MCAT Biological and Biochemical Foundations of Living Systems Flashcards: 1b Translation Post Translational Modification

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This deck focuses on 1b Translation Post Translational Modification, giving you a quick way to review the definitions, rules, and examples that matter most for MCAT Biological and Biochemical Foundations of Living Systems.

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Work through these flashcards in short sessions. Try to answer each prompt before flipping the card, then revisit any cards you miss until the explanation feels automatic.

Flashcard 1: What post-translational modification forms disulfide bonds, and where does it usually occur?

Answer: 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.

Flashcard 2: What is the function of aminoacyl-tRNA synthetase in translation?

Answer: 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.

Flashcard 3: What is the direction of mRNA translation by the ribosome?

Answer: mRNA is read $5' \rightarrow 3'$. Ribosomes decode mRNA in the 5' to 3' direction to match the synthesis polarity of nucleic acids.

Flashcard 4: Which ribosomal site binds the incoming aminoacyl-tRNA during elongation?

Answer: A site. The A site accommodates the next charged tRNA, allowing codon-anticodon pairing and peptide bond formation.

Flashcard 5: Which ribosomal site holds the tRNA carrying the growing polypeptide chain?

Answer: P site. The P site secures the tRNA with the nascent chain, positioning it for transfer to the incoming amino acid.

Flashcard 6: Which ribosomal site holds the deacylated tRNA just before it exits the ribosome?

Answer: E site. The E site temporarily holds the empty tRNA after peptide transfer, facilitating its release from the ribosome.

Flashcard 7: What is the start codon for translation, and which amino acid does it encode?

Answer: AUG; methionine (Met) (fMet in bacteria). AUG initiates translation by recruiting the initiator tRNA, specifying methionine as the first residue in the polypeptide.

Flashcard 8: What are the three stop codons in the standard genetic code?

Answer: UAA, UAG, UGA. These codons do not code for amino acids but signal the end of translation by binding release factors.

Flashcard 9: Which molecule recognizes stop codons and terminates translation?

Answer: Release factor (RF/eRF). Release factors mimic tRNA to bind stop codons, triggering hydrolysis and polypeptide release from the ribosome.

Flashcard 10: What enzyme activity in the large ribosomal subunit forms peptide bonds?

Answer: Peptidyl transferase (rRNA ribozyme activity). This ribosomal RNA catalyzes the formation of peptide bonds between amino acids during elongation.

Flashcard 11: What is the key difference between prokaryotic and eukaryotic ribosomes by subunits?

Answer: Prokaryotic $70S=50S+30S$; eukaryotic $80S=60S+40S$. Prokaryotic and eukaryotic ribosomes differ in size and subunit composition, reflecting evolutionary divergence.

Flashcard 12: What is the Shine-Dalgarno sequence used for in prokaryotic translation initiation?

Answer: 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.

Flashcard 13: What is the Kozak sequence used for in eukaryotic translation initiation?

Answer: Consensus around AUG that promotes start site recognition. The Kozak sequence optimizes initiation by enhancing ribosome binding and scanning efficiency around the AUG codon.

Flashcard 14: Identify the eukaryotic mRNA feature required for cap-dependent ribosome recruitment.

Answer: $5'$ mRNA cap (m$^7$G cap). The 5' cap recruits initiation factors and the 40S subunit, enabling scanning for the start codon.

Flashcard 15: What is the role of the poly(A) tail in eukaryotic translation efficiency?

Answer: 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.

Flashcard 16: Which process allows one mRNA to be translated by multiple ribosomes simultaneously?

Answer: Polysome (polyribosome) formation. Polysomes increase protein synthesis efficiency by allowing concurrent translation of a single mRNA by multiple ribosomes.

Flashcard 17: What is the immediate energetic cost to add one amino acid during elongation (high-energy bonds)?

Answer: $2$ 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.

Flashcard 18: Which factor and nucleotide drive ribosomal translocation during elongation in eukaryotes?

Answer: eEF2 with GTP. eEF2-GTP hydrolysis powers the movement of the ribosome along mRNA by one codon during elongation.

Flashcard 19: Which factor and nucleotide deliver aminoacyl-tRNA to the A site in eukaryotes?

Answer: eEF1A with GTP. eEF1A-GTP forms a ternary complex with aminoacyl-tRNA, ensuring accurate delivery and proofreading at the A site.

Flashcard 20: Which cellular compartment is the primary site of translation for secreted proteins?

Answer: 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.

Flashcard 21: What is the function of the signal recognition particle (SRP) in protein targeting?

Answer: 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.

Flashcard 22: What is the role of chaperone proteins in post-translational processing?

Answer: 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.

Flashcard 23: What is ubiquitination, and what is its most common consequence for a protein?

Answer: Covalent ubiquitin tagging; targets protein for proteasomal degradation. Ubiquitination marks proteins for degradation by attaching ubiquitin chains, recognized by the proteasome for proteolysis.

Flashcard 24: Which post-translational modification commonly regulates enzyme activity by adding a phosphate group?

Answer: Phosphorylation of Ser/Thr/Tyr residues. Phosphorylation adds phosphate groups to specific residues, modulating protein function, activity, or localization through conformational changes.