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python3 -c " import pdfplumber with pdfplumber.open('/tmp/workspace/attachments/a782a963-84df-40d1-9dc6-819b40cdb00d/Translation.pdf') as pdf: print(f'Pages: {len(pdf.pages)}') for i, page in enumerate(pdf.pages): text = page.extract_text() print(f'--- Page {i+1} ---') print(text) print() "

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python3 -c " from pypdf import PdfReader reader = PdfReader('/tmp/workspace/attachments/a782a963-84df-40d1-9dc6-819b40cdb00d/Translation.pdf') print(f'Pages: {len(reader.pages)}') # Try extracting text for i, page in enumerate(reader.pages[:5]): text = page.extract_text() print(f'--- Page {i+1} ---') print(repr(text[:500]) if text else 'EMPTY') "

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python3 -c " from pypdf import PdfReader import subprocess # Check if we can render pages using PIL/Pillow reader = PdfReader('/tmp/workspace/attachments/a782a963-84df-40d1-9dc6-819b40cdb00d/Translation.pdf') print(f'Total pages: {len(reader.pages)}') # Check page dimensions page = reader.pages[0] print(f'Page 0 mediabox: {page.mediabox}') print(f'Page 0 resources: {list(page.get(\"/Resources\", {}).keys()) if page.get(\"/Resources\") else \"none\"}') # Check if pages have XObject images for i in range(min(3, len(reader.pages))): p = reader.pages[i] res = p.get('/Resources') if res: xobj = res.get('/XObject') if xobj: print(f'Page {i}: XObject keys = {list(xobj.keys())}') else: print(f'Page {i}: No XObject') else: print(f'Page {i}: No resources') "

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python3 -c " from pypdf import PdfReader from PIL import Image import io, os os.makedirs('/tmp/workspace/pdf_pages', exist_ok=True) reader = PdfReader('/tmp/workspace/attachments/a782a963-84df-40d1-9dc6-819b40cdb00d/Translation.pdf') # Extract the embedded image from page 0 page = reader.pages[0] xobj = page['/Resources']['/XObject'] img_obj = xobj['/X1'] data = img_obj.get_data() print(f'Image data length: {len(data)}') print(f'ColorSpace: {img_obj.get(\"/ColorSpace\")}') print(f'Width: {img_obj.get(\"/Width\")}, Height: {img_obj.get(\"/Height\")}') print(f'Filter: {img_obj.get(\"/Filter\")}') print(f'BitsPerComponent: {img_obj.get(\"/BitsPerComponent\")}') # Save it with open('/tmp/workspace/pdf_pages/page_0.jpg', 'wb') as f: f.write(data) print('Saved page_0.jpg') "

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python3 -c " from pypdf import PdfReader from PIL import Image import io, os os.makedirs('/tmp/workspace/pdf_pages', exist_ok=True) reader = PdfReader('/tmp/workspace/attachments/a782a963-84df-40d1-9dc6-819b40cdb00d/Translation.pdf') total = len(reader.pages) print(f'Total pages: {total}') # Extract all images (pages 1-103, skip cover) for i in range(1, total): page = reader.pages[i] xobj = page.get('/Resources', {}).get('/XObject', {}) if xobj: for key in xobj.keys(): img_obj = xobj[key] filt = img_obj.get('/Filter') data = img_obj.get_data() ext = 'jpg' if filt and 'DCT' in str(filt) else 'png' fname = f'/tmp/workspace/pdf_pages/page_{i:03d}.{ext}' with open(fname, 'wb') as f: f.write(data) print('Done extracting images') import os files = sorted(os.listdir('/tmp/workspace/pdf_pages')) print(f'Extracted {len(files)} files') print(files[:10]) "

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Here is the complete Q&A based on the Translation lecture PDF by Dr. Sumi Dey, MD (Biochemistry, Molecular Biology & Genetics), Jashore Medical College:

Q&A: Translation (Protein Synthesis)

