13.12
Translation
Translation is the process of synthesizing proteins from the genetic information carried by messenger RNA (mRNA). Following transcription, it constitu…
Translation is the process by which a protein is made using a messenger RNA template.
The process begins when the small ribosomal subunit, along with a methionine tRNA and initiation factors, binds to the five-prime end of the mRNA.
The complex moves along the mRNA in the five-prime to three-prime direction until it reaches an AUG start codon, which codes for methionine, the first amino acid.
The methionine tRNA pairs with the AUG codon, and the large ribosomal subunit joins to form a complete ribosome.
Protein synthesis starts when a tRNA carrying an amino acid pairs with the next codon on the mRNA using its anticodon, which is complementary to the codon.
This positioning brings methionine close to the new amino acid, allowing a peptide bond to form between them.
The ribosome then moves to the next codon on the mRNA. This movement, called translocation, continues until the ribosome reaches a stop codon.
Stop codons do not match the anticodon of any tRNA.
During termination, release factors bind to the stop codon. This action releases the completed protein and separates the ribosome from the mRNA.
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Q1: How does translation begin at the ribosome?
Translation begins when the small ribosomal subunit, complexed with methionine tRNA and initiation factors, binds to the five prime end of mRNA. The complex scans the mRNA in the five prime to three prime direction until it encounters an AUG codon, which serves as the translation initiation site. The methionine tRNA then pairs with this AUG codon and recruits the large ribosomal subunit to form the complete translation machinery.
Q2: What role does tRNA play in protein synthesis?
Transfer RNA molecules deliver the correct amino acid to the ribosome based on mRNA codon sequences. At one end, aminoacyl-tRNA synthetases attach specific amino acids to the tRNA, while the anticodon at the other end ensures correct pairing with mRNA codons. Some tRNA molecules can bind multiple codons due to the wobble effect, which occurs because tRNA has lower binding specificity to the third nucleotide in the codon.
Q3: How are amino acids linked together during translation?
When a tRNA carrying its amino acid binds to an mRNA codon through its anticodon, the new amino acid is positioned close to the previously incorporated amino acid. A peptide bond then forms between the two amino acids, linking them together. This process repeats as the ribosome moves along the mRNA sequence during translocation, progressively building the polypeptide chain.
Q4: What happens when the ribosome reaches a stop codon?
Stop codons do not have corresponding tRNA molecules. Instead, proteins called release factors recognize and bind to stop codons, triggering the ribosome to release the newly synthesized protein. The ribosome then dissociates from the mRNA, completing the translation process and allowing the finished polypeptide to fold into its functional protein form.
Q5: Where does translation occur in eukaryotic cells?
In eukaryotes, translation occurs in the cytoplasm or on the rough endoplasmic reticulum after mRNA is transcribed in the nucleus and shuttled out. The mRNA is translated into a polypeptide chain, which may be further modified on the rough endoplasmic reticulum. In contrast, prokaryotes lack a nucleus, so translation occurs in the cytoplasm while transcription synthesis rna from dna is still happening.
Q6: How do mutations in translation components cause disease?
Mutations affecting translation machinery can cause inherited diseases. For example, hyperferritinemia results from mutations in the five prime untranslated region of mRNA, causing abnormally high ferritin translation and iron buildup. Diamond-Blackfan anemia stems from mutations in the RPS19 gene, which encodes a small ribosomal subunit component, disrupting normal protein synthesis.
Q7: Why is the codon sequence in mRNA important for translation?
The sequence of codons in mRNA determines the order of amino acids in the resulting polypeptide. Each codon corresponds to one of the 20 amino acids or signals a stop. This genetic code ensures that the correct amino acids are delivered in the correct order by tRNA molecules, ultimately determining the protein's structure and function.