- mRNA is read 5'→3' in non-overlapping triplets (codons).
- AUG is the start codon and codes for Met.
- Each codon maps to one amino acid (use the genetic code table).

2) Step-by-step reasoning
Take the given mRNA sequence and group into codons in the 5'→3' direction. The sequence is:
5' ... U U A U G C G C A A A ... 3'

There are three possible reading frames; list them:
- Frame 1 (start at first base): UUA | UGC | GCA | ...
- Frame 2 (start at second base): UAU | GCG | CAA | ...
- Frame 3 (start at third base): AUG | CGC | AAA | ...

3) Mapping codons to amino acids (intermediate steps)
- Frame 1: UUA → Leu; UGC → Cys; GCA → Ala → Leu-Cys-Ala.
- Frame 2: UAU → Tyr; GCG → Ala; CAA → Gln → Tyr-Ala-Gln.
- Frame 3: AUG → Met; CGC → Arg; AAA → Lys → Met-Arg-Lys.

4) Why the correct answer is correct
Because translation normally initiates at an AUG start codon, the functional polypeptide produced from this mRNA segment would begin at AUG (frame 3). Reading codons AUG → CGC → AAA gives Methionine, Arginine, Lysine (Met-Arg-Lys). This matches how ribosomes recognize the start codon to set the reading frame.

5) Why the other options are incorrect
- Leu-Cys-Ala and Tyr-Ala-Gln are the peptides produced by frames that begin at the first or second nucleotide, but these frames do not start at the canonical AUG start codon.
- Lys-Arg-Met would correspond to AAA-CGC-AUG, which is the reverse order of codons and is not the correct 5'→3' reading of this segment.

6) Conclusion
Using the start codon convention and reading 5'→3', the translated amino acids are Met-Arg-Lys. This confirms the provided correct answer: Met-Arg-Lys.

Using the genetic code provided below, which polypeptide could be made from the following piece of an mRNA: 5’-...UUAUGCGCAAA...-3’ ? (first three amino acids are indicated in the followings) 单项选择题

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