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Biochemistry
C6H12O6
?? Complete Textbook · Molecular Edition

Biochemistry

From amino acids to CRISPR — a rigorous, beautifully illustrated guide to the molecular machinery of life, with diagrams, video lectures, and audio in 9 languages.

12Chapters
58Topics
12Videos
12Diagrams
9Languages
Introduction ??

Introduction to Biochemistry

The Chemistry of Life

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???  Structural Diagram
The cell — the biochemical factory where all life chemistry occurs
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The cell — the biochemical factory where all life chemistry occurs

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Introduction to Biochemistry

Introduction to Biochemistry — Educational Video

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Audio Explanation — Introduction to Biochemistry
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01What Is Biochemistry?

Biochemistry is the study of the chemical processes and substances that occur within living organisms. It bridges biology and chemistry, seeking to explain biological phenomena in molecular terms. The word itself comes from the Greek bios (life) and the Arabic kimiya (chemistry). Biochemists ask questions like: How does DNA store and transmit genetic information? How do enzymes catalyze reactions with such extraordinary precision? How do cells generate and use energy? The answers to these questions have transformed medicine, agriculture, and biotechnology.

02The Molecular Logic of Life

Life is fundamentally chemical. The four major classes of biomolecules — carbohydrates, lipids, proteins, and nucleic acids — are all built from simpler units linked by covalent bonds. What makes life remarkable is the information content of these molecules: DNA encodes a program; proteins execute it. The same 20 amino acids, 4 nucleotide bases, and handful of sugars are used across virtually all life on Earth, from bacteria to blue whales — a testament to the deep chemical unity of biology discovered by biochemistry.

03Water: The Solvent of Life

Water is not merely the solvent in which biochemistry happens — it is an active participant. Its unique properties arise from hydrogen bonding: each water molecule can form up to four hydrogen bonds, creating a dynamic three-dimensional network. The high dielectric constant (e = 78) weakens electrostatic interactions between ions, allowing ionic compounds to dissolve. The hydrophobic effect — the tendency of nonpolar molecules to aggregate in water — drives protein folding, membrane formation, and ligand-receptor binding. Water's high heat capacity buffers temperature fluctuations critical to life.

04pH, Buffers & Biological Significance

Most biochemical reactions are exquisitely sensitive to pH. Enzymes have optimal pH ranges; even small deviations inactivate them. The Henderson-Hasselbalch equation (pH = pKa + log [A?]/[HA]) governs acid-base equilibria in biological systems. The bicarbonate buffer system (H2CO3 ? H? + HCO3?, pKa = 6.1) maintains blood pH at 7.4 in concert with respiratory and renal compensation. Phosphate buffers (pKa = 6.8) are critical intracellularly. Imidazole groups of histidine residues (pKa ˜ 6.0) serve as local pH buffers within enzyme active sites.

05Thermodynamics in Biochemistry

Living organisms are open thermodynamic systems — they exchange matter and energy with their environment. The Gibbs free energy change (?G = ?H - T?S) determines spontaneity: reactions with negative ?G proceed spontaneously. At standard biochemical conditions (pH 7, 37°C), we use ?G°'. Coupling exergonic reactions (negative ?G, like ATP hydrolysis: ?G°' = -30.5 kJ/mol) with endergonic reactions drives otherwise unfavorable processes. This thermodynamic coupling — the central trick of bioenergetics — allows cells to build complex molecules, pump ions, and move muscles against thermodynamic gradients.

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Chapter 01 ??

Amino Acids & Proteins

The Building Blocks of Function

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???  Structural Diagram
General structure of an amino acid: amino group, carboxyl group, and variable R-group
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General structure of an amino acid: amino group, carboxyl group, and variable R-group

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Amino Acids & Protein Structure

Amino Acids & Protein Structure — Educational Video

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Audio Explanation — Amino Acids & Proteins
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01The 20 Standard Amino Acids

All 20 standard amino acids share a common backbone: a central a-carbon bonded to an amino group (–NH3?), a carboxyl group (–COO?), a hydrogen atom, and a variable side chain (R group). At physiological pH 7.4, amino acids exist as zwitterions. They are classified by R-group properties: nonpolar aliphatic (Gly, Ala, Val, Leu, Ile, Pro, Met), aromatic (Phe, Trp, Tyr), polar uncharged (Ser, Thr, Cys, Asn, Gln), positively charged (Lys, Arg, His), and negatively charged (Asp, Glu). Cysteine forms disulfide bonds (–S–S–) critical for protein stability.

