Medical School · Year 1 · Foundations · includes a quiz and discussion video

Lecture 12: Translation and Protein Synthesis

Unit 1.1: Foundations of Medicine & Medical Sciences

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the structure and function of ribosomes in protein synthesis
2. Explain the genetic code, including codon-anticodon recognition and wobble pairing
3. Outline the three phases of translation: initiation, elongation, and termination
4. Identify post-translational modifications and their functional significance
5. Describe the targeting and trafficking of newly synthesized proteins
6. Identify antibiotics that target translation and their mechanisms

The Genetic Code

Properties of the Code

Translation converts the language of nucleic acids into the language of proteins—transforming a sequence of nucleotides into a sequence of amino acids. This conversion follows the rules of the genetic code, one of the most fundamental discoveries in molecular biology.

The code is a triplet code, meaning three consecutive nucleotides (a codon) specify one amino acid. With four possible nucleotides at each position, this yields 64 possible codons (4³ = 64). Of these, 61 are sense codons that specify amino acids, while three are stop codons (UAA, UAG, and UGA) that signal termination of translation. Translation begins at the start codon AUG, which specifies methionine—making methionine the first amino acid of virtually every newly synthesized protein (though it may be removed later by proteolytic processing).

Degeneracy

The genetic code is degenerate, meaning most amino acids are encoded by more than one codon. This is not a flaw but rather an elegant buffer against mutation. Leucine, for instance, is specified by six different codons (UUA, UUG, CUU, CUC, CUA, CUG), while tryptophan has only a single codon (UGG). The variability concentrates at the third position of codons, called the wobble position. Many mutations at this position are silent—they change the codon but not the encoded amino acid. This redundancy provides partial protection against the deleterious effects of point mutations.

Universality

The genetic code is nearly universal across all domains of life, from bacteria to humans. This universality reflects the code's ancient origin, likely established before the divergence of the major branches of life. Minor variations exist: mitochondria use a slightly modified code (for example, UGA codes for tryptophan rather than stop), and certain organisms have reassigned specific codons. However, these exceptions are rare, and the standard code prevails throughout most of biology. This universality enables recombinant DNA technology—a human gene expressed in bacteria will produce the same protein because both organisms read the code identically.

<image>Panel A: Genetic code table with the first codon position (5' base) on the left edge, the second position across the top, and the third position (wobble position) on the right edge, with three-letter and one-letter amino acid abbreviations in each cell. Panel B: Start codon AUG highlighted in green labeled "Met (Start)" and three stop codons (UAA, UAG, UGA) highlighted in red labeled "Stop." Panel C: Color shading grouping codons encoding the same amino acid together, visually demonstrating degeneracy and wobble position variations. Panel D: Legend showing amino acids color-coded by property: hydrophobic (yellow), polar (green), basic (blue), and acidic (red).</image>

Reading Frame

The sequence of nucleotides is read in consecutive, non-overlapping triplets beginning from the start codon. The start codon establishes the reading frame—which of the three possible frames is used. Once set, the reading frame continues uninterrupted to the stop codon. This means that frameshift mutations, caused by insertions or deletions that are not multiples of three, are particularly devastating: every codon downstream of the mutation is altered, typically producing a completely nonfunctional protein and often encountering a premature stop codon.

Transfer RNA (tRNA)

Structure

Transfer RNAs are the adaptor molecules that decode genetic information, physically linking specific codons to their corresponding amino acids. Each tRNA is approximately 75 nucleotides long and folds into a distinctive cloverleaf secondary structure, which further compacts into an L-shaped three-dimensional form. Many nucleotides in tRNA are chemically modified after transcription, including pseudouridine (Ψ), dihydrouridine (D), and inosine (I)—these modifications contribute to structural stability and decoding accuracy.

The critical functional regions of tRNA include the anticodon loop, which contains three nucleotides complementary to an mRNA codon, and the 3' acceptor stem, which terminates in the sequence CCA and carries the amino acid attached to its terminal adenosine. The D loop and TΨC loop contribute to tRNA recognition by various processing and aminoacylation enzymes.

