Translation &
Post-Translational Modification
Master how ribosomes decode mRNA into proteins and how post-translational modifications regulate protein function: essential knowledge for MCAT success.
MCAT Biological Foundations
What Is Translation?
Translation is the process by which the ribosome reads the nucleotide sequence of mRNA and assembles a corresponding chain of amino acids to form a polypeptide. It is the second major step in the Central Dogma of molecular biology:
Translation occurs in the cytoplasm (eukaryotes) or at the ribosome directly coupled to transcription (prokaryotes). It requires mRNA, tRNA, ribosomes, amino acids, and various protein factors.
Key Players in Translation
mRNA
Carries the genetic code as a sequence of codons (3-nucleotide units) from the nucleus to the ribosome.
tRNA
Adapter molecule with an anticodon that base-pairs with mRNA and carries the correct amino acid.
Ribosome
Large + small subunits catalyze peptide bond formation. Has A, P, and E sites for tRNA binding.
Ribosome Architecture
Three Phases of Translation
1
Initiation: Small ribosomal subunit binds mRNA at the 5' cap (eukaryotes) and scans for the start codon AUG. Initiator tRNA (Met-tRNA) occupies the P site. Large subunit joins.
2
Elongation: Charged tRNA enters the A site, peptide bond forms (peptidyl transferase), ribosome translocates one codon toward the 3' end. Repeat cycle.
3
Termination: A stop codon (UAA, UAG, UGA) enters the A site. Release factors trigger hydrolysis of the polypeptide from the final tRNA. Ribosome subunits dissociate.
The Elongation Cycle
Codon Degeneracy and Wobble
The genetic code has 64 codons but only 20 amino acids. Most amino acids are encoded by multiple codons: this is called degeneracy. Variation usually occurs at the 3rd (wobble) position.
Wobble Hypothesis
The 1st position of the anticodon (reads 3rd codon position) can form non-standard base pairs: e.g., inosine (I) pairs with U, C, or A.
Silent Mutations
A nucleotide change at the wobble position often does not change the amino acid: this is why degeneracy provides a buffer against point mutations.
Prokaryotic vs Eukaryotic Translation
| Prokaryotes | Eukaryotes | |
|---|---|---|
| Ribosome | 70S (\(30S + 50S\)) | 80S (\(40S + 60S\)) |
| Initiation | Shine-Dalgarno sequence | 5' cap scanning / Kozak |
| First amino acid | fMet (formyl-methionine) | Met (methionine) |
| Coupling | Transcription & translation coupled | Separated (nucleus vs cytoplasm) |
| Polycistronic mRNA | Yes | No (monocistronic) |
| Antibiotic targets | Chloramphenicol, tetracycline | Cycloheximide, diphtheria toxin |
What Are Post-Translational Modifications?
After translation, proteins often require chemical modifications before they become fully functional. These post-translational modifications (PTMs) regulate protein activity, localization, interactions, and degradation.
PTMs can be reversible (phosphorylation, acetylation) or irreversible (proteolytic cleavage, ubiquitination leading to degradation).
Major Types of PTMs
Phosphorylation
Addition of \(\text{PO}_4^{3-}\) to Ser, Thr, or Tyr by kinases. Removed by phosphatases. Regulates signaling cascades.
Glycosylation
Addition of sugar chains. N-linked (Asn) starts in ER; O-linked (Ser/Thr) occurs in Golgi. Critical for cell recognition.
Ubiquitination
Attachment of ubiquitin tags targeting proteins for proteasomal degradation. Key for cell cycle regulation.
Proteolytic Cleavage
Removal of signal peptides or activation of zymogens (e.g., trypsinogen \(\rightarrow\) trypsin). Irreversible.
Acetylation / Methylation
Histone acetylation opens chromatin. Methylation of non-histone proteins affects localization and interactions.
Phosphorylation: The Master Switch
The Secretory Pathway
How to Solve Translation Problems
1
Identify the reading frame: Locate the AUG start codon. Read in triplets from that point (5' \(\rightarrow\) 3').
2
Translate codons: Use the codon table. Remember wobble: the 3rd position is most tolerant of substitutions.
3
Check for stop codons: UAA, UAG, UGA terminate translation. No amino acid is added for stop codons.
4
Consider PTMs: Ask what happens next: is the protein phosphorylated? Glycosylated? Does it have a signal peptide for the ER?
