Question

When a codon in an mRNA with the sequence \(5^{\prime}-\mathrm{UAA}-3^{\prime}\) enters the A site of a ribosome, it is not recognized by a tRNA with a complementary anticodon. Why not? What recognizes it instead?

Short answer

Answer: The 5'-UAA-3' codon is a stop codon, which signals the end of protein translation. It is not recognized by any tRNA molecule since no tRNA carries an amino acid corresponding to the stop codon. Instead, stop codons are recognized by release factors, which are protein factors that bind to the ribosome and lead to the termination of polypeptide synthesis and the release of the newly formed protein.

Step-by-step solution

The given mRNA sequence is \(5^{\prime}-\mathrm{UAA}-3^{\prime}\). This specific codon is one of three stop codons which signals the end of protein translation. Hence, it is not recognized by any tRNA molecule.

#Step 2: Understanding the tRNA recognition mechanism#

In the process of protein translation, tRNA molecules recognize mRNA codons through complementary base pairing between the codon in mRNA and the anticodon in the tRNA. The tRNAs carry amino acids to the ribosome, where they incorporate the growing polypeptide chain. However, in the case of a stop codon, no tRNA molecule carries an amino acid that the stop codon corresponds to. Therefore, no tRNA can recognize the stop codon through base pairing.

#Step 3: Identifying the alternative recognition molecule#

Since stop codons cannot be recognized by tRNA molecules, they are instead recognized by protein factors called release factors. In this case, the \(5^{\prime}-\mathrm{UAA}-3^{\prime}\) stop codon is recognized by a release factor, which binds to the ribosome and leads to the termination of polypeptide synthesis and the release of the newly formed protein.

Key concepts

Understanding mRNA Translation

The process of mRNA translation is a key part of gene expression where the genetic code carried by mRNA is translated into a sequence of amino acids, forming a protein. This process occurs within the ribosome, a cellular structure that acts as the site of protein synthesis. In translation, tRNA molecules serve as adaptors matching specific three-nucleotide codons in the mRNA with the corresponding amino acids.

During initiation, the ribosome assembles around the target mRNA. The mRNA sequence is read in triplets known as codons, and each codon specifies a particular amino acid. This sequence is read from the 5' end to the 3' end of the mRNA. Translation continues with elongation, where successive amino acids are added to the growing polypeptide chain as each codon passes through the ribosome. The process is highly regulated and precise, ensuring that the proteins are synthesized correctly based on the information encoded within the mRNA.

Deciphering tRNA Anticodon Recognition

Transfer RNA (tRNA) is responsible for interpreting the genetic code found on the mRNA through the mechanism of anticodon recognition. Each tRNA molecule has an anticodon, a triplet of nucleotides that is complementary to a corresponding mRNA codon. The tRNA anticodon binds to the mRNA codon via base pairing, which follows the rules of complementary base pairing: adenine (A) pairs with uracil (U) in RNA, and cytosine (C) pairs with guanine (G).

During the translation process, as the mRNA is threaded through the ribosome, tRNA molecules matched with the appropriate amino acid align their anticodons with the codons of the mRNA. This precise pairing is crucial for the correct addition of amino acids to the polypeptide chain. If the anticodon does not precisely match the codon, the tRNA is rejected by the ribosome, preventing mistakes in protein synthesis.

Release Factors and Protein Termination

Once the ribosome encounters one of the three stop codons—UAA, UAG, or UGA—recognized by release factors rather than tRNA, the process of translation termination is triggered. Release factors are proteins that recognize stop codons and promote the disassembly of the translation machinery. In the absence of an amino acid to add to the polypeptide chain, the release factor binds to the A site of the ribosome where the stop codon is located.

Upon the binding of the release factor, a series of reactions occur that result in the release of the newly synthesized protein from the ribosome. The release factors also facilitate the release of the mRNA and the disassociation of the ribosome into its subunits, effectively ending the translation process. These factors play a critical role in ensuring proteins are of the correct length and that the ribosome is available for another round of translation.