<image>Panel A: Classic cloverleaf secondary structure of tRNA showing the acceptor stem at the top ending in CCA-3' with an amino acid icon, the TPC loop (right arm), and the D loop (left arm). Panel B: Anticodon loop at the bottom of the cloverleaf with three nucleotides of the anticodon highlighted and labeled, and the variable loop between the anticodon arm and TPC arm, with modified bases indicated by special symbols. Panel C: L-shaped three-dimensional ribbon diagram showing the anticodon triplet at one end (bottom) and the CCA acceptor end with attached amino acid at the other end (top), approximately 75 Angstroms apart. Panel D: Arrows indicating how the cloverleaf secondary structure folds into the L-shaped tertiary structure, with functional regions (anticodon and amino acid attachment site) at opposite ends.</image>

Aminoacyl-tRNA Synthetases

The accuracy of translation depends critically on aminoacyl-tRNA synthetases, the enzymes that attach amino acids to their corresponding tRNAs. There are 20 of these enzymes, one for each amino acid, and each must discriminate both the correct amino acid and the correct tRNA(s) from all others. The reaction proceeds in two steps: first, the amino acid is activated by reaction with ATP to form an aminoacyl-adenylate intermediate; second, the amino acid is transferred to the 3' end of the appropriate tRNA, releasing AMP. The overall reaction consumes two high-energy phosphate bonds (ATP → AMP + PPᵢ, followed by pyrophosphate hydrolysis).

The fidelity of aminoacyl-tRNA synthetases is remarkable, with error rates of approximately 1 in 10,000. Many synthetases have evolved editing sites that hydrolyze incorrectly charged tRNAs, providing a proofreading mechanism analogous to DNA polymerase's proofreading. Errors at this step would result in the wrong amino acid being incorporated into proteins wherever the corresponding codon appears, so high accuracy is essential.

Wobble Pairing

Although there are 61 sense codons, cells do not require 61 different tRNAs. The wobble hypothesis, proposed by Francis Crick, explains how fewer tRNAs can decode all codons. The first two positions of the codon-anticodon interaction follow strict Watson-Crick pairing (A-U, G-C), but the third position (the 5' position of the anticodon pairing with the 3' position of the codon) allows non-standard, or "wobble," pairing. Inosine (I) in the anticodon can pair with U, C, or A in the codon. G can pair with U. This flexibility means a single tRNA can often recognize multiple codons specifying the same amino acid, reducing the total number of tRNAs needed.

Ribosomes

Structure

Ribosomes are the molecular machines that catalyze protein synthesis, reading mRNA and joining amino acids into polypeptide chains. Each ribosome consists of two subunits, designated by their sedimentation coefficients. Prokaryotic ribosomes are 70S particles composed of a 30S small subunit (containing 16S rRNA) and a 50S large subunit (containing 23S and 5S rRNAs). Eukaryotic ribosomes are larger 80S particles, with a 40S small subunit (18S rRNA) and a 60S large subunit (28S, 5.8S, and 5S rRNAs). Note that sedimentation coefficients are not additive—they depend on shape as well as mass.

Functional Sites

Both subunits contribute to forming three tRNA binding sites. The A site (aminoacyl site) receives incoming charged tRNAs carrying new amino acids. The P site (peptidyl site) holds the tRNA attached to the growing polypeptide chain. The E site (exit site) is occupied by deacylated tRNAs before they dissociate from the ribosome. The small subunit provides the decoding center where codon-anticodon recognition occurs, while the large subunit contains the peptidyl transferase center where peptide bonds are formed.

The Ribosome as Ribozyme

A revolutionary discovery was that ribosomal RNA, not protein, catalyzes peptide bond formation. The peptidyl transferase activity resides in the 23S rRNA (or 28S in eukaryotes), making the ribosome fundamentally a ribozyme—an RNA enzyme. Ribosomal proteins, though numerous, play primarily structural and regulatory roles rather than catalytic ones. This finding supports the "RNA world" hypothesis, suggesting that life's earliest catalysts were RNA molecules, with proteins arriving later in evolution.

