Translation & Protein Synthesis

Translation for the MCAT

Part 1 of 4 — The Genetic Code & tRNA

Properties of the Code (Each One Is a Question Type)

• Triplet: 3 bases = 1 codon; 64 codons cover 20 amino acids + stop.
• Degenerate (redundant): most amino acids have multiple codons — usually differing at the THIRD position. Consequence: many third-position mutations are silent.
• Unambiguous: one codon never means two amino acids.
• Nonoverlapping & comma-free: read 3 at a time from a fixed start; no punctuation between codons. Consequence: insertions/deletions not in multiples of 3 shift the frame for everything downstream.
• Nearly universal: mitochondria and a few organisms deviate slightly — the standard exception clause.
• Start: AUG (methionine) sets the reading frame. Stops: UAA, UAG, UGA — no tRNA reads them (release factors do).
• Reading frame: any mRNA has 3 possible frames; the AUG chosen fixes which one is real. Passages that shift a start site are testing whether you re-read the frame.

tRNA — The Adapter

• Cloverleaf secondary structure, L-shaped 3D structure; two business ends:
  • Anticodon (bottom loop): base-pairs with the mRNA codon, antiparallel — codon 5'-AUG-3' pairs anticodon 3'-UAC-5'.
  • 3' CCA acceptor end: the amino acid is esterified to the terminal A's ribose — a high-energy ester bond whose hydrolysis later powers peptide bond formation.
• Wobble: the codon's third base pairs loosely with the anticodon's first (5') base — nonstandard pairs such as G-U are allowed. One tRNA can read several synonymous codons; this is WHY the code's redundancy concentrates at position 3, and why cells need far fewer tRNA species than the 61 sense codons.

• Inosine, a modified base at the anticodon's wobble position, pairs with U, C or A — which is how roughly 40 tRNA species cover all 61 sense codons.

Aminoacyl-tRNA Synthetases — Where Accuracy Actually Lives

• One synthetase per amino acid (20 total); each recognizes its amino acid AND all its cognate tRNAs.
• Charging is a two-step, 2-ATP-equivalent reaction: amino acid + ATP forms aminoacyl-AMP (PP$_i$ released and hydrolyzed), then transfer to the tRNA's 3' end.
• Many synthetases have editing sites that hydrolyze near-miss amino acids (isoleucine vs valine) — proofreading before the ribosome ever sees the cargo.
• The critical experiment: the ribosome checks codon-anticodon pairing, NOT the amino acid. A cysteine chemically converted to alanine while attached to tRNA$^{Cys}$ gets inserted wherever CYSTEINE codons appear. Mischarged tRNA = wrong protein; the synthetases are the true guardians of the code.

Worked Example — Decoding and Breaking a Message

Passage-style problem. A short mRNA reads: 5'-GCAUGGCUUGGUGA...-3'. (Codon reference: AUG = Met, GCU = Ala, UGG = Trp, UGA = stop, GCA = Ala, UGC = Cys.)

Question 1 — Find the reading frame and translate. Scan for the first AUG: it starts at nucleotide 3. Read triplets from there: AUG (Met) - GCU (Ala) - UGG (Trp) - UGA (stop). Peptide: Met-Ala-Trp, then release. The two 5' bases (GC) are untranslated leader — bases before the start codon never appear in protein. Counting from base 1 instead of the AUG is the designed error.

Question 2 — A mutation inserts a single C immediately after the AUG. Translate again. New message after the start: AUG - CGC - UUG - GUG - A... The frame is shifted; every downstream codon changes (Arg-Leu-Val...), and the original UGA stop is no longer in frame — translation continues until a new in-frame stop appears. Frameshift = new sequence AND usually new length. Contrast inserting THREE bases: one amino acid added, frame preserved.

Question 3 — The tRNA reading UGG (Trp) has what anticodon, and could it also read UGA? Written antiparallel, the anticodon is 3'-ACC-5' (conventionally written 5'-CCA-3' — always state orientation). UGA differs at the third codon position (G to A); wobble tolerates some third-position mismatches, but a normal tryptophan tRNA does not read UGA — UGA is a stop recognized by release factors.

