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Place where peptidyl tRNAs reside, within the ribosome, immediately following their formation during translation.
Starting out an an aminoacyl tRNAs in the A site, a chain of one or ore additional amino acids is added, forming the peptidyl tRNA. In the course of this process the growing polypeptide and associated tRNAs progress from the A site to the P site. The tRNA which had been occupying the P site (formerly peptidyl tRNA) then proceeds to the E site before leaving the ribosome altogether.
Note that these various movements occur in association with a ratcheting of the mRNA through the ribosome.
The "P" in P site of course stands for Peptide, Polypeptide and/or Peptidyl. One could also loosely consider the "P" as standing for Protein.
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