18.2 Nucleotide Metabolism, DNA Replication, Transcription, Translation & Repair

18.2 Nucleotide Metabolism, DNA Replication, Transcription, Translation & Repair

Independent Study Guide Notice: Independent study guide by OpenExamPrep. This educational resource is developed independently by OpenExamPrep and is not sponsored, endorsed, or affiliated with the National Board of Podiatric Medical Examiners (NBPME) or Meazure Learning.

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Introduction to Molecular Genetics

The central dogma of molecular biology dictates the directional flow of genetic information: DNA replication preserves genomic integrity across cellular generations, transcription copies DNA into messenger RNA, and ribosomal translation synthesizes polypeptide chains. Crucially, environmental mutagens, ultraviolet radiation, and spontaneous chemical alterations constantly damage the genome, necessitating specialized, high-fidelity DNA repair pathways. For podiatric medical candidates taking the APMLE Part I examination, mastery of the enzymatic machinery of replication, the pharmacology of topoisomerase and protein synthesis inhibitors, the pathogenesis of UV-induced cutaneous malignancies in DNA repair deficiencies, and the molecular genetics of sickle cell disease and muscular dystrophy is vital.

                         The Central Dogma & Genomic Flow

        REPLICATION ────┐
        (DNA Polymerase)│
                        ▼
                  ┌───────────┐
                  │   DNA     │ ◄─── DNA REPAIR (NER, BER, MMR, NHEJ, HR)
                  └─────┬─────┘
                        │
                        │ TRANSCRIPTION (RNA Polymerases I, II, III)
                        ▼
                  ┌───────────┐
                  │ Pre-mRNA  │
                  └─────┬─────┘
                        │ POST-TRANSCRIPTIONAL PROCESSING
                        │ (5' Cap, 3' Poly-A Tail, Splicing [snRNPs])
                        ▼
                  ┌───────────┐
                  │Mature mRNA│
                  └─────┬─────┘
                        │
                        │ TRANSLATION (Ribosomes, tRNAs, eIFs, eEFs, eRFs)
                        ▼
                  ┌───────────┐
                  │  PROTEIN  │
                  └───────────┘

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DNA Replication Machinery & Telomerase Biology

Eukaryotic and prokaryotic DNA replication is semiconservative (each daughter duplex contains one original parental strand and one newly synthesized daughter strand) and bidirectional, initiating at designated AT-rich origins of replication.

                      The Active Replication Fork

                                  Leading Strand Synthesis (Continuous, 5' -> 3')
                             5' ───────────────────────────────────────────────> 3'
  Parental DNA (3' -> 5')   3' ═════════════════════════════════════════════════ 5'
                                                 ▲
                                                 │ Topoisomerase (Relieves Strain)
                                 [ Helicase ] ───┴─── (Unwinds DNA at Fork)
  Parental DNA (5' -> 3')   5' ═════════════════════════════════════════════════ 3'
                             3' <─────── 5'     3' <─────── 5'   (Okazaki Fragments)
                                  RNA Primer        RNA Primer
                                  Lagging Strand Synthesis (Discontinuous, 5' -> 3')