However, a MUTANT tRNA with an altered anticodon can suppress stop codons (nonsense suppressors), inserting an amino acid at UGA and producing read-through protein — a classic passage twist.

Question 4 — Why did the exam give you both GCU and GCA as Ala? To make degeneracy concrete: a GCU-to-GCA change at codon 2 would be silent (third-position wobble), while GCU-to-GAU would not. Track WHICH position mutated before predicting the effect.

The Code & tRNA 🎯

Key Takeaways — Part 1

• Code properties as question types: triplet, degenerate (synonyms differ at position 3 — silent mutations), unambiguous, nonoverlapping/comma-free (indels not divisible by 3 = frameshift), nearly universal (mitochondria deviate)
• AUG starts and sets the frame; UAA/UAG/UGA stop via release factors, not tRNAs; translation begins at the first AUG, not the first base
• tRNA: anticodon pairs the codon antiparallel (complement THEN reverse); amino acid rides the 3' CCA end on a high-energy ester bond that later pays for the peptide bond
• Wobble at the third position: one tRNA reads a codon family (far fewer tRNAs than 61 codons); wobble never makes protein ambiguous
• Synthetases are the code's guardians: one per amino acid, two-step charging costing 2 ATP equivalents, editing sites reject near-miss substrates
• Chapeville: the ribosome checks only codon-anticodon pairing — mischarged tRNA delivers the wrong amino acid unchallenged

• Low-yield extras: inosine at the wobble position pairs with U, C or A, letting ~40 tRNA species read all 61 sense codons; mutant (nonsense-suppressor) tRNAs with altered anticodons can read stop codons and cause read-through

Translation for the MCAT

Part 2 of 4 — Ribosome Mechanics: Initiation, Elongation, Termination

The Machine

• Prokaryotic ribosome: 70S = 30S small + 50S large subunit. Eukaryotic: 80S = 40S + 60S. (S values do not add — sedimentation is shape-dependent; a favorite detail.)
• Three tRNA slots spanning both subunits: A site (aminoacyl — incoming), P site (peptidyl — holds the growing chain), E site (exit). Mnemonic: APE, reading in the direction tRNAs move.
• Peptidyl transferase — the catalyst forming peptide bonds — is ribosomal RNA of the large subunit: the ribosome is a ribozyme.

Initiation — Where Prokaryotes and Eukaryotes Diverge Most

• In eukaryotes the cap is bound by the initiation factor eIF4E, and the scanning 40S favors an AUG embedded in a favorable surrounding sequence (the Kozak context).

Order of assembly (both systems): small subunit + initiator tRNA find the start codon FIRST; the large subunit joins LAST, with the initiator tRNA landing directly in the P site — the only tRNA that ever enters there.

Elongation — The Three-Step Loop

1. Delivery: a GTP-bound elongation factor escorts aminoacyl-tRNA into the A site; correct codon-anticodon pairing triggers GTP hydrolysis and release — the pairing check IS the proofreading step.
2. Peptide bond: the P-site chain is transferred onto the A-site amino acid (peptidyl transferase, rRNA-catalyzed, no extra energy input — the ester bond pays).
3. Translocation: a second elongation factor + GTP ratchets the ribosome one codon 3'-ward; tRNAs shift A-to-P and P-to-E; the E-site tRNA leaves.

• The elongation factors by name: delivery is EF-Tu (eukaryotic eEF1); translocation is EF-G (eukaryotic eEF2).

Energy audit per peptide bond: 2 ATP equivalents (charging) + 1 GTP (delivery) + 1 GTP (translocation) = ~4 high-energy bonds per residue — protein synthesis is the cell's biggest energy line item.

Termination & Recycling

• A stop codon enters the A site; no tRNA fits. Release factors (proteins mimicking tRNA shape) bind and make peptidyl transferase hydrolyze the chain onto water — releasing the protein.
• Subunits dissociate and recycle. Multiple ribosomes read one mRNA simultaneously = a polysome (more protein per transcript per unit time; the 5'-most ribosome started LAST).

Worked Example — Locating a Block in the Elongation Cycle

Passage-style problem. Four in vitro translation reactions use identical mRNA and components except one inhibitor each. Products are analyzed for peptide length; ribosome-bound tRNAs are probed.

