When both the P and A site are occupied and a third tRNA anticodon comes to the ribosome, what happens to the first anticodon?

The first anticodon leaves the ribosome through the E-site

Explanation:

Translation is the second stage of gene expression. It occurs in the ribosomes (organnelles of protein synthesis) where amino acid sequence is synthesized using a mRNA template. The tRNA (transfer RNA) is responsible for reading the mRNA codon using its ANTICODON, which is complementary to the mRNA codon.

The tRNA reads the mRNA codon and carries the amino acid that corresponds what it reads. tRNA has three binding sites on the ribosome; A-site, P-site and E-site. A tRNA molecule with the complementary anticodon binds to the codon on the P-site, and carries its corresponding amino acid. Another tRNA with complementary anticodon occupies the A-site, carrying the corresponding amino acid again.

Once both sites are occupied, the tRNA on the P-site transfers its amino acid to the one on the A-site to form a peptide bond. This causes the ribosome to shift the tRNA on the P-site, allowing the one on the A-site to be free. When this happens, the anticodon of the first tRNA on the P-site leaves the ribosome via the E-site in order to bind to another complementary mRNA codon and continue the translation process.

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