Translation: The Genetic Code and Protein Synthesis

Chapter 19: Translation

Introduction to Translation

Translation is the process by which the genetic information encoded in messenger RNA (mRNA) is used to synthesize proteins. This process is fundamental to gene expression and involves the decoding of nucleotide sequences into amino acid sequences.

• Gene: A functional unit of DNA that encodes one or more polypeptides or functional RNA.

• Redundancy of the Genetic Code: Multiple codons can encode the same amino acid, providing a buffer against mutations.

Triplet Code

The genetic code is read in sets of three nucleotides, called codons, each of which specifies a particular amino acid.

• Codon: Sequence of three nucleotides in mRNA that codes for an amino acid.

• Number of Codons: 64 possible codons (43 combinations).

• Start Codon: AUG (codes for methionine).

• Stop Codons: UAA, UAG, UGA (signal termination of translation).

• Reading Frame: The way nucleotides are grouped into codons; shifting the frame changes the resulting amino acid sequence.

Mutations Affecting Translation

Mutations are changes in the DNA sequence that can affect protein synthesis in various ways.

• Missense Mutation: A single nucleotide change results in a codon that codes for a different amino acid.

• Nonsense Mutation: A codon is changed to a stop codon, leading to premature termination of translation.

• Insertion/Deletion (Indels): Addition or removal of nucleotides can cause frameshift mutations if not in multiples of three, altering the reading frame.

• Non-stop Mutation: A stop codon is converted into an amino acid codon, causing translation to continue beyond the normal endpoint.

• Silent Mutation: A change in the DNA sequence that does not alter the amino acid sequence due to the redundancy of the genetic code.

• Duplication/Translocation: Segments of DNA are duplicated or moved, potentially altering gene function.

Types of RNA in Translation

Three main types of RNA are involved in translation, each with a specific role.

• Messenger RNA (mRNA): Carries genetic information from DNA to the ribosome, where amino acids are assembled in the correct order.

• Transfer RNA (tRNA): Brings the correct amino acids to the ribosome during translation. Each tRNA has an anticodon that pairs with the mRNA codon.

• Ribosomal RNA (rRNA): A major component of ribosomes, rRNA helps guide and catalyze the assembly of polypeptides.

Ribosome Structure and Function

Ribosomes are the molecular machines that synthesize proteins. They are composed of rRNA and proteins and have distinct sites for tRNA binding and peptide bond formation.

• Composition: Ribosomes consist of a large and small subunit, each made of rRNA and proteins.

• A Site (Aminoacyl site): Binds incoming aminoacyl-tRNA carrying an amino acid.

• P Site (Peptidyl site): Holds the tRNA with the growing polypeptide chain.

• E Site (Exit site): Where discharged tRNAs leave the ribosome.

tRNA Charging and Accuracy

Each tRNA must be charged with the correct amino acid by aminoacyl-tRNA synthetases. Accuracy is critical for proper protein synthesis.

• Each cell has 20 different synthetases, one for each amino acid.

• These enzymes attach the correct amino acid to the matching tRNA, forming a high-energy ester bond.

• Proofreading: Synthetases check for correct matches before the tRNA is released.

Wobble Hypothesis

The wobble hypothesis explains how some tRNAs can recognize more than one codon due to flexible base pairing at the third position of the codon.

• Codon Recognition: Many tRNAs can bind to more than one codon, increasing efficiency.

• Pairing Flexibility: The third position of the codon allows for non-standard base pairing, reducing the number of tRNAs needed.

Translation Initiation

Translation begins with the assembly of the ribosome on the mRNA and the recruitment of the initiator tRNA.

• Initiator tRNA: Carries methionine and binds to the start codon (AUG).

• Initiation Factors: Proteins (eIFs in eukaryotes) that help assemble the ribosome and position the mRNA and tRNA correctly.

• GTP Hydrolysis: Provides energy for the assembly process.

Steps of Translation

Translation proceeds through three main stages: initiation, elongation, and termination.

1. Initiation: Ribosome assembles on the mRNA, and the initiator tRNA binds to the start codon.

2. Elongation: tRNAs bring amino acids to the ribosome, peptide bonds are formed, and the ribosome moves along the mRNA.

3. Termination: When a stop codon is reached, release factors promote the release of the newly synthesized polypeptide.

Peptide Bond Formation

The peptide bond between amino acids is catalyzed by peptidyl transferase activity, which is a function of rRNA in the ribosome.

• Peptidyl Transferase: Catalyzes peptide bond formation, making the ribosome a ribozyme (an RNA molecule with catalytic activity).

Protein Folding and Chaperones

After translation, proteins must fold into their correct three-dimensional shapes. Chaperones assist in this process.

• Chaperones: Proteins such as Hsp70 and Hsp60 help newly synthesized proteins fold correctly and prevent aggregation.

• ATP Hydrolysis: Provides energy for chaperone function.

Summary Table: Types of Mutations Affecting Translation

Mutation Type	Effect on Protein	Example
Missense	One amino acid replaced by another	Sickle cell anemia
Nonsense	Premature stop codon, truncated protein	Cystic fibrosis (some cases)
Frameshift	Altered reading frame, abnormal protein	Tay-Sachs disease
Silent	No change in amino acid sequence	Synonymous codon change

Key Equations and Concepts

• Number of possible codons: $4^3 = 64$

• Peptide bond formation: Catalyzed by rRNA, not protein

Additional info:

• Chaperonins (e.g., GroEL/GroES) provide an isolated environment for protein folding, using ATP hydrolysis to assist in the process.

• Protein folding is essential for proper function; misfolded proteins can lead to disease.