Q.40 The first step in protein synthesis is the binding of the small ribosomal tmit to the ribosome binding site of the
mRNA that contains one of the following codons:
1. UUA
2. UAU
3. AAU
4. AUG
The first step in protein synthesis involves the small ribosomal subunit binding to the mRNA at the start codon AUG.
Option Analysis
This multiple-choice question tests knowledge of translation initiation in eukaryotes, where the small ribosomal subunit (40S) scans mRNA to recognize the start codon.
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UUA: Codes for leucine; it’s an internal elongation codon, not involved in initiation as it lacks the specific recognition by initiator tRNA.
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UAU: Codes for tyrosine; functions only during elongation, with no role in ribosomal binding or start signal recognition.
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AAU: Codes for asparagine; another standard elongation codon, irrelevant to the initiation complex formation.
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AUG: Correct answer—the universal start codon (methionine) where the small subunit binds with initiator tRNAiMet, forming the initiation complex.
The first step in protein synthesis starts when the small ribosomal subunit binds to the AUG codon on mRNA, launching translation of genetic code into proteins.
Initiation Process
In eukaryotes, the 40S subunit with initiation factors scans 5′ cap to find the AUG start codon. Initiator tRNA carrying methionine base-pairs with AUG, signaling protein chain start; prokaryotes use 30S with Shine-Dalgarno sequence.
Codon Roles Explained
UUA (leucine) and UAU (tyrosine) serve elongation, adding amino acids mid-chain. AAU (asparagine) follows suit. Only AUG triggers initiation universally, though rare prokaryotic exceptions like GUG exist.
Biological Importance
This precise AUG recognition ensures correct reading frame, preventing faulty proteins. Mutations here disrupt synthesis, linking to diseases; key for biotech like recombinant proteins.
