Premed · Premed · General Biology 1
Lecture 22: Translation and the Genetic Code
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the features of the genetic code (triplet, degenerate, universal, non-overlapping)
- Explain the structure and function of tRNA and aminoacyl-tRNA synthetases
- Describe the structure and function of ribosomes
- Explain the three stages of translation: initiation, elongation, and termination
- Describe post-translational modifications and protein targeting
Lecture Content
I. The Genetic Code
The genetic code is the set of rules by which the nucleotide sequence of mRNA is translated into the amino acid sequence of a protein. The code is read in codons--three-nucleotide sequences in the mRNA, each specifying one amino acid or a stop signal. With four possible nucleotides at each of three positions, there are 4^3 = 64 possible codons. Of these, 61 specify amino acids while three--UAA, UAG, and UGA--are stop codons that signal the end of translation. The codon AUG serves a dual role: it codes for methionine and acts as the universal start codon, establishing where translation begins.
The genetic code has several important properties. It is triplet (three nucleotides per codon) and non-overlapping (codons are read sequentially without sharing nucleotides). It is comma-free, meaning there are no gaps or punctuation between codons; instead, the reading frame is established by the start codon and maintained throughout. The code is degenerate (redundant)--most amino acids are specified by more than one codon, though each codon specifies only one amino acid. This redundancy is concentrated at the third position of the codon, known as the wobble position, where variation is most tolerated. Wobble base pairing allows a single tRNA to recognize multiple codons that differ only at this third position, reducing the total number of tRNA species a cell needs. Finally, the code is nearly universal--virtually all organisms from bacteria to humans use the same code, with only minor exceptions in mitochondria and some protists. This universality is among the strongest pieces of evidence for a single common ancestor of all life on Earth.
The code was cracked through a series of elegant experiments. Nirenberg and Matthaei (1961) used synthetic mRNA composed entirely of uracil (poly-U) in a cell-free translation system and discovered that it directed the synthesis of polyphenylalanine, establishing that UUU codes for phenylalanine. Khorana extended this work by synthesizing mRNAs with repeating di- and trinucleotide sequences, systematically decoding additional codons until the entire table was complete.
II. Transfer RNA (tRNA)
Transfer RNAs are the adapter molecules that bridge the nucleic acid language of mRNA codons and the amino acid language of proteins. Each tRNA is a relatively small molecule of approximately 76-90 nucleotides that folds into a characteristic cloverleaf secondary structure, which in three dimensions adopts a compact L-shape. Several structural features are critical to tRNA function. The anticodon, a three-nucleotide sequence located on the anticodon loop, is complementary to the mRNA codon and determines which codon the tRNA recognizes. At the opposite end of the L-shaped molecule, the 3' acceptor stem terminates in the universally conserved sequence CCA, and the amino acid is covalently attached to the 3'-OH of the terminal adenine. The physical separation between the anticodon and the amino acid attachment site--approximately 7.5 nm--is bridged by the ribosome during translation. tRNAs also contain many modified bases, including inosine, pseudouridine, and dihydrouridine, which contribute to structural stability and fine-tune codon recognition.
The enzymes responsible for loading each tRNA with its correct amino acid are the aminoacyl-tRNA synthetases. There are 20 different synthetases, one for each amino acid, and their specificity is what truly "translates" the genetic code--if a synthetase attached the wrong amino acid to a tRNA, the ribosome would have no way to detect the error. The charging reaction proceeds in two steps: first, the amino acid is activated by ATP to form an aminoacyl-AMP intermediate (with release of pyrophosphate); then, the amino acid is transferred to the 3' end of the appropriate tRNA. The overall reaction is: amino acid + tRNA + ATP -> aminoacyl-tRNA + AMP + PPi. To ensure accuracy, many synthetases possess an editing (proofreading) site that hydrolyzes incorrectly attached amino acids, providing a second check on fidelity.
<image>A two-panel figure on tRNA. Panel A: The cloverleaf secondary structure of tRNA showing the acceptor stem (3' CCA end where the amino acid attaches), the D loop (dihydrouridine), the anticodon loop (with the three-nucleotide anticodon), the T-psi-C loop, and the variable loop. Modified bases are indicated. Panel B: The 3D L-shaped structure of tRNA, with the anticodon at one end and the amino acid attachment site at the other end, approximately 7.5 nm apart. An aminoacyl-tRNA synthetase is shown recognizing both the anticodon region and the acceptor stem of its specific tRNA, catalyzing the attachment of the correct amino acid.</image>
III. Ribosomes
Ribosomes are the molecular machines that catalyze protein synthesis. Each ribosome is composed of ribosomal RNA (rRNA) and ribosomal proteins, organized into two subunits that come together during translation. Prokaryotic ribosomes sediment at 70S, comprising a 50S large subunit and a 30S small subunit, while eukaryotic ribosomes are larger at 80S, with a 60S large subunit and a 40S small subunit. This size difference is clinically significant because it allows antibiotics to target bacterial ribosomes selectively.