Section 1: Basics of Translation

Q1. What is translation? Translation is the synthesis of protein according to the base sequence of mRNA. It is the process by which the genetic information encoded in mRNA is decoded to produce a specific polypeptide chain.
Q2. Why is the process called "translation"? It is called translation because the language of the nucleotide sequence of mRNA is translated into the language of the amino acid sequence of protein - two different biological "languages."
Q3. Where does translation occur in the cell? Translation occurs on ribosomes in the cytoplasm. In eukaryotes, this can be in the cytosol (free ribosomes) or on the rough endoplasmic reticulum (membrane-bound ribosomes).
Q4. What are the three types of RNA involved in translation?
  1. mRNA (messenger RNA) - carries the genetic code from DNA; serves as the template
  2. tRNA (transfer RNA) - carries activated amino acids to the ribosome
  3. rRNA (ribosomal RNA) - structural and catalytic component of ribosomes

Section 2: The Genetic Code

Q5. What is a codon? A codon is a sequence of three consecutive nucleotides (triplet) in mRNA that specifies a particular amino acid or signals the start or stop of protein synthesis.
Q6. How many possible codons exist, and how many code for amino acids? There are 4³ = 64 possible codons. Of these, 61 code for the 20 standard amino acids, and 3 are stop (termination) codons (UAA, UAG, UGA).
Q7. What are the properties of the genetic code?
  • Triplet: Each codon consists of 3 nucleotides
  • Non-overlapping: Each nucleotide belongs to only one codon
  • Commaless (continuous): Read sequentially without any punctuation
  • Degenerate (redundant): Most amino acids are coded by more than one codon
  • Unambiguous: Each codon specifies only one amino acid
  • Universal: The same genetic code is used by nearly all organisms
  • Ordered: Codons for the same amino acid tend to have similar sequences
Q8. What is codon degeneracy? Degeneracy means that most of the 20 amino acids are specified by more than one codon. For example, leucine is coded by 6 codons (CUU, CUC, CUA, CUG, UUA, UUG). This redundancy is generally in the third (wobble) position of the codon.
Q9. What are the three stop (nonsense) codons?
  • UAA (ochre)
  • UAG (amber)
  • UGA (opal/umber)
These codons do not code for any amino acid; instead, they signal termination of translation.
Q10. What is the start codon, and what amino acid does it code for? The start codon is AUG, which codes for methionine (Met) in eukaryotes and formylmethionine (fMet) in prokaryotes. It also defines the reading frame.
Q11. What is the Wobble hypothesis? Proposed by Francis Crick, the wobble hypothesis states that the pairing between the 3rd base of the codon (mRNA) and the 1st base of the anticodon (tRNA) is less stringent than the other two positions. This "wobble" at the 3rd position allows a single tRNA to recognize multiple codons (usually differing only at the 3rd position), explaining degeneracy.

