Translation

Translation is the process by which the nucleotide sequence of an mRNA is decoded into the amino acid sequence of a protein — the final step of gene expression (DNA → RNA → Protein). For the MCAT, focus on the logic of each stage, why each molecular player is built the way it is, and how prokaryotic and eukaryotic translation differ.

Priority labels: Must know = cold; Know the logic = mechanism not names; Passage-level = recognize, don't memorize; Optional = skippable.

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The Big Picture: Translation in Context

Must know

Anchor translation in the central dogma: it is the RNA-to-Protein step. Three RNAs do distinct, non-interchangeable jobs — mRNA is the blueprint (read 5′→3′), tRNA is the adaptor that brings the right amino acid to each codon, and rRNA (in the ribosome) is the enzyme that forms peptide bonds.

The ribosome reads codons on mRNA 5′→3′ and pairs them with tRNA anticodons antiparallel (3′→5′) — the same base-pairing rule as DNA and transcription.

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Roles of mRNA, tRNA, and rRNA

Messenger RNA (mRNA): The Blueprint

Must know

mRNA carries a continuous string of codons, each a nucleotide triplet encoding one amino acid (or a stop signal).

The genetic code:

• Degenerate (redundant): most amino acids have more than one codon; synonymous codons usually differ at the third (wobble) position.
• Non-overlapping and comma-free: each nucleotide is read once, three at a time, with no spacers.
• Nearly universal: the same code operates in almost all organisms (argument for common ancestry).
• Start codon AUG — methionine in eukaryotes; formyl-methionine (fMet) in prokaryotes.
• Stop codons UAA, UAG, UGA — no tRNA reads them; release factors bind instead.

Untranslated/regulatory features: the 5′ UTR holds the ribosome-binding site (prokaryotes base-pair the 5′ UTR to small-subunit rRNA; eukaryotes recognize the 5′ 7-methylguanosine (m⁷G) cap and scan to the first AUG). The 3′ UTR and eukaryotic poly-A tail affect mRNA stability. Eukaryotic mRNA is usually monocistronic (one protein); prokaryotic mRNA is often polycistronic (multiple proteins).

Transfer RNA (tRNA): The Adaptor

Must know

tRNA is the adaptor that physically links a codon to its amino acid. It folds into a cloverleaf secondary structure and an L-shaped 3D structure. The two functional ends:

1. Anticodon loop: its three-nucleotide anticodon base-pairs antiparallel with the mRNA codon.
2. 3′ CCA-OH acceptor end: the universal site where the amino acid is covalently attached.

Charging: aminoacyl-tRNA synthetases (aaRS) attach the correct amino acid to the correct tRNA (one per amino acid). This costs ATP ($\text{ATP} \rightarrow \text{AMP} + \text{PP}_i$, then $\text{PP}_i \rightarrow 2\,\text{P}_i$ — net two high-energy bonds). The charged product is an aminoacyl-tRNA. This is the key fidelity step: a mischarged tRNA puts the wrong amino acid into the protein.

Know the logic

Wobble: the codon's third position pairs loosely with the first anticodon position, so one tRNA can read several synonymous codons (fewer than 61 tRNAs needed). Inosine in the anticodon is especially flexible (pairs with U, C, or A).

Ribosomal RNA (rRNA): The Catalytic Core

Must know

rRNA is a ribozyme — it catalyzes peptide bond formation, supporting the RNA World hypothesis.

Reference — subunit composition:

Feature	Prokaryote	Eukaryote
Whole ribosome	70S	80S
Large subunit	50S (23S + 5S rRNA)	60S (28S + 5.8S + 5S rRNA)
Small subunit	30S (16S rRNA)	40S (18S rRNA)

"S" = Svedberg units (sedimentation rate, not additive — hence 30S + 50S = 70S).

Functions by subunit: the small subunit (30S/40S) decodes the mRNA (checks codon-anticodon pairing); the large subunit (50S/60S) holds the peptidyl transferase center (PTC) — the rRNA ribozyme active site — and the exit tunnel for the growing polypeptide.

Quick check: Why is the ribosome considered a ribozyme?

Answer: The peptidyl transferase activity that catalyzes peptide bond formation resides in the rRNA of the large subunit, not in any ribosomal protein. Since RNA does the catalysis, the ribosome is a ribozyme.

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Role and Structure of Ribosomes

The Three Sites: A, P, and E

Must know

• A site (Aminoacyl): accepts the incoming aminoacyl-tRNA; where codon-anticodon pairing is checked.
• P site (Peptidyl): holds the tRNA carrying the growing chain; the initiator tRNA starts here.
• E site (Exit): holds the uncharged tRNA as it leaves.

After each peptide bond, the ribosome translocates one codon (3 nt) 5′→3′, moving tRNAs A → P → E.

Polysomes

Must know

A single mRNA can be translated by multiple ribosomes at once — a polyribosome (polysome) — amplifying protein output from one mRNA.

Quick check: If a mutant tRNA has a deletion at its 3′ CCA end, what happens?