02Peptide Bonds & Primary Structure

Amino acids link via peptide bonds formed by condensation between the carboxyl group of one amino acid and the amino group of the next, releasing water. The peptide bond has partial double bond character due to resonance — it is planar, rigid, and predominantly in the trans conformation. Polypeptide chains have directionality: the N-terminus (free amino group) to C-terminus (free carboxyl group). Primary structure is the linear sequence of amino acids — encoded by DNA and determining all higher levels of protein structure through thermodynamic folding.

03Secondary Structure: a-Helix & ß-Sheet

Secondary structure refers to local regular folding stabilized by backbone hydrogen bonds. The a-helix: right-handed coil with 3.6 residues per turn, pitch 5.4 Å; each carbonyl oxygen (C=O) hydrogen bonds to the amide nitrogen (N–H) 4 residues ahead. a-Helices are found in muscle proteins (myosin, keratin) and membrane-spanning segments (helix is hydrophobic). The ß-pleated sheet: extended chains lying side by side, hydrogen bonding between adjacent strands; parallel (N-termini same direction, less stable) or antiparallel (opposite, more stable). Found in silk fibroin, immunoglobulins, amyloid fibrils.

04Tertiary & Quaternary Structure

Tertiary structure is the complete three-dimensional folding of a single polypeptide, stabilized by: hydrophobic interactions (dominant driving force — nonpolar side chains cluster inside), hydrogen bonds, ionic interactions (salt bridges), disulfide bonds (Cys–Cys), and van der Waals forces. Quaternary structure describes the assembly of multiple polypeptide subunits (protomers). Hemoglobin (a2ß2 tetramer) is the classic example — subunit interactions enable cooperativity and allosteric regulation. Chaperone proteins (Hsp70, GroEL/GroES) prevent misfolding and aggregation during synthesis.

05Protein Folding, Misfolding & Disease

Protein folding is guided by the energy landscape hypothesis: the native (correctly folded) state occupies the global free energy minimum. Anfinsen's experiment demonstrated that primary structure encodes tertiary structure — denatured ribonuclease refolds spontaneously. Misfolding causes disease: Alzheimer's (ß-amyloid plaques from amyloid precursor protein), Parkinson's (a-synuclein Lewy bodies), prion diseases (PrP? ? PrP?? conformational conversion — uniquely, misfolded proteins are infectious). Chaperones, the proteasome (ubiquitin-tagging ? 26S proteasome degradation), and autophagy maintain proteostasis.

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Chapter 02 ?

Enzymes

Biological Catalysts & Kinetics

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Induced fit model: enzyme undergoes conformational change upon substrate binding
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Induced fit model: enzyme undergoes conformational change upon substrate binding

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Enzymes & Enzyme Kinetics

Enzymes & Enzyme Kinetics — Educational Video

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Audio Explanation — Enzymes
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01Properties of Enzymes

Enzymes are biological catalysts — typically proteins (some RNAs, called ribozymes, also catalyze reactions). They accelerate reaction rates by factors of 106 to 10²³, without being consumed. Enzymes are remarkably specific: substrate specificity (often binding only one molecule or class), reaction specificity (catalyze only one type of reaction), and stereospecificity (distinguish enantiomers — L-amino acid oxidase acts only on L-amino acids). They lower the activation energy (Ea) by stabilizing the transition state — not by changing the equilibrium position. The active site is the small region where substrate binds and catalysis occurs.