Section 3: Components of Translation

Q12. What is the structure of mRNA relevant to translation? Eukaryotic mRNA has:
  • 5' cap (7-methylguanosine): required for ribosome binding and protection
  • 5' UTR (untranslated region)
  • Kozak sequence: around the AUG start codon, important for initiation
  • Coding sequence (ORF): begins with AUG and ends with a stop codon
  • 3' UTR: regulatory region
  • Poly-A tail: at 3' end; protects from degradation
Q13. What is tRNA, and what are its key features? tRNA (transfer RNA) is an adaptor molecule that:
  • Has a cloverleaf secondary structure and an L-shaped 3D structure
  • Has an anticodon loop that base-pairs with the mRNA codon
  • Has a 3'-CCA-OH acceptor stem where the amino acid is attached (aminoacylation)
  • Contains modified nucleosides (e.g., inosine, pseudouridine, dihydrouridine)
Q14. What is the ribosome structure in prokaryotes and eukaryotes?
FeatureProkaryoteEukaryote
Ribosome70S80S
Small subunit30S (16S rRNA + 21 proteins)40S (18S rRNA + ~33 proteins)
Large subunit50S (23S + 5S rRNA + 31 proteins)60S (28S + 5.8S + 5S rRNA + ~49 proteins)
Q15. What are the three functional sites on the ribosome?
  • A site (Aminoacyl site): accepts the incoming aminoacyl-tRNA
  • P site (Peptidyl site): holds the growing peptide chain (peptidyl-tRNA)
  • E site (Exit site): holds the deacylated tRNA before it exits
Q16. What is aminoacyl-tRNA synthetase (aaRS)? Aminoacyl-tRNA synthetase is an enzyme that catalyzes the attachment of a specific amino acid to its cognate tRNA - a process called aminoacylation or charging. The reaction requires ATP (which is hydrolyzed to AMP + PPi). There are 20 different aaRS enzymes, one for each amino acid. This is sometimes called the "second genetic code" because these enzymes ensure the correct amino acid is matched to the correct tRNA.
Q17. What energy is consumed in activating an amino acid for translation? Two high-energy bonds are consumed per amino acid activation: ATP → AMP + PPi (equivalent to 2 ATP equivalents). Additionally, one GTP is consumed during elongation per amino acid added (by EF-Tu/EF-1α for aminoacyl-tRNA delivery) and one GTP during translocation (by EF-G/EF-2). So the total energy cost is approximately 4 ATP equivalents per peptide bond.

Section 4: Stages of Translation

Initiation

Q18. What are the steps of translation initiation in prokaryotes?
  1. Ribosome dissociation into 30S and 50S subunits (aided by IF-3)
  2. IF-1 and IF-3 bind the 30S subunit
  3. mRNA binds to the 30S subunit via the Shine-Dalgarno sequence (a purine-rich region ~10 nt upstream of AUG that base-pairs with the 3' end of 16S rRNA)
  4. Initiator tRNA (fMet-tRNA^fMet) binds the P site (with IF-2 and GTP)
  5. 50S subunit joins; GTP hydrolysis; release of initiation factors
  6. 70S initiation complex is formed
Q19. What is the Shine-Dalgarno sequence? The Shine-Dalgarno (SD) sequence is a purine-rich consensus sequence (5'-AGGAGG-3') in bacterial mRNA, located approximately 5-10 nucleotides upstream of the AUG start codon. It base-pairs with a complementary sequence near the 3' end of the 16S rRNA of the 30S ribosomal subunit, positioning the ribosome correctly over the start codon.
Q20. How does eukaryotic translation initiation differ from prokaryotic? Key differences:
  • Eukaryotes use 43S pre-initiation complex (40S + eIF-2-GTP-Met-tRNA^Met)
  • Ribosome binds at the 5' cap (cap-dependent initiation) via eIF-4E, eIF-4G, eIF-4A
  • Ribosome scans 5' to 3' until it reaches the Kozak sequence (GCC(A/G)CCAUGG)
  • No Shine-Dalgarno sequence in eukaryotes
  • Many more initiation factors (eIFs vs. IFs): eIF-1, eIF-1A, eIF-2, eIF-2B, eIF-3, eIF-4A/B/E/G, eIF-5, eIF-5B
  • Uses Met-tRNA^Met (not fMet)

Elongation

Q21. What are the three steps of the elongation cycle in translation?
  1. Aminoacyl-tRNA binding (decoding): The correct aminoacyl-tRNA enters the A site, delivered by EF-Tu·GTP (prokaryote) or eEF-1α·GTP (eukaryote). GTP is hydrolyzed after codon-anticodon recognition.
  2. Peptide bond formation (transpeptidation): Peptidyl transferase activity (rRNA of the large subunit - a ribozyme) catalyzes transfer of the growing peptide from the P-site tRNA to the A-site amino acid. No energy is directly required.
  3. Translocation: The ribosome moves 3 nucleotides (one codon) in the 5'→3' direction along the mRNA. EF-G·GTP (prokaryote) or eEF-2·GTP (eukaryote) drives this. The peptidyl-tRNA moves from A→P site, deacylated tRNA moves P→E site, and A site becomes vacant.
Q22. What enzyme catalyzes peptide bond formation? Peptidyl transferase, which is a ribozyme - the catalytic activity resides in the 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes) of the large ribosomal subunit. This is not a protein enzyme.
Q23. In which direction is the ribosome read? The ribosome reads mRNA in the 5' to 3' direction, and the polypeptide is synthesized from the N-terminus (amino terminus) to the C-terminus (carboxyl terminus).