Answer: Aminoacyl-tRNA synthetases require the intact CCA-3′ end to charge the tRNA. Without it, the tRNA can't be charged or deliver an amino acid — translation is blocked for that codon.

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Initiation, Elongation, Termination, and Co-factors

Initiation

Must know

Initiation assembles the ribosome around the start codon with the initiator tRNA in the P site (the most regulated step).

In prokaryotes, the 30S subunit (with initiation factors) is positioned over AUG by base-pairing between the mRNA 5′ region and small-subunit rRNA; the fMet initiator tRNA enters the P site, the 50S joins, and GTP hydrolysis completes the 70S complex.

In eukaryotes, the 40S subunit (with initiation factors and the Met initiator tRNA) is recruited to the 5′ m⁷G cap and scans to the start AUG; the 60S joins to form the 80S complex.

Key difference: prokaryotes position the ribosome by base-pairing in the 5′ UTR; eukaryotes recognize the 5′ cap and scan. This is why prokaryotic mRNAs can be polycistronic. (You do not need the individual initiation-factor names.)

Elongation

Must know

A cyclic three-step process repeated per codon:

1. Decoding: an elongation factor + GTP delivers the correct aminoacyl-tRNA to the A site; correct pairing triggers GTP hydrolysis; wrong tRNAs are rejected (fidelity).
2. Peptide bond formation: the peptidyl transferase center (rRNA ribozyme) transfers the chain from the P-site tRNA to the A-site amino acid. The P-site tRNA is now uncharged.
3. Translocation: the ribosome moves one codon 5′→3′ (elongation factor + GTP); peptidyl-tRNA A→P, deacylated tRNA P→E and out, A site open for the next aa-tRNA.

Energy cost: ~4 high-energy phosphate bonds per amino acid (2 to charge the tRNA + 2 GTP for decoding and translocation). Translation is expensive, so cells regulate it tightly.

Termination

Must know

When a stop codon enters the A site, no tRNA reads it; a protein release factor binds (mimicking a tRNA) and triggers the PTC to hydrolyze the polypeptide off the P-site tRNA. The protein is released and the ribosome dissociates for reuse.

Quick check: A drug mimics a stop codon and occupies the A site irreversibly. What happens?

Answer: The ribosome stalls — no aa-tRNA can enter, no peptide bond forms, no translocation, and the polypeptide can't be released (no release factor can access the A site). Translation is blocked. (Because 70S and 80S ribosomes differ, many antibiotics selectively block the bacterial ribosome — the basis of selective toxicity.)

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Post-Translational Modification of Proteins

Must know

Translation makes a linear polypeptide; most proteins need further processing. These post-translational modifications (PTMs) expand the protein's chemistry beyond the 20 amino acids.

Proteolytic Cleavage

Must know

Many proteins are made as inactive precursors (zymogens/proproteins) activated by cleavage. Passage-level examples: insulin (preproinsulin → proinsulin → mature insulin); digestive zymogens (trypsinogen → trypsin); clotting factors (coagulation cascade).

Phosphorylation

Must know

Protein kinases add phosphate to the hydroxyl of serine, threonine, or tyrosine (using ATP); phosphatases remove it. The most common reversible PTM and central to cell signaling.

Glycosylation

Must know

Adding carbohydrate chains. Two types:

• N-linked: sugar on the amide nitrogen of asparagine (consensus Asn-X-Ser/Thr, X ≠ Pro); begins in the rough ER, refined in the Golgi.
• O-linked: sugar on the hydroxyl of serine or threonine; mainly in the Golgi.

Surface glycoproteins/glycolipids form the glycocalyx (cell recognition, blood group antigens).

Ubiquitination

Know the logic

Attaching ubiquitin to lysine (via E1/E2/E3 enzymes). Polyubiquitination tags proteins for degradation by the 26S proteasome — central to controlling protein levels and the cell cycle.

Acetylation

Must know

Adding an acetyl group (from acetyl-CoA) to the N-terminus or to lysine. Histone acetylation neutralizes histone charge, loosening histone-DNA contacts and generally opening chromatin for transcription; HDACs reverse this, silencing genes.

Disulfide Bond Formation

Must know

Oxidation of two cysteine residues to a disulfide bond (–S–S–), stabilizing structure (antibodies, insulin). Forms in the oxidizing ER lumen, not the reducing cytoplasm.

Methylation and Lipidation

Must know

Methylation (often on lysine/arginine) is a key histone epigenetic mark that can activate or repress transcription. Lipidation attaches a lipid to anchor a protein to a membrane (e.g., tethering Ras to the plasma membrane).

Signal Peptides and Protein Targeting

Must know

Proteins bound for the ER/Golgi/lysosomes/membrane/secretion carry an N-terminal signal peptide (~15–30 hydrophobic residues). The Signal Recognition Particle (SRP) recognizes it, halts translation, and docks the ribosome to the rough ER. Translation resumes co-translationally into the ER lumen (or membrane), and signal peptidase cleaves the signal peptide. Proteins without a signal are made on free ribosomes and stay in the cytosol.

