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Translation Termination and Release Factors

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translation termination stop-codons release-factors

Core Idea

Stop codons (UAA, UAG, UGA) are recognized by release factors (RF1, RF2 in prokaryotes; eRF1 in eukaryotes) rather than tRNAs. Release factors catalyze hydrolysis of the peptidyl-tRNA bond, releasing the completed polypeptide. RF3 (prokaryotes) and eRF3 (eukaryotes) are GTPases that facilitate release factor dissociation.

How It's Best Learned

Compare stop codon recognition with elongation codon recognition. Understand why stop codons have no corresponding tRNAs. Study how release factors recognize stop codons and catalyze hydrolysis instead of aminoacyl transfer.

Common Misconceptions

Explainer

You have followed a polypeptide through translation elongation — watching aminoacyl-tRNAs deliver amino acids to the ribosomal A site, peptide bonds form in the peptidyl transferase center, and the ribosome translocate along the mRNA one codon at a time. But this cycle cannot continue forever. The cell needs a signal that says "the protein is complete, stop here." That signal is a stop codon — one of three triplets (UAA, UAG, or UGA) that encode no amino acid and instead trigger the release of the finished polypeptide.

The key difference between stop codons and sense codons is what occupies the A site when the ribosome encounters them. During elongation, each sense codon is recognized by an aminoacyl-tRNA whose anticodon is complementary. But no tRNA in any organism has an anticodon for UAA, UAG, or UGA. Instead, proteins called release factors recognize stop codons directly. In prokaryotes, RF1 recognizes UAA and UAG, while RF2 recognizes UAA and UGA — note that UAA is recognized by both, which is one reason it is the most common stop codon. In eukaryotes, a single factor called eRF1 recognizes all three stop codons. These release factors are shaped roughly like a tRNA, allowing them to fit into the A site, but instead of delivering an amino acid they position a critical glutamine residue in the peptidyl transferase center.

Once a release factor is seated in the A site, it triggers hydrolysis rather than peptide bond formation. Instead of an amino group from a new amino acid attacking the ester bond linking the polypeptide to the P-site tRNA, a water molecule performs the attack. This breaks the bond between the completed polypeptide and the final tRNA, releasing the protein from the ribosome. A GTPase — RF3 in prokaryotes, eRF3 in eukaryotes — then uses GTP hydrolysis to dissociate the release factor from the ribosome.

After termination, the ribosome is still sitting on the mRNA with a deacylated tRNA in the P site. Ribosome recycling factor (RRF) and EF-G in prokaryotes, or ABCE1 in eukaryotes, split the ribosome into its large and small subunits so they can be reused. The entire process — from stop codon recognition through polypeptide release to ribosome disassembly — takes only a fraction of a second, yet errors here have outsized consequences. A failure to terminate produces an abnormally long protein that is usually non-functional and potentially toxic, which is why quality control mechanisms like nonsense-mediated mRNA decay exist to catch mRNAs with premature stop codons and aberrant termination events.

Practice Questions 5 questions

Prerequisite Chain

Counting to 10 → Counting to 20 → Understanding Zero → The Number Zero → Counting to Five → One-to-One Correspondence → Combining Small Groups Within 5 → Addition Within 10 → Addition Within 20 → Two-Digit Addition Without Regrouping → Two-Digit Addition with Regrouping → Addition Within 100 → Repeated Addition as Multiplication → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Two-Digit by One-Digit Division → Division with Remainders → Remainders and Quotients in Division → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Reading and Writing Decimals → Comparing and Ordering Decimals → Adding and Subtracting Decimals → Multiplying Decimals → Dividing Decimals → Dividing Fractions → Mixed Number Arithmetic → Order of Operations → Integer Order of Operations → Variable Expressions → Combining Like Terms → One-Step Equations → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Iterated Integrals and Fubini's Theorem → Double Integrals in Cartesian Coordinates → Double Integrals over Rectangular Regions → Double Integrals in Polar Coordinates → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Green's Theorem → Surface Integrals and Flux of Vector Fields → Surface Integrals and Flux of Vector Fields → Divergence Theorem: Flux and Outflow → Divergence Theorem → Electric Flux → Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → The Electromagnetic Spectrum → Blackbody Radiation and Planck's Law → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → Heisenberg Uncertainty Principle → Wavefunction and the Born Rule → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Scattering Theory → Introduction to Scattering Theory → Partial Wave Analysis in Scattering → Spin Angular Momentum → Electron Spin and Intrinsic Magnetic Moment → Stern-Gerlach Experiment: Spin Quantization and Measurement → Electron Diffraction and Matter Wave Properties → Davisson-Germer Experiment: Crystal Diffraction of Electrons → Electron Diffraction and Matter Wave Interference → Wavefunctions and Probability Density Interpretation → Quantum Superposition and Linear Combinations of States → Quantum Operators and Observables → Canonical Commutation Relations and Uncertainty → Heisenberg Uncertainty Principle and Measurement Limits → Time-Independent Schrödinger Equation and Eigenvalues → Hydrogen Atom in Quantum Mechanics → Spectral Lines and Energy Transitions → Selection Rules for Atomic Transitions → LS and jj Coupling Schemes in Multi-Electron Atoms → Pauli Exclusion Principle and Antisymmetric Wavefunctions → Electron Configuration and the Aufbau Principle → The Periodic Table and Atomic Electronic Structure → The Periodic Table → Electron Configuration → Periodic Trends → Ionization Energy → Ionic Bonding → Lewis Structures → Resonance Structures and Delocalized Electrons → Resonance and Formal Charge → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Organic Reaction Mechanisms and Arrow Pushing → Electrophilic Addition to Alkenes → Aromaticity and Benzene → DNA Structure → Central Dogma of Molecular Biology → Transcription: DNA to RNA → RNA Types and Structure → RNA Processing and Splicing → Translation: RNA to Protein → Translation: Initiation and Elongation → Translation Initiation: Start Codons and Ribosomal Scanning → mRNA Translation Start Sites and Initiation → Ribosomal Initiation Factors and Initiator tRNA → Translation Elongation and Elongation Factors → Translation Termination and Release Factors

Longest path: 181 steps · 786 total prerequisite topics

Prerequisites (2)

Translation Elongation and Elongation Factorshard Translation Elongation and Termination: Peptide Bond Formationsoft

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