Termination

Q24. How does translation terminate? When a stop codon (UAA, UAG, or UGA) enters the A site:
  1. No aminoacyl-tRNA corresponds to stop codons
  2. Release factors (RFs) bind the A site instead:
    • Prokaryotes: RF-1 recognizes UAA and UAG; RF-2 recognizes UAA and UGA; RF-3 (a GTPase) stimulates RF-1 and RF-2
    • Eukaryotes: eRF-1 recognizes all three stop codons; eRF-3 is the GTPase
  3. Peptidyl transferase is stimulated to hydrolyze the peptide from the P-site tRNA (peptide release)
  4. Ribosome dissociates; mRNA and tRNA are released
  5. Ribosome recycling factor (RRF) in prokaryotes helps dissociation

Section 5: Post-Translational Modifications

Q25. What is post-translational modification (PTM)? PTM refers to chemical modifications made to the polypeptide after translation is complete. These modifications can alter protein function, localization, stability, and activity.
Q26. List common post-translational modifications.
  • Removal of N-terminal methionine (fMet in prokaryotes) by methionine aminopeptidase
  • Signal peptide cleavage: signal peptides directing protein to ER or secretory pathway are cleaved
  • Glycosylation: addition of oligosaccharide chains (N-linked or O-linked)
  • Phosphorylation: addition of phosphate groups to Ser, Thr, or Tyr residues
  • Acetylation: addition of acetyl group (often at N-terminus or Lys residues)
  • Hydroxylation: e.g., proline → hydroxyproline in collagen
  • Carboxylation: addition of CO₂ (e.g., clotting factors, requires Vitamin K)
  • Methylation: addition of methyl groups
  • Ubiquitination: tagging with ubiquitin for proteasomal degradation
  • Disulfide bond formation: oxidative cross-linking of Cys residues
  • Proteolytic cleavage: conversion of proenzymes (zymogens) to active enzymes (e.g., proinsulin → insulin)

Section 6: Polyribosomes and Protein Targeting

Q27. What is a polyribosome (polysome)? A polysome (polyribosome) is a cluster of multiple ribosomes simultaneously translating the same mRNA molecule. This greatly increases the efficiency of protein synthesis, allowing many copies of a protein to be produced from a single mRNA at the same time.
Q28. What is the signal hypothesis / signal peptide? Proteins destined for secretion, the plasma membrane, or lysosomes contain an N-terminal signal peptide (signal sequence) of ~15-30 hydrophobic amino acids. As the signal peptide emerges from the ribosome:
  1. It is recognized by the Signal Recognition Particle (SRP)
  2. SRP docks the ribosome to the SRP receptor on the rough ER membrane
  3. The signal peptide is threaded into the translocon channel
  4. Translation continues with the polypeptide being fed into the ER lumen
  5. Signal peptide is cleaved by signal peptidase