1. Essential Enzymatic Components of the Replication Fork

• Origins of Replication (Ori): Specific nucleotide sequences enriched in Adenine-Thymine ($A-T$) base pairs. Because $A-T$ pairs share only two hydrogen bonds (compared to three in $G-C$ pairs), less thermal/mechanical energy is required to denature the duplex. Bacteria possess a single origin (oriC), whereas linear eukaryotic chromosomes contain multiple tandem replication origins to accelerate complete genome duplication during the S phase.
• DNA Helicase: An ATP-dependent motor enzyme that binds replication origins and breaks the hydrogen bonds holding the antiparallel strands together, unzipping the double helix and establishing the replication fork.
  • Clinical Pathology: Mutations in the BLM gene encoding a RecQ-family DNA helicase cause Bloom Syndrome (autosomal recessive). Defective helicase function leads to hyper-recombination, marked genomic instability, elevated sister chromatid exchanges, severe proportional dwarfism, microcephaly, photosensitive facial telangiectatic erythema in a butterfly distribution, and high risk of diverse malignancies.
• Single-Stranded DNA-Binding Proteins (SSBs / RPA): Prokaryotic SSBs and eukaryotic Replication Protein A (RPA) bind cooperatively to exposed single-stranded template DNA, preventing premature re-annealing and protecting the fragile single strands from cleavage by nucleases.
• DNA Topoisomerases (Gyrases): As helicase unwinds DNA, severe positive torsional supercoiling accumulates ahead of the advancing replication fork. Topoisomerases relieve this torsional strain:
  • Topoisomerase I: Cleaves a single strand of the DNA phosphodiester backbone, allows the free strand to rotate around the intact strand, and religates the nick without requiring ATP. Inhibited by antineoplastic agents irinotecan and topotecan.
  • Topoisomerase II (DNA Gyrase in bacteria): Introduces transient double-strand breaks into the DNA backbone, passes an intact DNA segment through the break, and religates both strands in an ATP-dependent reaction. Relieves both positive and negative supercoiling.
    • Fluoroquinolones (Ciprofloxacin, Levofloxacin, Moxifloxacin): Inhibit bacterial DNA Gyrase (Topoisomerase II) and Topoisomerase IV, stabilizing toxic double-strand cleaved complexes and causing bacterial cell death. Extensively prescribed in podiatric medicine for severe diabetic foot infections, Pseudomonas osteomyelitis, and complex cellulitis. Known board-tested adverse effects include Achilles tendinitis and Achilles tendon rupture (due to collagen degradation and chondrocyte cytotoxicity).
    • Etoposide & Teniposide: Inhibit human Topoisomerase II in eukaryotic cells, trapping cleaved DNA complexes and inducing apoptotic arrest in testicular and small cell lung carcinomas.
• Primase: A specialized DNA-dependent RNA polymerase that synthesizes a short (~10 nucleotide) RNA primer complementary to the single-stranded DNA template. DNA polymerases are completely unable to initiate de novo synthesis and require the free 3'-OH group provided by the RNA primer to add subsequent deoxyribonucleotides.
• DNA Polymerases:
  • Prokaryotic: DNA Polymerase III performs the primary leading- and lagging-strand synthesis in the 5' $\rightarrow$ 3' direction and possesses 3' $\rightarrow$ 5' exonuclease activity for immediate proofreading. DNA Polymerase I degrades the RNA primers via its unique 5' $\rightarrow$ 3' exonuclease activity, fills the resulting gap with DNA in the 5' $\rightarrow$ 3' direction, and proofreads via 3' $\rightarrow$ 5' exonuclease activity.
  • Eukaryotic: DNA Polymerase $\delta$ (synthesizes the lagging strand and proofreads), DNA Polymerase $\epsilon$ (synthesizes the leading strand and proofreads), DNA Polymerase $\alpha$ (associates with primase to synthesize initial RNA-DNA hybrid primers), and DNA Polymerase $\beta$ (participates in base excision repair).
• DNA Ligase: Catalyzes the formation of a phosphodiester bond between the adjacent 3'-OH group of one Okazaki fragment and the 5'-phosphate group of the adjacent fragment, sealing all single-strand nicks (requires ATP in eukaryotes, $NAD^+$ in bacteria).

2. The End-Replication Problem & Telomerase

Because DNA polymerases require an RNA primer with a 3'-OH group and synthesize exclusively in the 5' $\rightarrow$ 3' direction, removal of the terminal RNA primer on the lagging strand leaves an unreplicated gap at the 3' end of linear eukaryotic chromosomes. With each cycle of replication, chromosomes progressively shorten, eventually triggering cellular senescence (the Hayflick limit) or apoptosis:

• Telomeres: Non-coding, repetitive hexameric tandem repeats (5'-TTAGGG-3' in humans) located at the terminal ends of eukaryotic chromosomes, stabilized by the shelterin protein complex to prevent DNA damage response machinery from misidentifying chromosome ends as double-strand breaks.
• Telomerase: A specialized ribonucleoprotein reverse transcriptase consisting of two key components:
  1. TERT (Telomerase Reverse Transcriptase): The catalytic protein subunit that synthesizes DNA from an RNA template.
  2. TERC (Telomerase RNA Component): An internal, non-coding RNA molecule providing the built-in template ($3'\text{-AAUCCC-}5'$) to synthesize new 5'-TTAGGG-3' telomeric repeats onto the 3' single-stranded overhang.
• Clinical Significance: Telomerase is highly active in embryonic stem cells, adult germline cells, and hematopoietic progenitor cells, but is silenced in differentiated human somatic cells. Reactivation of telomerase is observed in 85% to 90% of human malignancies, conferring infinite proliferative capacity and cellular immortality upon neoplastic cells.

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DNA Repair Mechanisms & Board-Relevant Genodermatoses

Genomic stability is safeguarded by five primary DNA repair pathways, each dedicated to correcting specific physical or chemical lesions.

                    Five Major DNA Repair Pathways

     NUCLEOTIDE EXCISION REPAIR (NER)             BASE EXCISION REPAIR (BER)
     - G1 phase; excises BULKY lesions            - All phases; excises SINGLE modified base
     - Endonucleases cleave both sides            - Glycosylase -> AP-Endonuclease -> 
     - DEFECT: Xeroderma Pigmentosum                Lyase -> Pol-beta -> Ligase
     
     MISMATCH REPAIR (MMR)                        NON-HOMOLOGOUS END JOINING (NHEJ)
     - S/G2 phase; repairs replication slips      - DSB repair; NO template used
     - MSH2, MLH1, MSH6, PMS2                     - Error-prone (loss of nucleotides)
     - DEFECT: Lynch Syndrome (HNPCC)             - DEFECT: Ataxia-Telangiectasia (ATM)
     
     HOMOLOGOUS RECOMBINATION (HR)
     - Late S/G2 phase; uses sister chromatid template (error-free)
     - DEFECT: BRCA1, BRCA2 mutations (Hereditary Breast & Ovarian Cancer)

1. Nucleotide Excision Repair (NER)

• Timing & Lesion: Operates primarily during the G1 phase of the cell cycle. Repairs bulky, helix-distorting DNA lesions, most notably pyrimidine dimers (covalent cyclobutane thymine-thymine dimers) induced by ultraviolet (UV) radiation (UV-B light).
• Mechanism:
  1. Specific endonucleases (excinucleases) recognize the helical distortion and cleave the damaged strand several nucleotides upstream and downstream of the dimer, releasing an oligonucleotide fragment (approx. 24-32 nucleotides).
  2. DNA polymerase fills the resulting single-stranded gap.
  3. DNA ligase seals the phosphodiester backbone.
• Defective Disease: Xeroderma Pigmentosum (XP):
  • Autosomal recessive mutations in any of at least eight distinct NER genes (XPA through XPG).
  • Inability to repair UV-induced thymine dimers causes severe genomic instability in sun-exposed cutaneous tissues.
  • Clinical Manifestations: Severe photosensitivity (severe blistering sunburns upon minimal sun exposure), diffuse poikiloderma (atrophy, telangiectasias, mottled hyperpigmentation/freckling), severe xerosis, and corneal ulcerations. Patients exhibit a >1000-fold increased risk of cutaneous malignancies, including melanoma, squamous cell carcinoma, and basal cell carcinoma, often presenting before 10 years of age.