<image>Panel A: Complete 80S/70S ribosome cutaway cross-section with the small subunit (40S/30S, blue) on top and the large subunit (60S/50S, purple) below, with mRNA threading through a channel. Panel B: Three tRNA molecules in their respective binding sites: charged tRNA (green) entering the A site, peptidyl-tRNA (yellow) in the P site carrying the growing polypeptide chain (string of colored circles), and uncharged tRNA (gray) exiting through the E site. Panel C: Decoding center in the small subunit showing mRNA codon-anticodon interactions with base-pairing detail. Panel D: Exit tunnel through the large subunit depicting the channel through which the nascent polypeptide emerges, with the peptidyl transferase center labeled at the catalytic site.</image>

Translation: Initiation

Overview

Initiation is the phase where the ribosome assembles on mRNA, locates the start codon, and positions the first aminoacyl-tRNA. This phase is the most regulated step of translation, allowing cells to control which mRNAs are translated and at what rate. The mechanisms differ significantly between prokaryotes and eukaryotes.

Prokaryotic Initiation

In bacteria, the small 30S subunit binds directly to the mRNA at a purine-rich sequence called the Shine-Dalgarno sequence, located 5-10 nucleotides upstream of the start codon. This sequence base-pairs with a complementary sequence in the 16S rRNA, precisely positioning the start codon at the P site. The initiator tRNA, which carries N-formylmethionine (fMet-tRNAᶠᴹᵉᵗ), binds directly to the P site—uniquely among all aminoacyl-tRNAs, which normally enter at the A site. Three initiation factors (IF-1, IF-2, IF-3) facilitate this process, and their release triggers joining of the 50S subunit to form the complete 70S initiation complex.

Eukaryotic Initiation

Eukaryotic initiation is more complex, involving a dozen or more initiation factors (designated eIFs). The process begins at the 5' cap, not at an internal sequence like in prokaryotes. The cap-binding complex (eIF4E, eIF4G, and the helicase eIF4A) recognizes the 7-methylguanosine cap and recruits the 43S preinitiation complex—a structure composed of the 40S subunit, the initiator Met-tRNAᵢᴹᵉᵗ, and several eIFs including eIF2 (which delivers the initiator tRNA in a GTP-dependent manner) and eIF3 (which prevents premature large subunit joining).

The 43S complex then scans along the mRNA in the 5' to 3' direction, searching for the start codon in an appropriate sequence context. The optimal context, called the Kozak sequence (consensus: ACCAUGG), has a purine at position -3 and a G immediately following the AUG. Once recognized, base pairing between the anticodon and AUG triggers events leading to release of initiation factors and joining of the 60S subunit to form the 80S initiation complex, ready for elongation.

<image>Panel A: Mature mRNA with 5' cap (m7G) and poly(A) tail labeled, showing the 5' UTR, start codon (AUG in Kozak context), coding sequence, and 3' UTR, with the cap-binding complex (eIF4E in orange, eIF4G scaffold, eIF4A helicase) bound to the cap. Panel B: The 43S preinitiation complex (40S subunit in blue, Met-tRNAi in green, eIF2-GTP, eIF3, and other factors) attached near the cap and scanning along the 5' UTR with eIF4A unwinding secondary structure. Panel C: Start codon recognition within the Kozak sequence (ACC-AUG-G) with base pairing between the anticodon and AUG highlighted. Panel D: Initiation factors released (dispersing) and the 60S subunit (purple) joining to form the complete 80S initiation complex with Met-tRNA positioned in the P site.</image>

Translation: Elongation

Overview

Elongation is the repetitive phase where amino acids are added one at a time to the growing polypeptide chain. Each cycle adds one amino acid and advances the ribosome by one codon (three nucleotides). In eukaryotes, elongation proceeds at approximately 15-20 amino acids per second—fast but not as rapid as in bacteria, where rates can exceed 40 amino acids per second.