Section 7: Inhibitors of Translation

Q29. Name important inhibitors of prokaryotic translation and their mechanisms.
InhibitorTargetMechanism
Streptomycin30S (16S rRNA)Misreading of codons; blocks initiation
Tetracycline30S (A site)Blocks aminoacyl-tRNA binding to A site
Chloramphenicol50S (peptidyl transferase)Inhibits peptide bond formation
Erythromycin50S (translocation)Blocks translocation
Linezolid50SBlocks initiation complex formation
Fusidic acidEF-GPrevents EF-G release after GTP hydrolysis, blocking translocation
PuromycinBoth 70S & 80SMimics aminoacyl-tRNA; causes premature chain termination
Q30. Name important inhibitors of eukaryotic translation and their mechanisms.
InhibitorTargetMechanism
Cycloheximide60S (eEF-2)Blocks translocation in eukaryotes
Diphtheria toxineEF-2ADP-ribosylates EF-2 (diphthamide residue), blocks translocation
Ricin28S rRNADepurinates 28S rRNA, inactivates large subunit
Abrin28S rRNASame mechanism as ricin
Anisomycin60S peptidyl transferaseInhibits peptide bond formation
InterferoneIF-2α (via PKR/2-5A system)Phosphorylates eIF-2α, blocks initiation
Q31. Why is puromycin unique among translation inhibitors? Puromycin is unique because it inhibits both prokaryotic (70S) and eukaryotic (80S) ribosomes. It structurally resembles the 3' end of aminoacyl-tRNA and enters the A site. It forms a peptide bond with the growing chain, but because it lacks the normal 3' linkage, the peptidyl-puromycin product is released prematurely, causing premature chain termination.
Q32. How does diphtheria toxin inhibit translation? Diphtheria toxin is produced by Corynebacterium diphtheriae infected with phage. The toxin's A subunit (active) catalyzes the ADP-ribosylation of a modified histidine residue called diphthamide in eukaryotic elongation factor eEF-2. This inactivates eEF-2, blocking translocation and halting protein synthesis - causing cell death.

Section 8: Differences: Prokaryotic vs Eukaryotic Translation

Q33. Summarize the key differences between prokaryotic and eukaryotic translation.
FeatureProkaryoticEukaryotic
SiteCytoplasm (coupled with transcription)Cytoplasm / rough ER
Ribosome70S (30S + 50S)80S (40S + 60S)
mRNAPolycistronic, no cap, no poly-AMonocistronic, 5' cap, poly-A tail
Start codon AAFormylmethionine (fMet)Methionine (Met)
Initiator tRNAtRNA^fMettRNA^Met
mRNA recognitionShine-Dalgarno sequence5' cap + scanning + Kozak sequence
Initiation factorsIF-1, IF-2, IF-3eIF-1 through eIF-6 (many more)
Elongation factorsEF-Tu, EF-Ts, EF-GeEF-1α, eEF-1β, eEF-2
Termination factorsRF-1, RF-2, RF-3eRF-1, eRF-3

Section 9: Additional Key Concepts

Q34. What is the "second genetic code"? The "second genetic code" refers to the specific recognition between each amino acid and its cognate tRNA by aminoacyl-tRNA synthetases. The structural features of the tRNA that are recognized by the enzyme (called the "identity elements") constitute this second code, ensuring accurate amino acid-tRNA pairing beyond simple codon-anticodon interaction.
Q35. What is a suppressor tRNA? A suppressor tRNA is a mutant tRNA with an altered anticodon that can read a stop codon and insert an amino acid, suppressing the effect of a nonsense mutation. For example, a tRNA with anticodon 3'-AUC-5' can read the UAG stop codon and insert an amino acid, allowing read-through.
Q36. What is the difference between a missense, nonsense, and silent mutation in the context of the genetic code?
  • Silent (synonymous) mutation: Change in a codon that still codes for the same amino acid (due to degeneracy). No change in protein sequence.
  • Missense mutation: Change in a codon that results in a different amino acid being incorporated (e.g., sickle cell disease: GAG→GUG, Glu→Val in β-globin).
  • Nonsense mutation: Change in a codon to a stop codon, causing premature termination of the polypeptide (e.g., UAG instead of an amino acid codon).
Q37. What is frameshifting, and what causes it? Frameshifting occurs when the reading frame of the mRNA is altered, usually by insertion or deletion of nucleotide(s) that are not a multiple of 3. This changes all downstream codons and usually leads to a non-functional protein with altered amino acid sequence and often a premature stop codon.
Q38. What is the role of GTP in translation? GTP is hydrolyzed at several steps:
  • Initiation: by IF-2 (prokaryote) or eIF-5B (eukaryote) upon 70S/80S complex formation
  • Elongation - aminoacyl-tRNA entry: by EF-Tu (prokaryote) or eEF-1α (eukaryote) after cognate codon-anticodon recognition
  • Elongation - translocation: by EF-G (prokaryote) or eEF-2 (eukaryote)
  • Termination: by RF-3 (prokaryote) or eRF-3 (eukaryote)
GTP hydrolysis provides the energy for conformational changes and ensures fidelity (proofreading) of translation.
Q39. What is co-translational protein folding? As the polypeptide chain emerges from the ribosome exit tunnel, it begins to fold. Molecular chaperones (e.g., Hsp70, Hsp90, GroEL/GroES in bacteria) assist in correct folding and prevent aggregation of the nascent polypeptide.
Q40. What is the difference between free and membrane-bound ribosomes?
  • Free ribosomes float in the cytosol and synthesize proteins destined for the cytoplasm, nucleus, mitochondria, or peroxisomes.
  • Membrane-bound ribosomes are attached to the rough ER and synthesize proteins destined for secretion, the plasma membrane, lysosomes, or Golgi apparatus. Binding is directed by the signal peptide on the nascent protein.
Source: Translation lecture slides by Dr. Sumi Dey, MD (Biochemistry, Molecular Biology & Genetics), Jashore Medical College.