• Reaction 1: only dipeptides form; ribosomes stall with peptidyl-tRNA in the A site and a deacylated (empty) tRNA still in the P site — the ribosome never moves to the next codon.
• Reaction 2: initiation complexes assemble on the start codon, but no first peptide bond ever forms; the A site never accepts aminoacyl-tRNA.
• Reaction 3: when a compound resembling the 3' end of an aminoacyl-tRNA is added, short peptides of random lengths are released from ribosomes as free peptides — full-length protein never appears.
• Reaction 4: on eukaryotic ribosomes only, elongation halts; analysis shows eEF2, the eukaryotic translocation factor, carries an ADP-ribose group.

Question 1 — Assign Reaction 1. Peptide bond formed (dipeptide exists) but no movement: translocation (the GTP-driven translocation factor) is blocked. The stuck geometry — grown chain still in the A site — is the fingerprint, because transfer happens before movement.

Question 2 — Assign Reaction 2. Initiation is fine; the failure is A-site delivery (a defective delivery factor, or a drug that blocks the A site). No delivery, no substrate for peptidyl transferase, no first bond. Distinguish this from a peptidyl-transferase inhibitor: there, the A site LOADS but the bond never forms.

Real drugs for each block: tetracycline keeps aminoacyl-tRNA out of the A site; chloramphenicol inhibits peptidyl transferase.

Question 3 — Assign Reaction 3 and name the principle. Premature release at random positions = forced early termination: the compound (puromycin behaves this way) enters the A site as an aminoacyl-tRNA look-alike, accepts the growing chain in a normal peptidyl transfer, and then falls off the ribosome carrying it. Random-length free peptides are the tell; a translocation block gives stuck ribosomes, not released chains.

Question 4 — Reaction 4 is diphtheria toxin. Explain the species specificity and the catastrophic potency. Diphtheria toxin ADP-ribosylates eEF2 — the eukaryotic translocation factor; the bacterial counterpart lacks the specially modified residue the toxin attacks, so bacterial ribosomes are spared. One toxin molecule is an ENZYME: it modifies thousands of eEF2 copies, shutting down all translation in the cell — why single molecules kill cells.

Further detail: NAD$^+$ is the ADP-ribose donor; the modified residue is diphthamide (a modified histidine); the vaccine targets the toxin, not the bacterium (toxoid); Pseudomonas exotoxin A shares the identical mechanism.

Ribosome Mechanics 🎯

Key Takeaways — Part 2

• Ribosomes: 70S (30S + 50S) prokaryotic vs 80S (40S + 60S) eukaryotic; S values do not add; peptidyl transferase is rRNA — the ribosome is a ribozyme
• Sites: A accepts, P holds the chain, E exits; initiator tRNA (fMet in bacteria) is the only tRNA to enter the P site directly, riding in with the SMALL subunit before the large one joins
• Start finding: Shine-Dalgarno pairs 16S rRNA (internal starts, polycistronic OK) vs cap-binding + scanning to the first AUG (monocistronic)
• Elongation loop: an elongation factor delivers to A (GTP; pairing check), rRNA forms the bond (pre-paid by the charging ester), a second factor translocates (GTP); ~4 high-energy bonds per residue
• Termination: release factors are tRNA-shaped proteins that make the ribosome hydrolyze the chain onto water; subunits recycle
• Polysomes: many ribosomes, one mRNA; 5'-most ribosome initiated last; bacteria couple transcription and translation because there is no nuclear envelope
• Inhibitor fingerprints: stuck chain in A site = translocation block; loaded A site, no bond = peptidyl transferase block (chloramphenicol); empty A site = delivery block (tetracycline); random released peptides = puromycin-type forced termination; ADP-ribosylated eEF2 = diphtheria toxin (catalytic, single-molecule lethal)

• Low-yield extras: eIF4E binds the cap and Kozak context marks the preferred AUG; the elongation factors are EF-Tu/eEF1 (delivery) and EF-G/eEF2 (translocation); diphtheria toxin uses NAD$^+$ to modify diphthamide, is countered by a toxoid vaccine, and shares its mechanism with Pseudomonas exotoxin A

Translation for the MCAT

Part 3 of 4 — Mutations, Folding & Post-Translational Modification

Mutation Taxonomy — Predict the Protein First

• The surveillance pathway that destroys premature-stop transcripts is nonsense-mediated decay.