2. Base Excision Repair (BER)

• Timing & Lesion: Operates throughout the entire cell cycle. Corrects non-bulky, small base lesions caused by spontaneous deamination (e.g., cytosine deaminating into uracil), depurination, alkylation, or oxidation (e.g., 8-hydroxyguanine).
• The Five-Step Enzymatic Cascade ("G-E-L-P-L"):
  1. DNA Glycosylase: Specifically recognizes and cleaves the $N$-glycosidic bond linking the altered base to the deoxyribose sugar, releasing the damaged base and leaving an intact apurinic or apyrimidinic (AP site).
  2. AP-Endonuclease: Recognizes the abasic site and nicks the phosphodiester backbone at the 5' end of the AP site.
  3. AP-Lyase (Deoxyribose-phosphate lyase): Cleaves the phosphodiester backbone at the 3' end, excising the remaining abasic deoxyribose-phosphate moiety.
  4. DNA Polymerase $\beta$: Inserts the single correct complementary deoxyribonucleotide into the gap.
  5. DNA Ligase: Seals the remaining phosphodiester nick.

3. Mismatch Repair (MMR)

• Timing & Lesion: Operates during the S and G2 phases, functioning immediately post-replication to catch base-pairing mismatches and insertion/deletion loops that escaped DNA polymerase proofreading.
• Mechanism: Eukaryotic MMR proteins (MSH2, MSH6, MLH1, PMS2) form complexes that scan newly synthesized DNA. They identify mismatched base pairs, discriminate the newly synthesized daughter strand from the methylated parental template, excise the mismatched tract, and recruit DNA polymerase and ligase to reconstruct the sequence.
• Defective Disease: Lynch Syndrome / HNPCC:
  • Autosomal dominant germline mutations in MSH2 (60%), MLH1 (30%), MSH6, or PMS2.
  • Loss of MMR results in Microsatellite Instability (MSI)—abnormal expansions or contractions of short repeating nucleotide sequences (tandem repeats) scattered throughout the genome.
  • Markedly elevated lifetime risk of Colorectal Carcinoma (typically right-sided/proximal colon, early onset <50 years) and extracolonic cancers (endometrial carcinoma, ovarian, gastric, urothelial).

4. Non-Homologous End Joining (NHEJ)

• Lesion: Repairs dangerous double-strand DNA breaks (DSBs) induced by ionizing radiation (X-rays, gamma rays), topoisomerase II inhibitors, and reactive oxygen species.
• Mechanism: Does not require a homologous DNA template. The Ku70/Ku80 heterodimer binds free double-strand DNA ends, recruits the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) and the Artemis endonuclease, which process damaged ends. DNA Ligase IV complexed with XRCC4 joins the blunt ends together.
• Mutational Risk: Because ends are processed without a template, nucleotides are frequently deleted or inserted at the junction. NHEJ is inherently error-prone and mutagenic. (NHEJ is also utilized physiologically in developing B and T cells for V(D)J somatic recombination).
• Defective Disease: Ataxia-Telangiectasia:
  • Autosomal recessive mutations in the ATM gene on chromosome 11q22, encoding a serine/threonine protein kinase that senses double-strand breaks and activates downstream cell-cycle checkpoint kinases (p53, Chk2).
  • Clinical Triad: Progressive cerebellar ataxia (wheelchair-bound by age 10), oculocutaneous telangiectasias (bulbar conjunctiva and sun-exposed skin), and severe immunodeficiency (marked IgA and IgG2 deficiency with recurrent sinopulmonary infections). Marked hypersensitivity to ionizing radiation (diagnostic X-rays should be minimized) and high risk of leukemias and lymphomas.

5. Homologous Recombination (HR)

• Lesion & Timing: Repairs double-strand breaks with high fidelity (error-free) during late S and G2 phases, when a fully replicated undamaged sister chromatid is available to serve as a physical template.
• Mechanism: Mediated by the MRN complex, BRCA1, BRCA2, and the Rad51 recombinase.
• Clinical Significance: Germline mutations in BRCA1 or BRCA2 impair homologous recombination, predisposing individuals to hereditary breast, epithelial ovarian, prostate, and pancreatic carcinomas. Defective HR renders tumors exquisitely sensitive to PARP inhibitors (e.g., olaparib) via synthetic lethality.