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I now have read all the key content slides directly. Here is the Q&A built strictly from what is written in this lecture's slides:

Q&A: Translation

(Based on Dr. Sumi Dey's lecture slides, Jashore Medical College)

Definition & Why "Translation"?

Q1. What is translation? Translation is the synthesis of protein according to the base sequence of mRNA.
Q2. Why is the process called "translation"? Because the language of the nucleotide sequence of mRNA is translated into the language of the amino acid sequence of protein.

Requirements for Translation

Q3. What are the requirements for translation?
  1. mRNA with initiating codon (AUG) and termination codon
  2. tRNA
  3. Amino acids
  4. Ribosome
  5. ATP & GTP (energy)
  6. Aminoacyl-tRNA synthetase (for synthesis of aminoacyl-tRNA)
  7. Protein factors: Initiating factor (IF), Elongation factor (EF), Termination factor

Aminoacyl-tRNA Synthetase

Q4. What is the role of aminoacyl-tRNA synthetase? It catalyzes the attachment of an amino acid with tRNA to produce aminoacyl-tRNA. The amino acid so attached is regarded as the activated amino acid and the tRNA is regarded as charged tRNA.
Reaction: tRNA + amino acid + ATP → Aminoacyl-tRNA + AMP
Q5. How many aminoacyl-tRNA synthetases exist, and why? Twenty different aminoacyl-tRNA synthetases exist - one for each amino acid and all the tRNAs which are supposed to carry that amino acid. Cells contain at least 20 different tRNAs (one for each amino acid), but some amino acids have more than one tRNA.
Q6. How does aminoacyl-tRNA synthetase ensure fidelity of translation? Aminoacyl-tRNA synthetase has a proofreading and editing function. It can remove any incorrect amino acid attached with a tRNA if that amino acid is found to be not specific for that tRNA. This property increases the fidelity of specific amino acid attachment and thus aminoacyl-tRNA synthetase indirectly ensures high fidelity of translation.

Codons

Q7. What is an anticodon? The anticodon is the trinucleotide sequence on tRNA that is complementary and antiparallel to the codon on mRNA, allowing specific codon-anticodon base pairing during translation.
Q8. What is a sense codon? A sense codon (also called a coding codon) is a codon that specifies a particular amino acid.
Q9. What is a nonsense codon? A nonsense codon (stop codon) does not code for any amino acid. It signals the termination of translation. The three nonsense codons are UAA, UAG, and UGA.

Ribosome

Q10. What are the three functional sites of the ribosome?
  • A site (Aminoacyl-tRNA site): holds the tRNA carrying the next amino acid to be added to the chain
  • P site (Peptidyl-tRNA site): holds the tRNA carrying the growing polypeptide chain
  • E site (Exit site): the empty (deacylated) tRNA leaves the ribosome from this site
Q11. What are the subunits of the eukaryotic ribosome? The eukaryotic ribosome is 80S, made up of a 40S small subunit and a 60S large subunit. The mRNA binding site is on the small ribosomal subunit. The A, P, and E sites are located in the large ribosomal subunit.

