Translation and Post-Translational Modification (1B) - Bio & Biochem

What are the three stop codons in the standard genetic code?

UAA, UAG, UGA. These codons do not code for amino acids but signal the end of translation by binding release factors.
What is the Shine-Dalgarno sequence used for in prokaryotic translation initiation?

Aligns mRNA on the $30S$ subunit via $16S$ rRNA pairing. The Shine-Dalgarno sequence base-pairs with 16S rRNA to position the start codon correctly in the P site.
What is the role of chaperone proteins in post-translational processing?

Assist folding and prevent aggregation without changing sequence. Chaperones bind nascent polypeptides to promote proper folding and inhibit misfolding or aggregation without altering the amino acid sequence.
What is the role of the poly(A) tail in eukaryotic translation efficiency?

Enhances translation and stabilizes mRNA via PABP interactions. The poly(A) tail binds poly(A)-binding proteins, which interact with initiation factors to circularize mRNA and promote reinitiation.
Which cellular compartment is the primary site of translation for secreted proteins?

Rough endoplasmic reticulum (RER)-bound ribosomes. Secreted proteins have signal sequences that direct ribosomes to the RER for co-translational translocation into the ER lumen.
What enzyme activity in the large ribosomal subunit forms peptide bonds?

Peptidyl transferase (rRNA ribozyme activity). This ribosomal RNA catalyzes the formation of peptide bonds between amino acids during elongation.
What is the immediate energetic cost to add one amino acid during elongation (high-energy bonds)?

$2$ high-energy bonds (ATP $\rightarrow$ AMP counts as $2$ ) plus $2$ GTP. Charging tRNA hydrolyzes ATP to AMP (two bonds), and elongation requires two GTP for tRNA delivery and translocation.
Identify the eukaryotic mRNA feature required for cap-dependent ribosome recruitment.

$5'$ mRNA cap (m $^7$ G cap). The 5' cap recruits initiation factors and the 40S subunit, enabling scanning for the start codon.
Which post-translational modification commonly regulates enzyme activity by adding a phosphate group?

Phosphorylation of Ser/Thr/Tyr residues. Phosphorylation adds phosphate groups to specific residues, modulating protein function, activity, or localization through conformational changes.
Which factor and nucleotide deliver aminoacyl-tRNA to the A site in eukaryotes?

eEF1A with GTP. eEF1A-GTP forms a ternary complex with aminoacyl-tRNA, ensuring accurate delivery and proofreading at the A site.
Which ribosomal site holds the tRNA carrying the growing polypeptide chain?

P site. The P site secures the tRNA with the nascent chain, positioning it for transfer to the incoming amino acid.
Which process allows one mRNA to be translated by multiple ribosomes simultaneously?

Polysome (polyribosome) formation. Polysomes increase protein synthesis efficiency by allowing concurrent translation of a single mRNA by multiple ribosomes.
What is the key difference between prokaryotic and eukaryotic ribosomes by subunits?

Prokaryotic $70S=50S+30S$ ; eukaryotic $80S=60S+40S$ . Prokaryotic and eukaryotic ribosomes differ in size and subunit composition, reflecting evolutionary divergence.
Which molecule recognizes stop codons and terminates translation?

Release factor (RF/eRF). Release factors mimic tRNA to bind stop codons, triggering hydrolysis and polypeptide release from the ribosome.
What is ubiquitination, and what is its most common consequence for a protein?

Covalent ubiquitin tagging; targets protein for proteasomal degradation. Ubiquitination marks proteins for degradation by attaching ubiquitin chains, recognized by the proteasome for proteolysis.
What is the start codon for translation, and which amino acid does it encode?

AUG; methionine (Met) (fMet in bacteria). AUG initiates translation by recruiting the initiator tRNA, specifying methionine as the first residue in the polypeptide.
What is the Kozak sequence used for in eukaryotic translation initiation?

Consensus around AUG that promotes start site recognition. The Kozak sequence optimizes initiation by enhancing ribosome binding and scanning efficiency around the AUG codon.
Which factor and nucleotide drive ribosomal translocation during elongation in eukaryotes?

eEF2 with GTP. eEF2-GTP hydrolysis powers the movement of the ribosome along mRNA by one codon during elongation.
Which ribosomal site binds the incoming aminoacyl-tRNA during elongation?

A site. The A site accommodates the next charged tRNA, allowing codon-anticodon pairing and peptide bond formation.
Which ribosomal site holds the deacylated tRNA just before it exits the ribosome?

E site. The E site temporarily holds the empty tRNA after peptide transfer, facilitating its release from the ribosome.
What is the function of aminoacyl-tRNA synthetase in translation?

Charges tRNA with its correct amino acid using ATP. This enzyme ensures accurate amino acid attachment to tRNA, enabling proper codon recognition during protein synthesis.
What is the direction of mRNA translation by the ribosome?

mRNA is read $5' \rightarrow 3'$ . Ribosomes decode mRNA in the 5' to 3' direction to match the synthesis polarity of nucleic acids.
What post-translational modification forms disulfide bonds, and where does it usually occur?

Disulfide bond formation; typically in ER lumen (oxidizing environment). Disulfide bonds stabilize protein structure through cysteine oxidation, occurring in the ER due to its oxidizing conditions.
What is the function of the signal recognition particle (SRP) in protein targeting?

Binds signal peptide, pauses translation, targets ribosome to RER. SRP recognizes the N-terminal signal peptide, halts elongation, and docks the ribosome to the ER translocon for protein insertion.

Study these flashcards