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Transcription, Eukaryotic RNA Polymerases & Pre-mRNA Processing

Transcription is the enzymatic synthesis of single-stranded RNA from a complementary DNA template strand, proceeding strictly in the 5' $\rightarrow$ 3' direction (reading the DNA template 3' $\rightarrow$ 5').

                  Eukaryotic Pre-mRNA Architecture & Processing

   Pre-mRNA (hnRNA):
   5' ────[ 5' Cap ]──[ Exon 1 ]──[ Intron 1 ]──[ Exon 2 ]──[ Poly-A Signal ]─── 3'
                           │            │           │             │
                           │            ▼           │             ▼
                           │       SPLICING         │       POLYADENYLATION
                           │    (snRNPs U1-U6)      │    (Poly-A Polymerase)
                           ▼    (Excises Intron)    ▼             ▼
   Mature mRNA:
   5' ──[ m7G Cap ]──────[ Exon 1 ]───────[ Exon 2 ]─────────[ Poly-A Tail (200 A's) ]── 3'

1. Eukaryotic RNA Polymerases Comparison

Unlike prokaryotes, which utilize a single RNA polymerase core enzyme ($\alpha_2\beta\beta'\omega$) bound to a sigma ($\sigma$) initiation factor, eukaryotes utilize three distinct nuclear RNA polymerases:

1. RNA Polymerase I: Located in the nucleolus. Synthesizes the large 45S precursor ribosomal RNA (rRNA), which is processed into mature 28S, 18S, and 5.8S rRNAs. Ribosomal RNA is the most abundant type of RNA in the cell by mass ("rRNA is Ramped"). Resistant to $\alpha$-amanitin.
2. RNA Polymerase II: Located in the nucleoplasm. Synthesizes messenger RNA (mRNA) precursors (heterogeneous nuclear RNA / hnRNA), small nuclear RNAs (snRNAs), and microRNAs (miRNAs). It is the largest RNA type by molecular length ("mRNA is Massive").
   • Inhibition by $\alpha$-Amanitin: RNA Pol II is extraordinarily sensitive to and potently inhibited by $\alpha$-amanitin, a bicyclic octapeptide toxin produced by Amanita phalloides (the "death cap" mushroom). Following ingestion, $\alpha$-amanitin binds tightly to RNA Polymerase II, completely halting eukaryotic mRNA transcription and protein synthesis. Patients present with severe gastrointestinal distress (watery diarrhea, vomiting) followed 48 hours later by fulminant hepatic necrosis, jaundice, and acute liver failure.
3. RNA Polymerase III: Located in the nucleoplasm. Synthesizes transfer RNA (tRNA) and 5S ribosomal RNA. It represents the most abundant RNA type by molecule count ("tRNA is Tiny"). Inhibited by $\alpha$-amanitin only at extraordinarily high concentrations.

(Mnemonic for Polymerases in order I, II, III: R-M-T $\rightarrow$ rRNA, mRNA, tRNA).

2. Eukaryotic Promoters, Enhancers & Silencers

• Promoter: Non-coding DNA sequences located immediately upstream from the transcription start site (+1). Contains the conserved TATA box (Hogness box) at position -25 bp and the CAAT box at -75 bp. Bound by general transcription factors (e.g., TFIID via TATA-Binding Protein / TBP) to assemble RNA Pol II into the pre-initiation complex. Mutations in the promoter typically cause a dramatic reduction in transcription volume.
• Enhancers: DNA sequences that bind sequence-specific transcriptional activator proteins. Enhancers can be located thousands of base pairs upstream, downstream, or within the introns of the gene they regulate. DNA looping allows enhancer-bound activators to physically contact promoter-associated transcription factors to drastically increase transcription rate.
• Silencers: DNA regulatory sequences that bind repressor proteins to decrease or halt transcription.

