AP Biology: Translation – Ribosome, tRNA, Codons, Anticodons, Wobble, Elongation, Termination

Translation – Ribosome, tRNA, Codons, Anticodons, Wobble, Elongation, Termination

Concept Summary

• Translation: Process by which mRNA is decoded by ribosomes to synthesize a polypeptide chain, directly linking genotype to phenotype.
• Ribosome: Ribonucleoprotein complex (large + small subunits) that catalyzes peptide bond formation; A site (aminoacyl-tRNA entry), P site (peptidyl-tRNA holding growing chain), E site (exit for deacylated tRNA).
• tRNA: Adapter molecule with anticodon (complementary to mRNA codon) and 3’ CCA tail (covalently binds specific amino acid via aminoacyl-tRNA synthetase).
• Codon: Triplet of mRNA nucleotides encoding one amino acid (or stop signal); universal (same across most organisms) but redundant (multiple codons per amino acid).
• Wobble Hypothesis: Flexible base-pairing at the 3rd codon position (e.g., inosine in tRNA can pair with U, C, or A), reducing the number of tRNAs needed for 61 codons.

Core Questions

WHAT (definitional)

Q: What is a codon? A: A sequence of three mRNA nucleotides that specifies a single amino acid or a stop signal during translation. Trap/Clarification: Codons are read 5’?3’ on mRNA, not tRNA; the start codon (AUG) encodes methionine, not "start."

Q: What is the role of the ribosome’s P site? A: The P site holds the peptidyl-tRNA (tRNA carrying the growing polypeptide chain) and positions it for peptide bond formation with the incoming aminoacyl-tRNA in the A site. Trap/Clarification: The P site is not where the first tRNA enters (that’s the A site); it’s occupied by the initiator tRNA (Met-tRNA) during initiation.

WHY (causal/explanatory)

Q: Why is the genetic code redundant but not ambiguous? A: Multiple codons can specify the same amino acid (redundancy), but each codon specifies only one amino acid (no ambiguity), ensuring precise translation. Trap/Clarification: Redundancy-ambiguity; e.g., GUU and GUC both code for valine (redundant), but GUU never codes for leucine (unambiguous).

Q: Why is wobble pairing important? A: Wobble allows a single tRNA to recognize multiple codons (e.g., one tRNA with anticodon 3’-UAI-5’ can pair with AUU, AUC, or AUA), reducing the number of tRNAs required for translation. Trap/Clarification: Wobble occurs only at the 3rd codon position (5’ end of anticodon); the first two bases follow strict Watson-Crick pairing.

HOW (process/application)

Q: How does elongation proceed in translation? A: 1) Aminoacyl-tRNA enters the A site (codon-anticodon pairing), 2) Peptidyl transferase (rRNA ribozyme) catalyzes peptide bond formation, transferring the polypeptide from P-site tRNA to A-site tRNA, 3) Translocation: ribosome moves 5’?3’ along mRNA, shifting tRNAs to P and E sites (E-site tRNA exits). Trap/Clarification: Elongation requires GTP hydrolysis (not ATP) for tRNA entry and translocation; the polypeptide grows N-terminus-C-terminus.

Q: How is translation terminated? A: A release factor (protein, not tRNA) recognizes a stop codon (UAA, UAG, UGA) in the A site, triggering hydrolysis of the polypeptide from the P-site tRNA and ribosome disassembly. Trap/Clarification: Stop codons do not encode amino acids; release factors mimic tRNA shape to enter the A site.

CAN (conditions/possibilities)

Q: Can a tRNA with an anticodon 3’-UAC-5’ pair with a codon other than 5’-AUG-3’? A: No; the first two bases (U-A, A-U) follow strict Watson-Crick pairing, and wobble only affects the 3rd position (e.g., 3’-UAI-5’ could pair with 5’-AUG-3’ or 5’-AUC-3’). Trap/Clarification: Wobble rules apply only to the 3rd codon base; the first two positions are invariant.

Q: Under what conditions does a ribosome stall during translation? A: Ribosomes stall if 1) a stop codon is missing (no release factor), 2) a rare codon lacks corresponding tRNA (e.g., in heterologous expression), or 3) the mRNA lacks a poly-A tail (reduces efficiency in eukaryotes). Trap/Clarification: Stalling is not caused by wobble pairing; wobble actually prevents stalling by allowing flexibility in codon recognition.

Quick Facts & Traps

• Fact: Peptidyl transferase is an rRNA ribozyme (23S rRNA in prokaryotes, 28S rRNA in eukaryotes), not a protein enzyme.
• Trap: "tRNA carries codons."-Reality: tRNA carries anticodons (complementary to mRNA codons).
• Fact: Initiation factors (e.g., IF-2 in prokaryotes, eIF-2 in eukaryotes) deliver the initiator tRNA to the P site, not the A site.
• Trap: "All tRNAs have the same structure."-Reality: tRNAs vary in sequence but share a cloverleaf secondary structure and L-shaped tertiary structure.
• Fact: Polyribosomes (multiple ribosomes on one mRNA) increase translation efficiency; visible in electron micrographs as "beads on a string."
• Trap: "Translation starts at the 5’ cap in eukaryotes."-Reality: The ribosome scans from the 5’ cap to find the first AUG (Kozak sequence context matters).

Rapid-Fire True/False

• Statement: The anticodon 3’-GGC-5’ can pair with the codon 5’-CCU-3’. Answer: TRUE Why the common mistake happens: Students forget that anticodons are reverse-complementary to codons (GGC pairs with CCG, but wobble allows U to pair with G).

• Statement: Termination of translation requires a tRNA with an anticodon complementary to a stop codon. Answer: FALSE Why the common mistake happens: Stop codons are recognized by release factors (proteins), not tRNAs; students confuse them with sense codons.

• Statement: In prokaryotes, translation can begin before transcription is complete. Answer: TRUE Why the common mistake happens: Students assume all processes are temporally separated; prokaryotes lack a nucleus, enabling coupled transcription-translation.