Step 1: Aminoacyl-tRNA Binding

Each elongation cycle begins with delivery of a charged tRNA to the empty A site. In bacteria, the elongation factor EF-Tu (eEF1A in eukaryotes) forms a ternary complex with GTP and the aminoacyl-tRNA. This complex enters the A site, where the anticodon tests for complementarity with the exposed codon. Correct codon-anticodon matching triggers a conformational change in the ribosome that stimulates GTP hydrolysis by EF-Tu. This provides an opportunity for kinetic proofreading: incorrect tRNAs, with weaker codon-anticodon interactions, tend to dissociate before GTP hydrolysis commits the ribosome to their incorporation. Following GTP hydrolysis, EF-Tu-GDP dissociates, and the aminoacyl-tRNA fully accommodates into the A site, positioning its amino acid near the peptidyl transferase center.

Step 2: Peptide Bond Formation

With charged tRNAs in both the A and P sites, peptide bond formation occurs spontaneously, catalyzed by the ribosomal RNA. The alpha-amino group of the amino acid in the A site performs a nucleophilic attack on the carbonyl carbon of the ester bond linking the peptide chain to the P-site tRNA. This transfers the entire growing polypeptide to the A-site tRNA, extending the chain by one amino acid. The P-site tRNA is now uncharged, while the A-site tRNA carries the nascent polypeptide.

Step 3: Translocation

The final step of each cycle moves everything by one position. The elongation factor EF-G (eEF2 in eukaryotes) binds in its GTP-bound form and promotes translocation: the ribosome moves 3 nucleotides along the mRNA, the peptidyl-tRNA shifts from the A site to the P site, the uncharged tRNA moves from the P site to the E site (and then exits), and a new codon is exposed in the A site. GTP hydrolysis by EF-G provides the energy for these movements. With the A site empty and a new codon positioned, the cycle repeats.

<image>Panel A: Aminoacyl-tRNA binding step showing the ribosome with peptidyl-tRNA (yellow, carrying 5 amino acids) in the P site, EF-Tu (green oval) bound to GTP delivering a charged tRNA (purple) to the empty A site with codon-anticodon base-pairing, and GTP hydrolysis releasing EF-Tu-GDP. Panel B: Peptide bond formation step with the 23S/28S rRNA active site highlighted, showing the nucleophilic attack by the amino group of the A-site amino acid on the carbonyl of the P-site ester bond, transferring the peptide chain to the A-site tRNA (now 6 amino acids). Panel C: Translocation step with EF-G-GTP (blue oval) binding and GTP hydrolysis moving the ribosome one codon: peptidyl-tRNA shifts to P site, uncharged tRNA exits via E site, and a new empty A site is exposed. Panel D: Overview of the complete elongation cycle with arrows indicating all molecular movements and the repetitive nature of the process until a stop codon is reached.</image>

Translation: Termination

Stop Codon Recognition

Translation terminates when the ribosome encounters one of three stop codons: UAA, UAG, or UGA. These codons are not recognized by any tRNA. Instead, protein factors called release factors recognize stop codons and trigger termination. In bacteria, RF-1 recognizes UAA and UAG, while RF-2 recognizes UAA and UGA; RF-3 is a GTPase that facilitates release factor function. Eukaryotes use a simpler system: eRF1 recognizes all three stop codons, and eRF3 provides GTPase activity.

Termination Events

When a stop codon enters the A site, the release factor binds in place of a tRNA. The release factor contains a conserved GGQ motif that enters the peptidyl transferase center and positions a water molecule for nucleophilic attack on the ester bond linking the polypeptide to the P-site tRNA. This hydrolysis releases the completed polypeptide chain. Subsequently, the release factors, mRNA, and ribosomal subunits all dissociate. In bacteria, ribosome recycling factor (RRF) and EF-G catalyze subunit separation, freeing the components for new rounds of translation.