3. Post-Transcriptional Processing of Pre-mRNA (hnRNA)

Nascent pre-mRNA undergoes three obligatory modifications within the eukaryotic nucleus prior to nuclear export:

1. 5' 7-Methylguanosine ($m^7G$) Capping: Added cotranscriptionally to the 5' triphosphate end via an unusual 5'-to-5' triphosphate linkage. Protects nascent mRNA from degradation by 5' exonucleases and provides the molecular recognition signal for the eukaryotic initiation factor 4F complex (eIF4E) during translation initiation.
2. 3' Polyadenylation: Cleavage and polyadenylation specificity factor (CPSF) recognizes the conserved consensus signal AAUAAA near the 3' end of the transcript. The RNA is cleaved ~10-30 nucleotides downstream, and poly-A polymerase (PAP) adds a tail of 200 to 250 adenine residues. Crucial board fact: Poly-A tail addition does NOT require a DNA template; it promotes mRNA stability and facilitates nuclear export.
3. Pre-mRNA Splicing & The Spliceosome: Removal of non-coding introns and covalent ligation of coding exons:
   • Conserved Splice Sites: Splicing adheres strictly to the GU-AG rule: the 5' splice donor site begins with GU, the branch point within the intron contains an invariant adenosine, and the 3' splice acceptor site terminates with AG.
   • Mechanism: The spliceosome is assembled from five small nuclear ribonucleoproteins (snRNPs: U1, U2, U4, U5, U6) complexed with pre-mRNA. The 2'-OH group of the branch-point adenosine launches a nucleophilic attack on the 5' GU donor phosphate, creating a unique 2'-5' phosphodiester bond that forms a loop-shaped lariat intermediate. The newly exposed 3'-OH of the 5' exon attacks the 3' AG acceptor, ligating the exons and releasing the lariat, which is degraded.
   • Clinical Autoimmune Connections: Autoantibodies directed against snRNP core proteins (anti-Smith / anti-Sm antibodies) are highly specific for Systemic Lupus Erythematosus (SLE). Autoantibodies against anti-U1 RNP are diagnostic of Mixed Connective Tissue Disease (MCTD).
   • Alternative Splicing: A regulated process wherein different combinations of exons are selectively included or skipped in the mature mRNA. Allows a single eukaryotic gene to encode multiple structurally and functionally distinct protein isoforms (e.g., membrane-bound vs secreted immunoglobulins; tissue-specific tropomyosin isoforms).

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The Genetic Code, Mutation Classifications & Translation

                          Types of Genetic Point Mutations

    Original Wild-Type Sequence:   5' ─── AUG   AAG   GAG   UAA ─── 3'
    Amino Acid Translation:              Met   Lys   Glu   Stop
    
    1. SILENT MUTATION:            5' ─── AUG   AAA   GAG   UAA ─── 3'
       (Wobble change, same aa)          Met   Lys   Glu   Stop
       
    2. MISENSE MUTATION:           5' ─── AUG   AAG   GUG   UAA ─── 3'
       (Different amino acid)            Met   Lys   Val   Stop  (e.g., HbS Sickle Cell)
       
    3. NONSENSE MUTATION:          5' ─── AUG   UAG   GAG   UAA ─── 3'
       (Premature stop codon)            Met   STOP               (Truncated, nonfunctional)
       
    4. FRAMESHIFT MUTATION:        5' ─── AUG   AGA   GGA   GUA   A ── 3'
       (1-bp insertion 'G')              Met   Arg   Gly   Val   ...
       (Completely altered reading frame downstream; e.g., Duchenne Muscular Dystrophy)

