Protein Synthesis

Protein Synthesis (Translation)

Protein synthesis is the process of translating the genetic code (mRNA) into a specific sequence of amino acids. It occurs at ribosomes and requires mRNA, tRNA, amino acids, ribosomes, initiation/elongation/termination factors, and GTP.

The Genetic Code

  • Triplet codons on mRNA specify amino acids. 64 codons total: 61 sense codons + 3 stop codons (UAA, UAG, UGA).
  • Degenerate (redundant): Multiple codons for the same amino acid (especially at 3rd position — wobble)
  • Non-overlapping, commaless, read 5'→3'
  • AUG = Start codon (Met in eukaryotes, fMet in prokaryotes)

tRNA Structure

Cloverleaf secondary structure with 4 arms. The anticodon loop base-pairs with mRNA codon (antiparallel). The 3' CCA end (acceptor stem) carries the amino acid. Aminoacyl-tRNA Synthetases charge specific tRNAs with their correct amino acid (uses ATP; hydrolysis to AMP + PPi — equivalent to 2 ATP); this is the fidelity step.

Phases of Translation

Initiation (GTP used):

  • 43S preinitiation complex forms (small 40S subunit + Met-tRNAi + initiation factors eIF2·GTP)
  • Recruited to mRNA 5' cap by eIF4F cap-binding complex; scans 5'→3' for AUG
  • Large 60S joins (eIF5 hydrolyzes GTP) → 80S ribosome

Elongation (GTP used for each amino acid added):

  • A-site: Aminoacyl-tRNA binds (EF-1α·GTP in eukaryotes)
  • P-site: Peptide chain grows; Peptidyl transferase (rRNA ribozyme) forms peptide bond
  • E-site: Discharged tRNA exits
  • Translocation: Ribosome moves 3 nt in 5'→3' direction (EF-2·GTP)

Termination: Stop codon (UAA/UAG/UGA) recognized by Release Factors (eRF1+eRF3) → peptide released → ribosome dissociates.

Post-Translational Modifications

  • Glycosylation: Addition of carbohydrates (N-linked in ER; O-linked in Golgi)
  • Hydroxylation: Pro and Lys in collagen (requires Vitamin C)
  • Phosphorylation: Ser, Thr, Tyr by kinases — regulation
  • Ubiquitination: Tags proteins for proteasomal degradation
  • Acetylation, Methylation: Histones — gene regulation
  • Signal sequences target proteins to ER, mitochondria, peroxisomes (SRP recognition)

Antibiotics & Protein Synthesis Inhibitors

  • Prokaryotic 30S inhibitors: Aminoglycosides (Gentamicin — misreading), Tetracyclines (block A-site)
  • Prokaryotic 50S inhibitors: Chloramphenicol (peptidyl transferase — aplastic anemia), Macrolides (translocation — erythromycin), Linezolid, Clindamycin
  • Eukaryotic 80S inhibitors: Diphtheria toxin (inhibits EF-2 by ADP-ribosylation), Cycloheximide, Ricin (depurinates 28S rRNA)

Protein Folding & Chaperones

Chaperones (Hsp70, Hsp90, GroEL/GroES) assist proper protein folding by preventing aggregation. Misfolded proteins → unfolded protein response (UPR) in ER → if unresolvable → apoptosis. Prion diseases involve misfolded PrPc → PrPsc (β-sheet conformation) that templates misfolding of normal proteins — fatal neurodegenerative disorders.

Quiz - Exam Preparation Strategy

When studying Quiz for your final board exams, it is critical to focus on the core concepts and fundamental formulas. Relying strictly on NCERT textbook solutions and practicing previous year questions (PYQs) is the proven methodology for scoring high marks. Avoid rote memorization and instead focus on the logical application of the theories presented in this chapter.

⚠️ Common Mistakes to Avoid

❓ Frequently Asked Questions

How can I quickly memorize the concepts of Quiz?

The most effective way is to create short, handwritten revision notes and continuously test your knowledge using our interactive Mock Tests. Spaced repetition and active recall are much better than passive reading.

What type of questions are most commonly asked from Quiz?

Board exams tend to favor conceptual application questions and direct formula-based derivations from the NCERT syllabus. Ensure you have solved every single exercise in the official textbook.

Is reading the NCERT book enough for this chapter?

Yes, the NCERT textbook is the absolute gold standard for board exams. However, to improve your speed and accuracy during the actual exam, you must supplement your reading by solving timed mock tests and objective questions.