The ribosome contains three tRNA binding sites. The A site (aminoacyl site) is where each incoming aminoacyl-tRNA binds. The P site (peptidyl site) holds the tRNA carrying the growing polypeptide chain. The E site (exit site) is where deacylated tRNAs are released after donating their amino acids. The large subunit is responsible for catalyzing peptide bond formation. Remarkably, the catalytic activity resides not in the protein components but in the rRNA itself--specifically, the peptidyl transferase center located in the 23S rRNA (prokaryotes) or 28S rRNA (eukaryotes). The ribosome is therefore a ribozyme, an RNA enzyme, providing compelling evidence that RNA preceded proteins in early evolution. The small subunit, meanwhile, binds the mRNA and ensures correct codon-anticodon pairing, serving as the decoding center.
IV. Translation -- Initiation
Prokaryotic Initiation
Prokaryotic translation initiation requires mRNA, the 30S and 50S ribosomal subunits, a special initiator tRNA (fMet-tRNA^fMet carrying N-formylmethionine), three initiation factors (IF1, IF2, IF3), and GTP. The process begins when IF3 binds to the 30S subunit and prevents it from prematurely associating with the 50S subunit. The mRNA then binds to the 30S subunit through base-pairing between a purine-rich sequence on the mRNA called the Shine-Dalgarno sequence (located approximately 5-10 nucleotides upstream of the start codon) and a complementary region of the 16S rRNA. This interaction precisely positions the AUG start codon in the P site. IF2, bound to GTP, then delivers the initiator fMet-tRNA^fMet to the P site, where its anticodon pairs with the AUG codon. Finally, the 50S subunit joins, GTP is hydrolyzed, the initiation factors are released, and the complete 70S initiation complex is ready to begin elongation.
Eukaryotic Initiation
Eukaryotic initiation is considerably more complex, involving more than 12 initiation factors (designated eIFs). Eukaryotic mRNAs lack a Shine-Dalgarno sequence; instead, the 40S ribosomal subunit is recruited to the 5' cap of the mRNA. The 40S subunit, pre-loaded with eIFs and the initiator Met-tRNA^Met (which carries unformylated methionine, unlike the formylmethionine used in prokaryotes), then scans along the mRNA in the 5' to 3' direction until it encounters the first AUG in a favorable sequence context known as the Kozak sequence (consensus: 5'-ACCAUGG-3'). Once the start codon is recognized, the 60S subunit joins to form the 80S initiation complex, and elongation can proceed.
V. Translation -- Elongation
Elongation is a three-step cycle that repeats for each amino acid added to the growing polypeptide chain.
In the first step, codon recognition, an aminoacyl-tRNA is delivered to the A site by the elongation factor EF-Tu (prokaryotes) or eEF1A (eukaryotes) bound to GTP. When the correct codon-anticodon match is detected, GTP is hydrolyzed and EF-Tu is released. Incorrect tRNAs are rejected through a kinetic proofreading mechanism that exploits the time delay between initial binding and GTP hydrolysis--mismatched tRNAs dissociate before the irreversible hydrolysis step.
In the second step, peptide bond formation, the peptidyl transferase center in the large subunit rRNA catalyzes the transfer of the entire polypeptide chain from the tRNA in the P site to the amino acid on the tRNA in the A site, forming a new peptide bond. The P-site tRNA is now deacylated (empty), while the A-site tRNA carries the growing chain.
In the third step, translocation, the elongation factor EF-G (prokaryotes) or eEF2 (eukaryotes), powered by GTP hydrolysis, drives the ribosome one codon along the mRNA in the 5' to 3' direction. The tRNAs shift in register: the deacylated tRNA moves from the P site to the E site and exits, the peptidyl-tRNA moves from the A site to the P site, and a new codon is exposed in the now-empty A site. The cycle then repeats. Prokaryotic ribosomes elongate at a rate of approximately 15-20 amino acids per second, while eukaryotic ribosomes are somewhat slower at roughly 6 amino acids per second.
