When a codon in an mRNA with the sequence 5'-UAA-3' enters the A site of a ribosome, it is not recognized by a tRNA with a complementary anticodon. Why not? What recognizes it instead?

Verified step by step guidance

1. Understand that the sequence 5'-UAA-3' is a stop codon in mRNA.
2. Recall that stop codons are not recognized by tRNA molecules because they do not code for any amino acids.
3. Recognize that instead of tRNA, stop codons are recognized by release factors.
4. Release factors bind to the stop codon in the A site of the ribosome.
5. The binding of release factors leads to the termination of translation and release of the polypeptide chain.

Key Concepts

Codons and Anticodons

Codons are sequences of three nucleotides in mRNA that specify an amino acid or signal termination during protein synthesis. Each codon is matched by a complementary anticodon found on tRNA molecules, which carry specific amino acids. In the case of the UAA codon, it is a stop codon, meaning it does not have a corresponding tRNA anticodon, which typically pairs with codons that code for amino acids.

Stop Codons

Stop codons are specific codons in mRNA that signal the termination of protein synthesis. The three stop codons—UAA, UAG, and UGA—do not code for any amino acids and are recognized by release factors instead of tRNA. When a ribosome encounters a stop codon in the A site, it triggers the release of the newly synthesized polypeptide chain from the ribosome.

Release Factors

Release factors are proteins that recognize stop codons during translation. When a ribosome reaches a stop codon, such as UAA, a release factor binds to the A site, prompting the ribosome to release the completed polypeptide chain. This process is crucial for terminating protein synthesis and ensuring that proteins are properly released and folded for their functional roles.