Questions: Translation: RNA to Protein

Question 1

During translation elongation, what catalyzes the formation of the peptide bond between the growing polypeptide and the incoming amino acid?

• A. A protein enzyme called peptidyl transferase
• B. The rRNA component of the large ribosomal subunit
• C. A charged tRNA in the A site
• D. Translation elongation factor EF-Tu / EF-1α

Answer: (B)

Peptide bond formation is catalyzed by the 23S rRNA (in prokaryotes) or 28S rRNA (in eukaryotes) of the large ribosomal subunit — making the ribosome a ribozyme. This was a landmark discovery because it showed that RNA, not protein, performs this central catalytic function. The protein components of the ribosome play structural and regulatory roles.

Question 2

The peptide bond in a growing polypeptide is formed by a protein enzyme called peptidyl transferase.

• T. True
• F. False

Answer: False

Peptidyl transferase activity resides in the rRNA of the large ribosomal subunit, not a protein enzyme — the ribosome is a ribozyme. This fact also supports the RNA World hypothesis, since it implies that protein synthesis could have originated in an RNA-based system before protein enzymes evolved.

Question 3

What sets the reading frame during translation, and why does a single-nucleotide insertion in the middle of a coding sequence typically destroy protein function?

Model answer: The reading frame is established by the AUG start codon. A single nucleotide insertion shifts the triplet grouping of every codon downstream, producing a completely different amino acid sequence from that point forward — almost always including a premature stop codon.

The ribosome reads mRNA in non-overlapping three-nucleotide codons starting from the AUG. Insert one nucleotide and all downstream codons are misread: the mutation is not a single amino acid change but a garbled sequence for the entire remainder of the protein, almost always rendering it nonfunctional.