1. Characteristics of the Universal Genetic Code

• Degenerate / Redundant: 64 triplet codons encode only 20 standard amino acids; multiple distinct codons specify the same amino acid. (Exceptions: Methionine [AUG] and Tryptophan [UGG] are encoded by only a single codon each).
• Wobble Hypothesis: Pairing between the first two bases of the mRNA codon and the anticodon of the tRNA is rigid, but non-Watson-Crick pairing is permitted at the 3' base of the codon (the "wobble" position, corresponding to the 5' base of the tRNA anticodon). Allows cells to translate all 61 amino acid codons using fewer than 61 distinct tRNAs.
• Unambiguous: Each specific codon specifies one and only one amino acid.
• Universal: Highly conserved across all biological species (with rare minor exceptions in mitochondrial DNA, where UGA encodes tryptophan instead of a stop signal).
• Non-Overlapping & Commaless: Read continuously three nucleotides at a time without gaps or shared bases from a fixed start point.
• Start Codon: AUG encodes Methionine in eukaryotes and N-formylmethionine (fMet) in prokaryotes and mitochondria. (Bacterial fMet peptides are potent chemoattractants for human neutrophils via formyl peptide receptors during lower extremity soft tissue infections).
• Stop Codons: UAA, UAG, UGA (Mnemonic: U Are Away, U Are Gone, U Go Away). Recognize protein release factors rather than tRNAs.

2. Mutation Classifications & Clinical Prototypes

1. Silent (Synonymous) Mutation: A nucleotide substitution that generates an altered codon specifying the exact same amino acid (commonly occurring at the 3rd wobble position). Typically has no structural or functional consequence.
2. Missense Mutation: A nucleotide substitution that alters a codon to specify a different amino acid:
   • Conservative: Replaced by an amino acid with similar physicochemical properties (e.g., Asp replaced by Glu).
   • Non-Conservative: Replaced by an amino acid with completely different chemical charge or polarity. Prototypes:
     • Sickle Cell Disease (HbS): A single point mutation ($A \rightarrow T$ transversion) in codon 6 of the $\beta$-globin gene changes $GAG$ to $GTG$, substituting hydrophilic, negatively charged glutamic acid with hydrophobic valine ($Glu6Val$). Under deoxygenated conditions, hydrophobic valine inserts into a complementary pocket on an adjacent $\beta$-chain, causing hemoglobin tetramers to polymerize into rigid, insoluble crescent-shaped sickled erythrocytes. Leads to microvascular vaso-occlusion, excruciating pain crises, dactylitis (hand-foot syndrome) in infants, avascular necrosis of the femoral and talar heads, and heightened susceptibility to Salmonella enterica osteomyelitis.
     • Hemoglobin C (HbC): Codon 6 missense mutation substituting glutamic acid with basic, positively charged lysine ($Glu6Lys$; $GAG \rightarrow AAG$). Produces mild extravascular hemolytic anemia and characteristic intracellular HbC crystals on peripheral blood smear.
3. Nonsense Mutation: A nucleotide substitution that transforms an amino-acid-specifying codon into a premature stop codon (UAA, UAG, or UGA). Results in a truncated, incomplete polypeptide chain that is non-functional and rapidly degraded by cellular proteasomes (e.g., severe forms of $\beta^0$-thalassemia, cystic fibrosis).
4. Frameshift Mutation: Deletion or insertion of a number of nucleotides that is not divisible by 3, altering the reading frame for all downstream codons. Almost always results in an aberrant amino acid sequence terminating in a premature stop codon downstream. Prototypes:
   • Duchenne Muscular Dystrophy (DMD): X-linked recessive frameshift deletion in the massive dystrophin gene (anchoring actin cytoskeleton to extracellular matrix). Causes complete absence of functional dystrophin, leading to progressive muscle fiber necrosis, wheelchair dependence by age 12, pseudohypertrophy of calves, Gowers sign, and dilated cardiomyopathy.
   • Contrast with Becker Muscular Dystrophy (BMD): Caused by in-frame deletions or insertions (divisible by 3) in the dystrophin gene. Produces a truncated but partially functional dystrophin protein, resulting in a significantly milder, later-onset clinical phenotype.