Severity heuristics the exam rewards: earlier premature stops are worse; frameshifts are usually worse than missense; a nonsense mutation near the very END may be mild (most of the protein is made). Always ask WHERE in the gene, not just WHAT type.

Folding & Its Failures

• Primary sequence contains the folding information (Anfinsen's ribonuclease experiment: denatured enzyme refolds and regains activity spontaneously — no instructions needed beyond sequence).
• Chaperones (heat shock proteins, chaperonins) do not add information — they prevent aggregation and give slow folders a protected chamber. Induced by heat/stress.
• Misfolding diseases: aggregates (amyloid), prions (misfolded PrP TEMPLATES the misfolding of normal PrP — protein-only inheritance of shape), and loss-of-function via degradation (deltaF508 CFTR folds imperfectly and is destroyed by quality control even though it could conduct chloride).

Post-Translational Modifications — The Protein Isn't Done at the Stop Codon

• Two disposal systems, two jobs: proteasome = tagged, individual cytosolic/nuclear proteins (cell-cycle regulators like cyclins; misfolded ER proteins retrotranslocated out to the cytosol); lysosome = bulk material and things arriving by vesicle (endocytosed receptors, autophagy). Choosing the right one is a standard question.
• Zymogen logic: synthesize the knife folded but sheathed; unsheath (cleave) only at the destination. Pancreatitis = premature activation in the wrong compartment.

Worked Example — From Gel Bands to Mutation Mechanism

Passage-style problem. A secreted enzyme (mature form: 55 kDa) is studied in four patient cell lines. Western blots of cell LYSATE and MEDIUM (secreted fraction) show:

• Line A: lysate 55 kDa (weak), medium empty; mRNA level normal; adding a proteasome inhibitor makes a 55 kDa band accumulate strongly in the lysate.
• Line B: lysate 30 kDa band, medium empty; mRNA slightly reduced.
• Line C: lysate and medium both show 55 kDa at normal levels, but the secreted enzyme is catalytically dead.
• Line D: lysate 57 kDa, medium empty; the extra mass is removed by an enzyme that cleaves signal peptides.

Question 1 — Line B first (the easy anchor). A shortened protein with somewhat reduced mRNA is the signature of a nonsense or frameshift mutation mid-gene: translation stops early (30 kDa truncation), and the modest mRNA drop fits too — transcripts carrying a premature stop are partly degraded. The truncated protein also fails quality control for secretion.

Question 2 — Line A: full-length protein, normal mRNA, rescued by proteasome inhibition. Size is right, synthesis is right — the protein is being DEGRADED. A missense mutation causing misfolding lets ER quality control ubiquitinate the protein for proteasomal destruction. This is the deltaF508-CFTR pattern: a functional-if-it-could-fold protein lost to surveillance. The proteasome-inhibitor rescue is the deciding experiment.

Question 3 — Line C: normal size, normal secretion, dead enzyme. Everything about synthesis, folding (enough to pass QC), and trafficking is intact — the lesion must sit in the active site: a missense substitution of a catalytic residue. Function and structure are separable claims; a protein can traffic perfectly and do nothing.

Question 4 — Line D: 2 kDa TOO BIG and stuck inside. The mature size assumes the signal peptide was cleaved during ER import. An uncleaved signal (mutated cleavage site) leaves the protein 1-3 kDa heavy and tethered/mislocalized, blocking secretion. Alternatively a wholly cytosolic 57 kDa form would suggest failed ER TARGETING — the passage's cleavage-enzyme result pins it to the cleavage step instead.

The method: order the checkpoints — transcribed? translated full-length? folded/stable? localized? catalytically active? — and let each line's ONE anomalous readout name the failed checkpoint.

