During translation, what happens when the ribosome shifts three nucleotides along the mRNA?

• A. The tRNA in the P site moves into the E site.
• B. The tRNA in the A site moves to the P site.
• C. The ribosome moves along without changing tRNA positions.
• D. The tRNA in the E site moves into the P site.

Answer: (A)

During translation, after a peptide bond has formed, the ribosome shifts along the mRNA by a full codon (three nucleotides). This translocation rearranges which tRNA sits in each site: the tRNA that was in the P site moves into the E site and exits, while the tRNA in the A site moves into the P site, leaving the A site open for the next aminoacyl-tRNA. The statement that the tRNA in the P site moves into the E site describes this essential repositioning that occurs as the ribosome advances along the mRNA, making it the best description of what happens during translocation. The other options describe related steps or incorrect movements, but they don’t capture the immediate rearrangement that accompanies the ribosome’s advance.