3. Ribosomal Translation Cascade

Ribosomal translation proceeds in three coordinated phases within the cytoplasm:

• Ribosome Subunits: Prokaryotes utilize 70S ribosomes (50S + 30S); Eukaryotes utilize 80S ribosomes (60S + 40S). Each ribosome contains three distinct tRNA binding pockets: the A site (Aminoacyl: accepts incoming charged aminoacyl-tRNA), the P site (Peptidyl: holds the tRNA carrying the growing polypeptide chain), and the E site (Exit: holds empty, deacylated tRNA prior to discharge).
• Phase 1: Initiation: Eukaryotic initiation factors (eIFs) bind the 5' cap and poly-A tail of mature mRNA. The small 40S ribosomal subunit complexed with initiator Met-$tRNA_i^{\text{Met}}$ scans the 5' UTR until it identifies the AUG start codon within the Kozak consensus sequence ($5'\text{-ACCAUGG-}3'$). GTP hydrolysis releases eIFs, and the large 60S subunit docks to form the 80S initiation complex, placing Met-$tRNA_i^{\text{Met}}$ directly into the P site.
• Phase 2: Elongation:
  1. Aminoacyl-tRNA Binding: Elongation factor eEF-1A hydrolyzes GTP to deliver the correct aminoacyl-tRNA into the vacant A site.
  2. Peptide Bond Formation: Catalyzed by the ribozyme peptidyl transferase (the 28S rRNA catalytic core of the 60S eukaryotic subunit). The amino group in the A site attacks the ester linkage in the P site, transferring the growing polypeptide onto the A-site tRNA.
  3. Translocation: Elongation factor eEF-2 hydrolyzes GTP to advance the ribosome by exactly three nucleotides toward the 3' end of the mRNA. The uncharged tRNA moves to the E site and exits; the peptidyl-tRNA shifts from the A site to the P site, vacating the A site for the next cycle.
• Phase 3: Termination: When a stop codon enters the A site, eukaryotic release factors (eRF1 and eRF3) bind the ribosome, stimulating peptidyl transferase to hydrolyze the ester bond linking the polypeptide to the P-site tRNA using a water molecule, releasing the completed protein and triggering ribosomal dissociation.

Nucleotide Synthesis, Salvage and Degradation

Purine and pyrimidine metabolism explains gout and the actions of many antimetabolite drugs.

De novo purine synthesis. The purine ring is built on PRPP (ribose-5-phosphate from the HMP shunt). The committed step is glutamine-PRPP amidotransferase, which is inhibited by purine nucleotides. The pathway uses glycine, glutamine, aspartate, CO2 and N10-formyl-tetrahydrofolate, producing IMP, which is converted to AMP and GMP. Mycophenolate inhibits IMP dehydrogenase, which blocks GMP synthesis in lymphocytes. Methotrexate limits purine and thymidine synthesis by inhibiting dihydrofolate reductase.

Purine salvage and breakdown.

• HGPRT recycles hypoxanthine and guanine back to IMP and GMP. Lesch-Nyhan syndrome (X-linked complete HGPRT deficiency) causes uric acid overproduction, gout, intellectual disability and self-mutilation.
• Adenosine deaminase (ADA) deficiency causes severe combined immunodeficiency.
• Purines are degraded to hypoxanthine, then xanthine oxidase forms xanthine and then uric acid. Allopurinol and febuxostat inhibit xanthine oxidase (11.3).

Pyrimidine synthesis.

• The ring is built first and then attached to PRPP.
• Carbamoyl phosphate synthetase II (cytosolic, using glutamine) is the regulated step.
• Dihydroorotate dehydrogenase is inhibited by leflunomide.
• UMP synthase deficiency causes orotic aciduria with megaloblastic anemia that does not respond to B12 or folate and without hyperammonemia, unlike ornithine transcarbamylase deficiency.

Deoxynucleotides.

• Ribonucleotide reductase converts ribonucleotides to deoxyribonucleotides and is inhibited by hydroxyurea.
• Thymidylate synthase converts dUMP to dTMP using N5,N10-methylene-THF and is inhibited by 5-fluorouracil.

Why gout follows from purine turnover. Gout results from urate overproduction or, more often, underexcretion. Overproduction sources include tumor lysis, myeloproliferative disease, psoriasis and HGPRT deficiency. Underexcretion causes include CKD, thiazide and loop diuretics, low-dose aspirin and alcohol (8.2).