<image>Panel A: Ribosome with the nascent polypeptide chain attached to tRNA in the P site and a stop codon (UAG) positioned in the A site, with no tRNA able to recognize the codon. Panel B: Release factor (eRF1/RF-1, purple shape mimicking tRNA's L-shape) entering the A site, with the GGQ motif highlighted near the peptidyl transferase center and a water molecule (H2O) positioned for nucleophilic attack. Panel C: Hydrolysis of the ester bond releasing the completed polypeptide chain (shown as a coiled protein floating free), with the chemical mechanism of bond cleavage depicted. Panel D: Ribosome dissociation into its subunits (40S and 60S separating), mRNA released, and deacylated tRNA exiting, with RRF and EF-G/eRF3 facilitating the dissociation process.</image>

Polyribosomes

A single mRNA molecule can be translated by multiple ribosomes simultaneously, forming a structure called a polyribosome or polysome. As each ribosome clears the start codon and moves into the coding region, another can initiate, resulting in a chain of ribosomes progressing along the message. Polysomes can be visualized by electron microscopy as beads on a string, with more ribosomes indicating more active translation. Ribosomes are typically spaced about 80 nucleotides apart.

Polyribosomes may be free in the cytoplasm or bound to the endoplasmic reticulum. Free polysomes synthesize proteins destined for the cytoplasm, nucleus, mitochondria, or peroxisomes. Membrane-bound polysomes produce proteins that will enter the secretory pathway—secreted proteins, membrane proteins, and lysosomal enzymes.

Post-Translational Modifications

Protein Folding

Protein folding often begins while the polypeptide is still being synthesized, with the N-terminal portion emerging from the ribosome exit tunnel and beginning to adopt secondary and tertiary structure. Molecular chaperones assist this process, preventing aggregation and guiding proper folding. The ribosome-associated chaperone Hsp70 (in eukaryotes) captures nascent chains as they emerge, while downstream chaperones like Hsp60/GroEL provide protected environments for complete folding. Quality control mechanisms identify misfolded proteins and target them for refolding attempts or, ultimately, degradation.

Proteolytic Processing

Many proteins undergo proteolytic cleavage as part of their maturation. Signal sequences that direct proteins to the ER, mitochondria, or other organelles are typically removed by signal peptidases after targeting is complete. Zymogens (inactive enzyme precursors) are activated by specific proteolytic cleavages—for example, the digestive enzyme trypsin is synthesized as inactive trypsinogen and activated by cleavage in the small intestine. Insulin provides another example: it is synthesized as preproinsulin, cleaved to proinsulin in the ER, and further processed to mature insulin (A and B chains connected by disulfide bonds) in secretory granules.

Covalent Modifications

Post-translational modifications dramatically expand the functional diversity of proteins. Phosphorylation, the addition of phosphate groups to serine, threonine, or tyrosine residues by protein kinases, is the most common regulatory modification, controlling enzyme activity, protein-protein interactions, and cellular signaling cascades. Glycosylation adds carbohydrate chains to asparagine (N-linked) or serine/threonine (O-linked) residues, affecting protein folding, stability, and recognition. Lipidation attaches lipid groups that anchor proteins to membranes—examples include the farnesylation of Ras proteins critical for their function in growth signaling.

Acetylation, particularly of histones, regulates chromatin structure and gene expression. Ubiquitination attaches the small protein ubiquitin to lysine residues; polyubiquitin chains typically mark proteins for degradation by the proteasome, though monoubiquitination serves signaling functions. Methylation of histones influences chromatin state, while methylation of other proteins can regulate their activity or localization.