Mutations & Protein Maturation 🎯

Key Takeaways — Part 3

• Mutation taxonomy predicts protein outcome: silent (position 3), missense (conservative vs NONconservative — argue from side-chain chemistry; sickle Glu to Val), nonsense (truncation, often with a degraded transcript), frameshift (garbled + premature stop), in-frame indel (deltaF508), splice-site (masquerades as either)
• Location matters as much as type: early stops are severe, near-end stops can be mild
• Anfinsen: sequence encodes structure (native state = energy minimum); chaperones prevent aggregation and assist kinetics without adding information
• Folding failures: degradation of fixable proteins (deltaF508 CFTR, ubiquitinated and sent to the proteasome — proteasome-inhibitor rescue is the fingerprint), aggregation/amyloid, prions (conformational templating — shape replicates without nucleic acid)
• PTM toolkit: phosphorylation = reversible switch; cleavage = irreversible switch (zymogens, insulin); N-glycosylation starts in the ER, O-linked in Golgi; ubiquitin = proteasome address label; lipid anchors = membrane attachment
• Disposal division: proteasome for selected, tagged, cytosolic/nuclear individuals (cyclins); lysosome for bulk and vesicle-delivered cargo
• Diagnostic method: run the checkpoints (full-length? stable? localized? active?) and let the single anomalous readout name the lesion

• Low-yield extras: premature-stop transcripts are destroyed by nonsense-mediated decay; the export-and-destroy route for misfolded ER proteins is called ERAD (ER-associated degradation)

Translation for the MCAT

Part 4 of 4 — Translational Control, Toxins & MCAT Integration

Regulating Translation Itself

Transcription sets the menu; translational control sets the serving speed — faster (seconds-minutes) and reversible.

1. Global control by initiation-factor phosphorylation: stress (unfolded proteins, viral infection, starvation) activates kinases that phosphorylate the initiation factor that delivers initiator tRNA, throttling initiation — bulk translation drops while select stress mRNAs continue.

• That factor is eIF2; the kinase network is called the integrated stress response, and the virus-sensing kinase is PKR (activated by double-stranded RNA).

1. mRNA-specific control — the iron story (a classic example): iron regulatory protein (IRP) binds iron response elements (IREs, hairpins) when iron is LOW:
   • Ferritin mRNA (iron storage): IRE in the 5' UTR — bound IRP blocks ribosome scanning → LESS ferritin when iron is low (no iron to store). High iron pulls IRP off → ferritin made.
   • Transferrin receptor mRNA (iron import): IREs in the 3' UTR — bound IRP protects the mRNA from degradation → MORE receptor when iron is low (import more!). High iron releases IRP → mRNA degraded.
   • Same protein, opposite outcomes, decided purely by WHERE the element sits: 5' UTR occupancy blocks initiation; 3' UTR occupancy controls stability.
2. miRNA/RNAi: small RNAs guide silencing complexes to complementary mRNA 3' UTRs — translational repression or cleavage. (Exploited experimentally as knockdown — biotechnology lesson.)
3. mRNA half-life, poly-A tail length: additional dials — regulation does not end at the promoter.

• Short upstream open reading frames (uORFs) in the 5' UTR are another dial: they capture scanning ribosomes before the main start codon.

The Toxin & Antibiotic Gallery (Ribosome Edition)

• Diphtheria toxin's mechanistic twin: Pseudomonas exotoxin A also ADP-ribosylates human eEF2.

Selectivity rule: antibiotics exploit the 70S vs 80S difference — but remember mitochondrial ribosomes are 70S-like, the accepted explanation for the mitochondrial toxicity of some ribosome-targeting antibiotics, such as aminoglycoside hearing loss (Organelles Part 3 crossover).

• The same logic covers chloramphenicol's bone-marrow suppression and linezolid's mitochondrial toxicity.

Passage Strategy for the Whole Central Dogma

• Level-of-control test: mRNA level changed? (transcription/stability — northern, qPCR). mRNA flat but protein changed? (translation or protein stability — polysome profile and pulse-chase decide between them). Protein flat but activity changed? (PTM — phosphatase treatment, mobility shifts).
• Polysome profiling: an mRNA shifting from light monosomes into heavy polysome fractions = translationally ACTIVATED, even with total mRNA unchanged.
• Pulse-chase: label newly made protein briefly, chase, and follow — separates synthesis rate (pulse intensity) from degradation rate (chase decay).