Chaperone-Assisted Folding

Must know

Molecular chaperones (e.g., Hsp70) bind exposed hydrophobic regions on the emerging polypeptide, prevent misfolding/aggregation, and use ATP to assist folding. Chaperonins are barrel-shaped chambers where a protein folds in isolation.

Quick check: A protein has a signal peptide but ends up in the cytoplasm due to a mutation that destroys SRP binding. What would you predict?

Answer: Without SRP targeting, it's translated on a free ribosome in the cytoplasm. It won't enter the ER, won't be N-glycosylated, won't reach its proper destination, and may misfold without the ER's oxidizing environment for disulfide bonds.

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Common Confusions & Tricks

1. Svedberg units are not additive.
30S + 50S = 70S (not 80S). Svedberg units reflect sedimentation (shape + mass), not mass alone. Don't add them.

2. fMet vs. Met.
Prokaryotes start with formyl-methionine (fMet); eukaryotes start with plain Met.

3. "The ribosome is an enzyme" — the RNA is the enzyme.
Peptidyl transferase activity is rRNA; ribosomal proteins are mainly structural (RNA World relevance).

4. Stop codons have no cognate tRNA.
There is no "stop tRNA." Release factors (proteins) recognize stop codons.

5. The anticodon reads 3′→5′ while the codon is read 5′→3′.
Antiparallel pairing, same as all nucleic acids. Don't flip them.

6. Wobble is at codon position 3 / anticodon position 1 (5′ end).
Flexibility is at the third codon nucleotide pairing the first anticodon nucleotide.

7. Start-codon recognition differs by domain.
Prokaryotes base-pair in the 5′ UTR; eukaryotes recognize the 5′ cap and scan. Both deliver the small subunit + initiator tRNA to AUG.

8. Signal peptides are removed.
The signal peptide is cleaved in the ER — it's not in the mature protein.

9. Phosphorylation targets Ser, Thr, Tyr.
Always the hydroxyl group. Kinases use ATP; phosphatases hydrolyze it off.

10. N-linked → Asn; O-linked → Ser/Thr.
Hook: N-linked → Nitrogen → asN (asparagine). O-linked → Oxygen → Ser/Thr (OH).

11. ER ribosomes vs. free ribosomes are the same ribosomes.
Identical 80S ribosomes — SRP just docks one to the ER when it's making a signal-peptide protein.

12. Antibiotic selectivity comes from 70S vs. 80S.
Many antibiotics block the bacterial ribosome without harming ours. (No need to memorize which drug hits which subunit.)

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Key Takeaways

The Three RNA Players

• mRNA: carries codons (5′→3′); AUG = start; UAA/UAG/UGA = stop; prokaryotes position via 5′ UTR, eukaryotes via 5′ cap + scanning.
• tRNA: anticodon pairs antiparallel with codon; CCA-3′ end carries the amino acid; charged by aminoacyl-tRNA synthetases ($\text{ATP} \rightarrow \text{AMP} + \text{PP}_i$); wobble at the 3rd codon position.
• rRNA: structural + catalytic core; rRNA = ribozyme (peptidyl transferase).

Ribosome Composition

	Prokaryote	Eukaryote
Full ribosome	70S	80S
Small subunit	30S	40S
Large subunit	50S	60S
Initiator aa	fMet	Met

Ribosome Sites (A → P → E)

• A site: incoming aminoacyl-tRNA
• P site: peptidyl-tRNA (chain grows here); initiator tRNA starts here
• E site: exiting, deacylated tRNA

Elongation and Termination

• Elongation: aa-tRNA enters A site (elongation factor + GTP) → peptide bond by peptidyl transferase center → translocation (elongation factor + GTP); ~4 high-energy bonds per amino acid.
• Termination: stop codons recognized by release factors (proteins, not tRNAs) → polypeptide released → ribosome dissociates.

Key Post-Translational Modifications

PTM	Residue(s)	Location	Reversible?
Phosphorylation	Ser, Thr, Tyr	Cytoplasm/nucleus	Yes (phosphatases)
N-glycosylation	Asn (Asn-X-Ser/Thr)	ER → Golgi	Mostly no
O-glycosylation	Ser, Thr	Golgi	Partially
Ubiquitination	Lys	Cytoplasm	Yes
Acetylation (histones)	Lys (histone tails)	Nucleus	Yes (HDACs)
Disulfide bonds	Cys–Cys	ER lumen / extracellular	Oxidizing env.
Methylation	Lys, Arg	Nucleus (histones)	Yes
Lipidation	Cys / N- or C-terminus	Membranes	Variable
Proteolytic cleavage	Peptide bonds	ER/extracellular	No
Signal peptide removal	N-terminus	ER	No

Essential Conceptual Anchors

• Peptidyl transferase = rRNA ribozyme (supports RNA World hypothesis)
• Antibiotic selectivity exploits 70S vs. 80S differences
• SRP routes signal-peptide proteins to the ER co-translationally
• Chaperones prevent misfolding and use ATP to assist folding
• Polysomes = multiple ribosomes on one mRNA
• The genetic code is degenerate, non-overlapping, comma-free, and nearly universal