02Michaelis-Menten Kinetics

The Michaelis-Menten model describes enzyme kinetics: E + S ? ES ? E + P. The Michaelis constant Km = (k?1 + k2) / k1 ˜ the substrate concentration at half-maximal velocity. It reflects enzyme-substrate affinity (low Km = high affinity). Vmax = kcat × [E]total, where kcat (turnover number) is the catalytic rate per active site per second (carbonic anhydrase: 600,000 s?¹; typical enzymes: 1–1000 s?¹). The Lineweaver-Burk (double-reciprocal) plot (1/V vs 1/[S]) linearizes the hyperbolic Michaelis-Menten curve, allowing graphical determination of Km and Vmax.

03Enzyme Inhibition

Competitive inhibition: inhibitor resembles substrate, binds active site, increases apparent Km, Vmax unchanged (can be overcome by excess substrate). Noncompetitive inhibition: inhibitor binds a site other than the active site (allosteric site), reduces Vmax without affecting Km. Uncompetitive inhibition: inhibitor binds only the ES complex, reduces both Vmax and Km. Irreversible inhibitors form covalent bonds: organophosphates (nerve agents, aspirin acetylates COX — aspirin irreversibly inhibits cyclooxygenase, blocking thromboxane A2 synthesis) and penicillin (acylates transpeptidase). Many drugs and toxins work as enzyme inhibitors.

04Coenzymes, Cofactors & Vitamins

Many enzymes require non-protein helpers. Metal ion cofactors: Zn²? (carboxypeptidase, carbonic anhydrase), Fe²?/Fe³? (cytochromes), Mg²? (kinases binding ATP-Mg²?). Coenzymes (organic cofactors): NAD?/NADH (niacin/B3 — electron carrier in oxidation/reduction), FAD/FADH2 (riboflavin/B2), CoA (pantothenic acid/B5 — acyl group carrier), thiamine pyrophosphate (B1 — decarboxylation), pyridoxal phosphate (B6 — amino acid metabolism), tetrahydrofolate (folate/B9 — one-carbon transfers), cobalamin (B12 — methyl transfers). Vitamin deficiencies cause specific enzyme failures: B1 deficiency ? pyruvate dehydrogenase deficiency ? Wernicke's encephalopathy.

05Allosteric Regulation & Cooperativity

Allosteric enzymes have regulatory sites distinct from the active site. Binding of an effector molecule (activator or inhibitor) at the allosteric site induces conformational changes that alter active site activity. They often show sigmoidal (S-shaped) kinetics rather than hyperbolic, modeled by the Hill equation: V = Vmax[S]n / (K'n + [S]n), where n is the Hill coefficient (cooperativity index). Hemoglobin is the textbook example: O2 binding to one subunit increases affinity of others (positive cooperativity, n = 2.8). ATCase (aspartate transcarbamoylase) is allosterically inhibited by its end product CTP (feedback inhibition) — a key regulatory principle in biosynthesis.

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Chapter 03 ??

Carbohydrates

Energy Currency & Structural Materials

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Glucose: Fischer projection to Haworth representation showing ring closure
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Glucose: Fischer projection to Haworth representation showing ring closure

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Carbohydrates in Biochemistry

Carbohydrates in Biochemistry — Educational Video

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Audio Explanation — Carbohydrates
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01Monosaccharides: Structure & Stereochemistry

Monosaccharides are the simplest carbohydrates, with the formula (CH2O)n. They are classified by: number of carbons (trioses C3, pentoses C5, hexoses C6), functional group (aldoses have aldehyde, ketoses have ketone). Glucose (C6H12O6) is the most abundant — the primary fuel for most cells. D-glucose and L-glucose are mirror images (enantiomers); most biological sugars are D-configuration. Epimers differ at one carbon: glucose (C2-OH equatorial in a-pyranose) and galactose (C4 epimer) differ only at C4, yet this difference is metabolically significant (galactosemia). Anomers (a and ß) differ at the anomeric carbon created by ring formation.

