Translation Flashcards

16S rRNA and Shine-Dalgarno Sequence

• 16S rRNA helps position the AUG start codon in the P-site of the ribosome by interacting with the ribosome binding site (RBS).
• The Shine-Dalgarno sequence (5′-GGAGG-3′) is crucial for translation initiation in prokaryotes, typically found 3 to 9 nucleotides upstream of the start codon.
• Recognition of the Shine-Dalgarno sequence by the small ribosomal subunit is facilitated by complementary sequences in 16S RNA.

Variability of the Shine-Dalgarno Sequence

• Variations in the Shine-Dalgarno sequence can affect translation efficiency.
• Sequences closely matching the ideal Shine-Dalgarno sequence display higher activity, while deviations generally reduce activity.
• Proper distance from the start codon also influences translation efficiency.

Eukaryotic Translation Initiation

• The 5′ cap on mRNA is recognized by eIFs, recruiting the small ribosomal subunit and assisting in scanning for the start codon.
• mRNA lacking a cap is vulnerable to degradation by exonucleases, underscoring the protective role of the cap.

Charging tRNA by Aminoacyl tRNA Synthetases

• Aminoacyl tRNA synthetases charge tRNAs in two steps: adenylation (binding amino acid to AMP) and tRNA charging (transferring the amino acid to tRNA).
• Some organisms have fewer than 20 synthetases yet can charge all amino acids by modifying others.

Role of EF-Tu in Translation

• Charged tRNA is bound to EF-Tu-GTP, which delivers it to the ribosome's A site.
• Upon correct codon-anticodon pairing, EF-Tu hydrolyzes GTP, releasing tRNA into the ribosome.

Stop Codon Recognition in Eukaryotes

• Stop codons are recognized by class I release factors RF1 and RF2 using a peptide anticodon that specifically binds to the codon.
• This recognition leads to the termination of protein synthesis by hydrolyzing the bond between the polypeptide and tRNA.

Termination of Translation

• Upon stop codon recognition, release factors facilitate polypeptide release and ribosome disassembly through a series of interactions, involving RF3 and RRF.
• EF-G helps displace deacylated tRNAs from the ribosome.

Translation Accuracy

• The error rate for translation (10^-3 to 10^-4) is higher than that for transcription (10^-4 to 10^-5) and DNA replication (as low as 10^-10).
• Higher error rates in translation are acceptable since they only affect individual proteins, unlike transcription which influences multiple proteins from a single mRNA.

Translation Elongation Events

• Key events for amino acid addition during elongation include the insertion of the correct aminoacyl-tRNA into the ribosome’s A site, peptidyl bond formation, and translocation of peptidyl-tRNA to the P site.

Differences in Translation between Prokaryotes and Eukaryotes

• While the processes are similar, distinct differences exist in translation elongation between prokaryotes and eukaryotes.

Ribosome Structure and Function

• Prokaryotic ribosome: 70S total (30S + 50S), Eukaryotic ribosome: 80S total (40S + 60S).
• The ribosome has three tRNA binding sites: A (aminoacyl), P (peptidyl), and E (exit).

Understanding tRNA

• Charged tRNA is one that has an amino acid attached, crucial for protein synthesis.
• tRNA structure features a 3' terminus with a CCA sequence and contains an anticodon that recognizes mRNA codons.

Peptidyl Transferase Reaction

• Catalyzed by the 23S rRNA of the large subunit, the reaction facilitates peptidyl bond formation.

Mechanisms Ensuring Accurate tRNA Insertion

• Hydrogen bonds between adenine residues in 16S rRNA and the anticodon-codon pair enhance specificity.
• Correct pairing allows for EF-Tu interaction with the ribosome, leading to subsequent peptide bond formation through tRNA accommodation.