<image>Panel A: Phosphorylation showing a protein kinase transferring a phosphate group (yellow circle with "P") from ATP to a serine/threonine/tyrosine hydroxyl group, with chemical structures of unmodified and phosphorylated serine. Panel B: N-linked glycosylation showing a branched oligosaccharide (hexagons, squares) attached to asparagine within the consensus sequence Asn-X-Ser/Thr. Panel C: Ubiquitination showing ubiquitin (small red shape labeled "Ub") attached to a lysine residue via an isopeptide bond, with a chain of four ubiquitins indicating proteasomal targeting. Panel D: Lipidation showing three types: myristoylation (14-carbon chain on N-terminal glycine), palmitoylation (16-carbon chain on cysteine), and prenylation (farnesyl or geranylgeranyl on cysteine), each anchoring the protein to a membrane (gray bar).</image>

Protein Targeting and Trafficking

The Signal Hypothesis

Proteins destined for secretion, the plasma membrane, or the ER/Golgi/lysosome system are directed by N-terminal signal sequences, typically 15-30 amino acids long with a hydrophobic core. As this signal sequence emerges from the ribosome, it is recognized by the signal recognition particle (SRP), a ribonucleoprotein complex. SRP binding slows or arrests translation and escorts the ribosome-mRNA-nascent chain complex to the ER membrane.

At the ER, SRP binds its receptor, and the ribosome docks at a protein channel called the translocon (Sec61 complex). Translation resumes, and the growing polypeptide is threaded through the translocon into the ER lumen. Signal peptidase cleaves the signal sequence, and the protein folds in the oxidizing environment of the ER, assisted by ER-resident chaperones. Membrane proteins are inserted laterally into the ER membrane via the translocon.

Protein Destinations and Sorting Signals

Different signals direct proteins to different cellular destinations. Proteins with an N-terminal signal peptide enter the ER and proceed through the secretory pathway unless they contain retention signals. The tetrapeptide KDEL (or HDEL) at the C-terminus retrieves proteins from the Golgi back to the ER. Mannose-6-phosphate tags, added in the Golgi, target proteins to lysosomes by binding mannose-6-phosphate receptors that divert them to late endosomes.

Nuclear localization signals (NLS), typically short sequences rich in basic amino acids (lysine and arginine), direct proteins through nuclear pore complexes. Mitochondrial targeting sequences, usually N-terminal amphipathic helices, guide proteins to mitochondrial import machinery. Peroxisomal targeting signals include the C-terminal tripeptide SKL (PTS1) and an N-terminal sequence (PTS2).

<image>Panel A: Rough ER with ribosomes attached synthesizing a protein with signal sequence, and an inset showing signal sequence recognition by SRP and docking at the translocon. Panel B: Secretory pathway traced with arrows from ER to Golgi (cis, medial, trans) to various destinations: secretory vesicles to the plasma membrane for secretion or membrane protein insertion, and KDEL-containing proteins showing retrograde transport back to ER. Panel C: Mannose-6-phosphate-tagged proteins directed to lysosomes via receptor-mediated sorting from the trans-Golgi network. Panel D: Cytosolic ribosomes making proteins for the cytoplasm, nucleus (with NLS), mitochondria (with N-terminal targeting sequence), and peroxisomes (with PTS), each destination labeled with its targeting signal.</image>

Antibiotics Targeting Translation

The ribosome is one of the most important antibiotic targets because bacterial and eukaryotic ribosomes differ enough to allow selective inhibition. Understanding these mechanisms is clinically essential.

30S Subunit Inhibitors

Aminoglycosides (streptomycin, gentamicin, tobramycin) bind the 30S subunit and cause misreading of the genetic code—incorrect amino acids are incorporated, producing nonfunctional proteins. They also interfere with initiation. Their positive charges prevent entry into mammalian cells but allow penetration of bacterial membranes. Toxicity (nephrotoxicity, ototoxicity) limits their use.

Tetracyclines (tetracycline, doxycycline) block the A site, preventing aminoacyl-tRNA binding. They are broad-spectrum antibiotics effective against many gram-positive and gram-negative organisms, as well as atypical bacteria.

50S Subunit Inhibitors

Chloramphenicol inhibits peptidyl transferase activity in the 50S subunit, preventing peptide bond formation. Its use is limited by bone marrow toxicity (aplastic anemia), but it remains important for certain infections.