<image>A three-step diagram of the translation elongation cycle on a ribosome. Step 1 (Codon recognition): An aminoacyl-tRNA (carried by EF-Tu-GTP) enters the A site and its anticodon pairs with the mRNA codon. GTP is hydrolyzed, and EF-Tu-GDP is released. Step 2 (Peptide bond formation): The peptidyl transferase center catalyzes transfer of the polypeptide chain from the P-site tRNA to the A-site amino acid, forming a new peptide bond. The growing chain is now on the A-site tRNA. Step 3 (Translocation): EF-G-GTP binds, causing the ribosome to shift one codon to the right. The deacylated tRNA moves from P to E site (and exits), the peptidyl-tRNA moves from A to P site, and a new empty A site is exposed. The cycle repeats.</image>
VI. Translation -- Termination
Translation terminates when a stop codon (UAA, UAG, or UGA) enters the A site. No tRNA molecules recognize stop codons. Instead, proteins called release factors enter the A site and trigger the end of translation. In prokaryotes, RF1 recognizes UAA and UAG, RF2 recognizes UAA and UGA, and RF3 is a GTPase that facilitates recycling of RF1 and RF2. In eukaryotes, a single release factor, eRF1, recognizes all three stop codons, assisted by the GTPase eRF3. The release factors stimulate the peptidyl transferase center to hydrolyze the bond between the completed polypeptide and the P-site tRNA, freeing the finished protein. The ribosome then dissociates into its subunits, aided by ribosome recycling factor (RRF) in prokaryotes, making the components available for new rounds of translation.
VII. Polyribosomes (Polysomes)
Multiple ribosomes can translate the same mRNA molecule simultaneously, forming a structure called a polyribosome (polysome). Each ribosome in the polysome independently produces a complete copy of the polypeptide, dramatically increasing the rate of protein production from a single mRNA. In prokaryotes, transcription and translation are coupled--ribosomes begin translating the mRNA while RNA polymerase is still transcribing it, since both processes occur in the cytoplasm. This coupling is impossible in eukaryotes, where transcription occurs in the nucleus and translation in the cytoplasm; the two processes are therefore spatially and temporally separated, with mRNA export through nuclear pores serving as an obligatory intermediate step.
VIII. Post-Translational Modifications and Protein Targeting
The polypeptide emerging from the ribosome is not yet a finished, functional protein. It must fold into its correct three-dimensional conformation and often undergoes covalent modifications. Post-translational modifications include proteolytic cleavage (removal of signal peptides or processing of polyproteins, as in the case of insulin), glycosylation (addition of carbohydrate chains--N-linked in the ER and O-linked in the Golgi), phosphorylation (addition of phosphate groups by kinases, serving regulatory functions), ubiquitination (tagging with ubiquitin chains to target the protein for proteasomal degradation), and various other modifications such as acetylation, methylation, and lipidation.
Equally important is protein targeting--directing each protein to its correct cellular destination. The signal hypothesis, for which Gunter Blobel received the Nobel Prize in 1999, explains how proteins destined for the ER, Golgi, lysosomes, plasma membrane, or secretion are routed through the endomembrane system. These proteins carry an N-terminal signal peptide of approximately 15-30 hydrophobic amino acids. As the signal peptide emerges from the ribosome, it is recognized by the signal recognition particle (SRP), which pauses translation and docks the ribosome at the SRP receptor on the rough ER membrane. The polypeptide is then threaded through a translocon (a protein-conducting channel) into the ER lumen, and the signal peptide is cleaved by signal peptidase. Proteins destined for the nucleus carry nuclear localization signals (NLS) and are imported through nuclear pores, while mitochondrial and chloroplast proteins carry transit peptides that direct them to the appropriate organelle.
IX. Antibiotics That Target Translation
The structural differences between prokaryotic (70S) and eukaryotic (80S) ribosomes provide a basis for selective antibiotic targeting. Tetracycline blocks the A site and prevents tRNA binding. Chloramphenicol inhibits the peptidyl transferase activity of the 50S subunit. Erythromycin binds the 50S subunit and blocks translocation. Streptomycin binds the 30S subunit and causes misreading of the mRNA, leading to incorporation of incorrect amino acids. Puromycin is unusual in that it structurally mimics aminoacyl-tRNA and is incorporated into the growing chain, causing premature termination; because it exploits the conserved chemistry of peptide bond formation, puromycin affects both prokaryotic and eukaryotic ribosomes and is used primarily as a research tool rather than a clinical antibiotic.