02Disaccharides & Glycosidic Bonds

Monosaccharides link via glycosidic bonds formed by condensation between the anomeric hydroxyl of one sugar and a hydroxyl of another. The type of bond profoundly affects properties: maltose (a-1,4-glucose–glucose — from starch digestion), lactose (ß-1,4-galactose–glucose — milk sugar; lactase deficiency causes lactose intolerance in ~65% of adults), sucrose (a-1,2-ß-glucose–fructose — table sugar; non-reducing since both anomeric carbons linked), trehalose (a-1,1 — remarkably stable, protects organisms in desiccation). Reducing sugars (free anomeric OH) reduce Cu²? in Benedict's test — used diagnostically.

03Polysaccharides: Storage & Structure

Starch (plant energy storage): amylose (unbranched a-1,4 chains, helical) + amylopectin (branched a-1,4 with a-1,6 branch points every 24–30 residues). Glycogen (animal energy storage): more branched than amylopectin (a-1,6 every 8–12 residues), allowing faster glucose release by multiple phosphorylase enzymes working simultaneously at branch ends. Cellulose (plant structural): ß-1,4-glucose chains — the ß linkage creates straight, ribbon-like chains that form hydrogen-bonded microfibrils of extraordinary tensile strength; humans lack cellulase. Chitin (ß-1,4-N-acetylglucosamine): structural polymer in insect exoskeletons and fungal cell walls.

04Glycolysis: The Central Pathway

Glycolysis (Greek: glykys = sweet, lysis = splitting) converts glucose (6C) to two pyruvate (3C) molecules in 10 enzymatic steps, occurring in the cytoplasm of all cells. Investment phase (steps 1–5): consumes 2 ATP to phosphorylate glucose (hexokinase) ? glucose-6-phosphate ? fructose-6-phosphate ? fructose-1,6-bisphosphate (phosphofructokinase-1: committed step, allosterically regulated by ATP, AMP, citrate) ? 2 glyceraldehyde-3-phosphate. Payoff phase (steps 6–10): produces 4 ATP + 2 NADH net. Key regulated enzymes: hexokinase (inhibited by G6P), PFK-1 (inhibited by ATP, citrate; activated by AMP, ADP, fructose-2,6-bisphosphate), pyruvate kinase (activated by fructose-1,6-bisphosphate).

05Gluconeogenesis & Glycogen Metabolism

Gluconeogenesis synthesizes glucose from non-carbohydrate precursors (pyruvate, lactate, glycerol, glucogenic amino acids) — primarily in liver and kidney. It is not simply glycolysis in reverse: three irreversible glycolytic steps are bypassed by four unique enzymes: pyruvate carboxylase (pyruvate ? oxaloacetate, requires biotin), PEPCK (OAA ? PEP), fructose-1,6-bisphosphatase (F1,6BP ? F6P), and glucose-6-phosphatase (liver/kidney only). Glycogen synthesis: glucose-1-phosphate ? UDP-glucose (UDP-glucose pyrophosphorylase) ? glycogen synthase adds to non-reducing end; branching enzyme creates a-1,6 branches. Glycogen phosphorylase cleaves a-1,4 bonds, releasing glucose-1-phosphate — regulated by phosphorylation (glucagon/epinephrine) and allosteric effectors.

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Chapter 04 ??

Lipids & Membranes

Hydrophobic Molecules & Biological Boundaries

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Fluid mosaic model of the cell membrane: phospholipid bilayer with embedded proteins
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Fluid mosaic model of the cell membrane: phospholipid bilayer with embedded proteins

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Lipids and Membrane Structure

Lipids and Membrane Structure — Educational Video

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Audio Explanation — Lipids & Membranes
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01Fatty Acids & Triacylglycerols

Fatty acids are carboxylic acids with long hydrocarbon chains (typically C14–C22). Saturated fatty acids have no double bonds (palmitic acid C16:0, stearic acid C18:0) — straight chains pack tightly ? high melting point, solid at room temperature. Unsaturated fatty acids have one (monounsaturated, e.g., oleic acid C18:1?9) or more (polyunsaturated, e.g., linoleic acid C18:2?9,12) double bonds — cis double bonds introduce kinks ? lower melting points, liquid oils. Essential fatty acids (linoleic ?-6, a-linolenic ?-3) cannot be synthesized de novo and must be obtained from diet. Triacylglycerols (triglycerides): glycerol + 3 fatty acids via ester bonds — major energy storage form, yielding ~38 kJ/g (vs 17 kJ/g for carbohydrates).