Worked Example — The Iron Passage, End to End

Passage-style problem. Researchers treat cultured cells with an iron chelator (desferrioxamine) or with excess iron, then measure ferritin protein, ferritin mRNA, transferrin receptor (TfR) protein, and TfR mRNA.

Results: chelator — ferritin protein DOWN with ferritin mRNA UNCHANGED; TfR protein UP with TfR mRNA UP. Excess iron — the mirror image (ferritin protein up, mRNA unchanged; TfR protein and mRNA down).

Question 1 — Ferritin protein falls without any mRNA change. Name the control level and mechanism. Translational control: low iron leaves IRP iron-free and able to clamp the IRE hairpin in ferritin's 5' UTR, physically blocking 40S scanning. The transcript sits intact but unread — exactly why the mRNA measurement stays flat. Any answer invoking transcription contradicts the unchanged mRNA.

Question 2 — TfR mRNA RISES with chelation, yet the passage states the TfR gene's transcription rate is constant. Reconcile. Steady-state mRNA = synthesis minus decay (the transcription lesson's bathtub). Constant synthesis with rising level means slowed DECAY: IRP bound to the 3' UTR IREs shields TfR mRNA from nucleases. Same sensor, different element position, different mechanism — stability, not initiation.

Question 3 — Predict the phenotype of a mutation deleting the IRE from ferritin's 5' UTR. Constitutive, iron-blind ferritin translation — high ferritin even in iron deficiency. Deleting a repressor's binding SITE de-represses permanently and cannot be rescued by more IRP (cis vs trans, again).

This exact class of mutation (in the related L-ferritin IRE) causes hereditary hyperferritinemia-cataract syndrome.

Question 4 — Why does the cell wire ferritin and TfR in opposite directions off one signal? Physiology check: low iron means import more (TfR up) and stop warehousing (ferritin down); high iron means the reverse — storage protects against free iron's reactive-oxygen chemistry. Regulation questions ultimately resolve to teleology-backed-by-mechanism: state the mechanism, then confirm the direction makes physiological sense. If your predicted direction serves no purpose, re-check which UTR you assigned.

Translational Control & Integration 🎯

Key Takeaways — Part 4

• Translational control is fast and reversible: global brake = phosphorylation of the initiator-tRNA-delivering initiation factor (stress, viral infection); specific dials = UTR elements, miRNAs, poly-A length
• The iron circuit is position logic: IRP on a 5' UTR IRE blocks initiation (ferritin, low iron = less storage); IRP on 3' UTR IREs blocks decay (TfR, low iron = more import); one sensor, opposite outcomes, physiologically coherent
• Deleting a repressor's site = constitutive expression, unrescuable in trans
• Ribosome pharmacology: 30S (tetracycline A-site block, aminoglycoside misreading) vs 50S (chloramphenicol peptidyl transferase, macrolide exit tunnel); human targets — eEF2 (diphtheria, ADP-ribosylation), 28S rRNA (ricin depurination), any ribosome (puromycin mimic)
• Mitochondrial 70S-like ribosomes breach antibiotic selectivity (aminoglycoside hearing loss) — endosymbiosis with clinical consequences
• Catalytic toxins beat stoichiometric ones: covalent, irreversible, processive — single-molecule lethality
• Level-of-control algorithm: mRNA changed = transcription/stability; mRNA flat + protein changed = translation (polysome shift) or turnover (pulse-chase); protein flat + activity changed = PTM

• Low-yield extras: the phosphorylated factor is eIF2 (integrated stress response; PKR senses viral dsRNA); uORFs are another 5' UTR dial; Pseudomonas exotoxin A is diphtheria toxin's mechanistic twin (ADP-ribosylates eEF2); chloramphenicol marrow suppression and linezolid toxicity are mitochondrial-ribosome effects; L-ferritin IRE mutations cause hereditary hyperferritinemia-cataract syndrome; free iron is dangerous because of Fenton chemistry (hydroxyl-radical generation)