5
Track the mutation impact: For mutation questions, check if the amino acid changes (missense), stays the same (silent), or creates a premature stop (nonsense).
Example: Translating an mRNA Sequence
1
Find AUG: Start codon is at position 1. Reading frame established.
2
Read triplets: AUG (Met) - CCU (Pro) - UAC (Tyr) - UGA (Stop)
3
Result: Met-Pro-Tyr (3 amino acid polypeptide). Translation terminates at UGA.
Example: Mutation Effects on Translation
Normal
GAG codes for glutamic acid (charged, hydrophilic). Normal hemoglobin function.
Mutant
GUG codes for valine (nonpolar, hydrophobic). Creates a hydrophobic patch on the protein surface.
Example: PTM in Insulin Processing
1
Signal peptide cleavage: Preproinsulin enters the ER. The signal peptide is cleaved \(\rightarrow\) proinsulin.
2
Disulfide bond formation: Two interchain and one intrachain disulfide bonds form in the ER (between A and B chains).
3
Proteolytic cleavage: In the Golgi, the C-peptide is excised by endoproteases \(\rightarrow\) mature insulin (A + B chains linked by disulfides).
4
Secretion: Mature insulin is packaged into secretory vesicles and released upon glucose stimulation.
Common Mistakes
✕ Confusing 70S and 80S
Prokaryotic ribosomes are 70S (\(30S+50S\)). Eukaryotic are 80S (\(40S+60S\)). Svedberg units do not add linearly.
✕ Reading Wrong Strand
mRNA is read 5' \(\rightarrow\) 3'. The template DNA strand is 3' \(\rightarrow\) 5'. Don't translate the coding (sense) DNA strand directly.
✕ Forgetting PTM Steps
The initial polypeptide is NOT the final protein. Signal peptide cleavage, folding, and modifications are required for function.
✕ Mixing Up Antibiotic Targets
Antibiotics targeting 30S/50S (prokaryotic) won't affect eukaryotic 40S/60S. This is the basis of selective toxicity.
✕ Ignoring Wobble Position
Not all point mutations change the amino acid. A substitution at the 3rd codon position is often silent due to degeneracy.
Summary: Translation & PTMs at a Glance
| Concept | Key Details | MCAT Focus |
|---|---|---|
| Initiation | AUG, small subunit, initiation factors | Shine-Dalgarno vs 5' cap |
| Elongation | A/P/E sites, peptidyl transferase, GTP | Energy cost per amino acid |
| Termination | UAA, UAG, UGA, release factors | Stop codons are not suppressor tRNAs |
| Phosphorylation | Kinase adds, phosphatase removes | Ser, Thr, Tyr residues |
| Glycosylation | N-linked (ER), O-linked (Golgi) | Cell surface recognition |
| Ubiquitination | Ubiquitin tags \(\rightarrow\) proteasome | Protein turnover, cell cycle |
Question 1 of 5
During translation, the peptidyl transferase activity that catalyzes peptide bond formation is associated with which component?
The ribosome is a ribozyme: the 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes) in the large subunit catalyzes peptide bond formation, not a protein enzyme.
Question 2 of 5
A mutation changes a codon from GAA to GAG. Both encode glutamic acid. What type of mutation is this?
Since both GAA and GAG code for glutamic acid, the amino acid sequence is unchanged. This is a silent (synonymous) mutation, typically occurring at the wobble (3rd) position of the codon.
Question 3 of 5
A protein destined for secretion is first translated with an N-terminal signal peptide. Where is this signal peptide cleaved?
Signal peptide recognition by SRP (signal recognition particle) directs the ribosome to the rough ER. Signal peptidase in the ER lumen cleaves the signal peptide as the polypeptide enters the ER.
Question 4 of 5
Which of the following is a reversible post-translational modification?
Phosphorylation is reversible: kinases add the phosphate group, and phosphatases remove it. All other options involve proteolytic cleavage, which permanently breaks peptide bonds and cannot be reversed.
Question 5 of 5
In prokaryotes, how does the small ribosomal subunit identify the correct start site on mRNA?
Prokaryotic mRNA lacks a 5' cap. Instead, the Shine-Dalgarno sequence (a purine-rich region ~8 nucleotides upstream of AUG) base-pairs with the 16S rRNA of the 30S subunit to position the ribosome at the correct start codon.