Each amino acid is added to a growing polypeptide chain through a process driven by complementary base pairing between tRNA and mRNA and facilitated by ribosomes.
The Step-by-Step Process
Here's a breakdown of how an amino acid is added to a polypeptide:
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mRNA Binding: The messenger RNA (mRNA) molecule, which contains the genetic code in the form of codons, binds to a ribosome. The ribosome provides the platform for translation.
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tRNA Activation: Transfer RNA (tRNA) molecules act as adaptors. Each tRNA is specific to a particular amino acid and has an anticodon sequence complementary to a codon on the mRNA. An enzyme called aminoacyl-tRNA synthetase attaches the correct amino acid to its corresponding tRNA molecule. This process is called "charging" or "aminoacylation" of the tRNA and requires ATP.
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Codon Recognition: The ribosome moves along the mRNA, exposing successive codons. A tRNA molecule carrying the appropriate amino acid enters the ribosome and its anticodon binds to the mRNA codon through complementary base pairing (A with U, and G with C). For example, if the mRNA codon is AUG, the tRNA with the anticodon UAC will bind to it.
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Peptide Bond Formation: Once the correct tRNA is in place, an enzymatic activity of the ribosome (peptidyl transferase activity) catalyzes the formation of a peptide bond between the amino acid attached to the incoming tRNA and the growing polypeptide chain. The polypeptide chain is then transferred from the tRNA in the P site to the tRNA in the A site.
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Translocation: The ribosome then translocates (moves) one codon down the mRNA. This shifts the tRNA that was in the A site (carrying the growing polypeptide chain) to the P site, and the tRNA that was previously in the P site moves to the E site, where it is released from the ribosome. The A site is now free for another tRNA to bind based on the next codon.
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Repetition: This cycle of codon recognition, peptide bond formation, and translocation repeats, adding one amino acid at a time to the growing polypeptide chain, until a stop codon is encountered on the mRNA.
Key Players
- mRNA: Provides the template with codons.
- tRNA: Carries the amino acid and recognizes the codon.
- Ribosomes: The site of protein synthesis; facilitates tRNA binding and peptide bond formation.
- Aminoacyl-tRNA synthetases: Enzymes that charge tRNAs with their correct amino acids.
Accuracy
The accuracy of protein synthesis is critical. The specificity of aminoacyl-tRNA synthetases ensures that the correct amino acid is attached to the correct tRNA. Furthermore, the ribosome has proofreading mechanisms to help minimize errors in codon-anticodon pairing.
Conclusion
In summary, the addition of an amino acid to a polypeptide chain is a precise process driven by complementary base pairing between tRNA anticodons and mRNA codons within the ribosome, ensuring accurate translation of the genetic code.