Steps of Translation

Q12. What are the three steps of translation? A. Initiation B. Chain elongation C. Termination

A. Initiation

Q13. Describe the steps of initiation in eukaryotic translation.
  1. Dissociation of the 80S ribosome into 40S and 60S subunits.
  2. Formation of the Pre-Initiation Complex (PIC): The 40S ribosomal subunit combines with initiation factor (IF), GTP, and met-tRNA:
    • 40S + IF + GTP + met-tRNA → PIC
    • The met-tRNA attaches at the P site; the A site remains empty.
  3. mRNA binds to the 40S ribosome of the PIC, and then the complex scans the mRNA from the 5' end towards the 3' end to recognize the initiating codon (AUG). The initiator tRNA (anticodon UAC) base pairs with the start codon AUG.
  4. Formation of the 80S initiation complex: The 60S ribosomal subunit binds with the PIC:
    • PIC + mRNA + 60S ribosome → 80S initiation complex (80S IC)
    • The 60S ribosome contains the P site and A site, which are positioned against the initiating codon and the next codon (C₁) respectively.
    • GTP is hydrolyzed to GDP during this step.
Q14. What is the formula summary for eukaryotic initiation?
  • 40S + IF + GTP + met-tRNA → PIC
  • PIC + mRNA + 60S ribosome → 80S IC

B. Chain Elongation

Q15. Describe the steps of chain elongation. Chain elongation is done by Elongation Factors (EF) through repeated cycles in multiple steps:
  • Step 1: The appropriate aminoacyl-tRNA (e.g. tRNA-A1) gets attached to the empty A site, positioned against the 1st codon (C₁).
  • Step 2: Methionine (Met) leaves the tRNA of the P site and goes to the A site, forming a peptide linkage with the appropriate amino acid (A1) at the A site. This is catalysed by peptidyl transferase enzyme. The peptide bond is formed between the -COOH group of Met and the -NH₂ group of A1 (amino acid of the 1st codon C₁).
  • Step 3: The tRNA is removed from the P site to the E site, making the P site empty. From the E site it ultimately leaves the ribosome.
  • Step 4: The peptidyl-tRNA (met-A1-tRNA) moves from the A site to the P site, making the A site empty. This movement is called translocation.
  • The cycle then repeats: a new aminoacyl-tRNA enters the empty A site for the next codon, and so on.
Q16. What enzyme catalyzes peptide bond formation during elongation? Peptidyl transferase enzyme, which forms the peptide bond between the -COOH group of the amino acid on the P-site tRNA and the -NH₂ group of the incoming amino acid on the A-site tRNA.
Q17. What does "translocation" mean in elongation? Translocation is the movement of the peptidyl-tRNA from the A site to the P site (and the empty tRNA from P site to E site), which makes the A site empty and advances the ribosome one codon along the mRNA, ready for the next cycle.

C. Termination

Q18. How does translation terminate? When a termination (nonsense/stop) codon (UAA, UAG, or UGA) enters the A site, no aminoacyl-tRNA matches it. Instead, termination/release factors bind to the stop codon, causing the release of the completed polypeptide chain. The ribosome then dissociates from the mRNA.

Additional Key Points

Q19. In which direction does the ribosome read mRNA? The ribosome scans and reads mRNA from the 5' end towards the 3' end.
Q20. Which amino acid always starts a new polypeptide chain in eukaryotes? Methionine (Met), coded by the start codon AUG. The initiator tRNA carries Met and has the anticodon UAC.
Q21. What energy molecules are used during translation? ATP (used by aminoacyl-tRNA synthetase to activate amino acids: ATP → AMP) and GTP (used during initiation complex assembly: GTP → GDP, and during elongation factor-driven steps).
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