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Translation Termination and Release Factors

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Translation Elongation and Elongation Factors Translation Elongation and Termination: Peptide Bond Formation→

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

Assuming stop codons are rare or non-essential; they delimit all coding sequences.
Not recognizing that read-through of stop codons (rare tRNAs with stop-codon anticodons) can extend proteins in specialized genes.
Thinking the peptide must be released from the ribosome immediately; recycling factors then dissociate the ribosomal subunits and release mRNA.

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

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → 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 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → 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 → Sine, Cosine, and Tangent Ratios → 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 → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → 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 → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with 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 → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → 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 → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Enzyme Structure and Function → Transcription: DNA to RNA → RNA Types and Structure → RNA Structure and Intramolecular Base Pairing → RNA Processing and Splicing → Translation: RNA to Protein → Ribosomes: Protein Synthesis Machines → 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: 216 steps · 1099 total prerequisite topics

Prerequisites (2)

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

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