Macrolides (erythromycin, azithromycin, clarithromycin) bind in the exit tunnel of the 50S subunit, physically blocking the passage of the growing polypeptide chain. This causes premature dissociation of peptidyl-tRNA from the ribosome.

Clindamycin also inhibits peptidyl transferase and binds a site overlapping with macrolides. Linezolid (an oxazolidinone) inhibits formation of the initiation complex by preventing association of the 50S subunit with the initiation factors.

Other Translation Inhibitors

Puromycin structurally mimics the aminoacyl end of charged tRNA. It enters the A site, accepts the peptide chain (forming a peptide-puromycin conjugate), but cannot translocate, causing premature termination and release of truncated proteins. Puromycin is used experimentally but is too toxic for clinical use.

Fusidic acid inhibits EF-G, blocking translocation. Diphtheria toxin, produced by Corynebacterium diphtheriae, ADP-ribosylates eukaryotic elongation factor 2 (eEF2), inactivating it and completely blocking protein synthesis in target cells—this is how the toxin kills cells.

<image>Panel A: 70S bacterial ribosome with the 30S subunit (light blue) and 50S subunit (light purple) distinguished, showing A, P, and E sites. Panel B: 30S-targeting antibiotics positioned at their sites of action: aminoglycosides (green) at the decoding center causing misreading and tetracyclines (yellow) blocking the A site to prevent aminoacyl-tRNA binding. Panel C: 50S-targeting antibiotics at their sites: chloramphenicol (red) at the peptidyl transferase center, macrolides (orange) in the exit tunnel, and linezolid (pink) at the 50S-tRNA interface preventing initiation. Panel D: Fusidic acid (brown) bound to EF-G blocking translocation, with arrows indicating the step each drug inhibits and a note that differences between 70S and 80S ribosomes enable selective toxicity.</image>

Clinical Correlations

Genetic Code Mutations

Mutations in coding sequences have varying effects depending on how they alter the genetic code. Missense mutations change one amino acid to another—the clinical impact ranges from benign to severe depending on the substitution. The classic example is sickle cell disease, where a single nucleotide change (GAG→GUG) converts glutamate to valine at position 6 of β-globin, causing hemoglobin to polymerize under low oxygen conditions.

Nonsense mutations introduce a premature stop codon, producing a truncated protein that is usually nonfunctional and often degraded by nonsense-mediated decay. Silent mutations change the codon but not the amino acid; most are phenotypically neutral, though some can affect splicing or mRNA stability. Frameshift mutations, caused by insertions or deletions not divisible by three, shift the reading frame and typically result in a nonfunctional protein and premature termination.

Ribosomopathies

A surprising group of diseases results from mutations in ribosomal proteins or ribosome biogenesis factors. Diamond-Blackfan anemia is caused by mutations in genes encoding ribosomal proteins (most commonly RPS19), leading to failure of red blood cell production and often physical abnormalities. Shwachman-Diamond syndrome results from defects in ribosome assembly, causing exocrine pancreatic insufficiency and bone marrow failure. These disorders reveal that certain tissues are particularly sensitive to reduced ribosome function.

I-Cell Disease

I-cell disease (mucolipidosis II) illustrates the importance of proper protein targeting. The disease results from deficiency of the enzyme that adds mannose-6-phosphate tags to lysosomal enzymes in the Golgi. Without this tag, lysosomal enzymes are secreted rather than delivered to lysosomes. The resulting lysosomal storage defects cause severe developmental abnormalities, skeletal malformations, and death in early childhood. The name "I-cell" refers to the inclusion-filled cells visible on microscopy, stuffed with undigested materials that functional lysosomes would normally degrade.

Summary

• The genetic code is triplet, degenerate, and nearly universal
• tRNAs decode mRNA codons via anticodon base-pairing
• Translation occurs in three phases: initiation, elongation, termination
• Ribosomes are ribozymes with rRNA catalyzing peptide bond formation
• Post-translational modifications expand protein diversity and function
• Signal sequences direct proteins to appropriate cellular destinations
• Many antibiotics target bacterial translation machinery

Key Terms

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