Lecture 25 - Translation

Genetic Code Characteristics

Written in non-overlapping, three-nucleotide codes (codons); it is nearly universal, unambiguous, and degenerate (multiple codons can code for the same amino acid)[cite: 33, 34].

Codon

A sequence of three nucleotides on mRNA that codes for one specific amino acid[cite: 34].

Start Codon

Typically AUG (or occasionally GUG/UUG in bacteria); signals the initiation of polypeptide synthesis and recruits the initiator tRNA carrying methionine[cite: 37, 40].

Stop Codon

UAA, UAG, or UGA; found at the end of structural genes and signals translation release factors to end polypeptide synthesis[cite: 37, 40].

Coding Sequence (Open Reading Frame)

The genetic sequence that specifies the amino acid sequence of the polypeptide[cite: 37, 40].

Aminoacyl-tRNA Synthetase

An enzyme responsible for charging tRNAs by linking the correct amino acid to its specific tRNA; there are 20 different enzymes (one for each amino acid)[cite: 43, 45].

tRNA Charging (Aminoacylation)

The two-step process using ATP where an amino acid is activated via AMP and then transferred to the 3' acceptor stem of the tRNA[cite: 44, 45].

Anticodon

A three-nucleotide sequence on the tRNA loop that base-pairs with the complementary codon on the mRNA[cite: 44].

Wobble Hypothesis

Explains that base-pairing of the 3rd base of a codon is more flexible, allowing one tRNA to recognize and bind to more than one codon and accounting for the degenerate nature of the code[cite: 46, 47].

Ribozyme

An RNA molecule that functions as an enzyme; ribosomes act as ribozymes to catalyze peptide bond formation[cite: 49].

Ribosome Binding Sites (EPA)

The three tRNA binding sites on the large ribosomal subunit: A site (aminoacyl-tRNA binding), P site (peptidyl-tRNA binding), and E site (exit site)[cite: 49].

Translation Initiation Steps

Involves the small ribosomal subunit binding to mRNA, placement of the initiator tRNA at the start codon in the P site, and recruitment of the large ribosomal subunit using GTP[cite: 51, 52].

Translation Elongation Steps

Consists of three repeating steps: (1) Codon recognition in the A site, (2) Peptide bond formation, and (3) Translocation driven by GTP hydrolysis[cite: 53, 54, 55, 56, 57].

Translation Termination Steps

Occurs when a stop codon reaches the A site, recruiting a release factor that hydrolyzes the bond and releases the free polypeptide and subunits[cite: 58, 59, 60].

Targeting Proteins to the ER

Proteins destined for the ER or secretion begin translation in the cytosol, where a signal peptide is recognized by a signal-recognition particle (SRP) that docks with an ER receptor protein[cite: 62].

Prokaryotic Site of Translation

Transcription and translation are coupled and occur simultaneously in the cytoplasm[cite: 38, 39, 63].

Prokaryotic Ribosome Structure

70S ribosome composed of a 30S small subunit (which binds to the Shine-Dalgarno sequence) and a 50S large subunit[cite: 38, 39, 63].

Prokaryotic mRNA Organization

Polycistronic mRNA containing multiple open reading frames (ORFs) that encode different proteins from a single mRNA molecule[cite: 37, 38].

Prokaryotic mRNA End Modifications

mRNAs have no modifications (no 5' cap or poly-A tail) on their ends; they feature a 5' PPP group[cite: 37, 38, 63].

Shine-Dalgarno Sequence

The ribosome binding site with a consensus sequence of 5'-AGGAGG-3', located about 8 base pairs upstream of each start codon[cite: 37].

Prokaryotic Start Codon Selection

Translation begins at the first AUG preceded by a Shine-Dalgarno sequence[cite: 37, 63].

Prokaryotic Translation Factors

Utilizes three initiation factors and two elongation factors[cite: 63].

Eukaryotic Site of Translation

Transcription/processing occur in the nucleus, and mature mRNA is transported to the cytoplasm for translation (not coupled)[cite: 42, 63].

Eukaryotic Ribosome Structure

80S ribosome composed of a 40S small subunit (which locates the 5' cap) and a 60S large subunit[cite: 42, 63].

Eukaryotic mRNA Organization

Monocistronic mRNA, where each mRNA molecule contains only one protein-coding segment (one gene per mRNA)[cite: 40, 63].

Eukaryotic mRNA End Modifications

Features a 5' methylated cap (G-PPP) for stability and nuclear transport, and a 3' poly-A tail[cite: 40, 63].

5' UTR & 3' UTR (Eukaryotic mRNA)

The 5' UTR helps the small ribosomal subunit locate the start codon, while the 3' UTR provides stability and regulatory elements for termination[cite: 40].

Eukaryotic Start Codon Selection

The first AUG codon following the 5' cap and 5' UTR is used as the start codon (no ribosome-binding site)[cite: 40, 63].

Eukaryotic Translation Factors

Utilizes multiple initiation factors and multiple elongation factors[cite: 63].