02Phospholipids & Membrane Assembly

Phospholipids are the fundamental building blocks of biological membranes. Structure: glycerol backbone, two fatty acid 'tails' (hydrophobic), phosphate-head group (hydrophilic) — amphipathic molecules. Common phospholipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS, negatively charged, inner leaflet), phosphatidylinositol (PI, signaling precursor). Sphingomyelin: sphingosine backbone. In water, phospholipids spontaneously form bilayers (critical vesicle diameter ~25 nm) — the hydrophobic effect drives tail sequestration, minimizing water-hydrocarbon contacts. Cholesterol inserts between phospholipids, modulating membrane fluidity (too fluid ? disrupts barrier; too rigid ? impairs protein function).

03Membrane Fluidity & the Fluid Mosaic Model

Singer and Nicolson (1972) proposed the fluid mosaic model: membranes are 2D fluids of lipids in which proteins are embedded or attached. Lateral diffusion (within one leaflet, rapid, D ˜ 10?8 cm²/s) versus transverse diffusion/flip-flop (across bilayer, very slow, requires flippases). Fluidity depends on: fatty acid saturation (more unsaturated ? more fluid), chain length (shorter ? more fluid), temperature (higher ? more fluid), cholesterol content (at physiological temps: orders lipid packing, reduces fluidity; above Tm: increases ordering). Lipid rafts — cholesterol and sphingolipid-enriched microdomains — concentrate signaling proteins and may act as scaffolds for signal transduction.

04ß-Oxidation of Fatty Acids

Fatty acid oxidation (ß-oxidation) occurs in the mitochondrial matrix (long-chain) and peroxisome (very long-chain, branched). Activation: fatty acid + CoA + ATP ? acyl-CoA + AMP + PPi (fatty acyl-CoA synthetase). Transport into mitochondria via carnitine shuttle: acyl-CoA ? acylcarnitine (carnitine acyltransferase I, outer membrane — rate-limiting, inhibited by malonyl-CoA preventing futile cycle with synthesis) ? inner membrane transporter ? acyl-CoA regenerated inside. Each round of ß-oxidation: 1 FAD ? FADH2 (1.5 ATP), 1 NAD? ? NADH (2.5 ATP), releases acetyl-CoA. Palmitate (C16): 7 rounds ? 7 FADH2 + 7 NADH + 8 acetyl-CoA ? net ~108 ATP (minus 2 for activation).

05Ketone Bodies & Lipid Storage Diseases

When oxaloacetate is limiting (starvation, diabetes), acetyl-CoA from ß-oxidation diverts to ketone body synthesis in liver mitochondria: acetoacetate, ß-hydroxybutyrate (exported to blood), and acetone (volatile, causes sweet breath in diabetic ketoacidosis). The brain (normally glucose-dependent) adapts to use ketone bodies during prolonged starvation — reducing reliance on glucose and protein catabolism. Lipid storage diseases (sphingolipidoses) result from lysosomal enzyme deficiencies: Gaucher disease (ß-glucocerebrosidase deficiency ? glucocerebroside accumulation in macrophages), Niemann-Pick (sphingomyelinase ? sphingomyelin), Tay-Sachs (hexosaminidase A ? GM2 ganglioside in neurons ? neurodegeneration).

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Chapter 05 ??

Nucleic Acids & DNA

Information Storage & Genetic Code

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???  Structural Diagram
The DNA double helix: antiparallel strands held by hydrogen bonds between base pairs
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The DNA double helix: antiparallel strands held by hydrogen bonds between base pairs

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DNA Structure & Nucleic Acids

DNA Structure & Nucleic Acids — Educational Video

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Audio Explanation — Nucleic Acids & DNA
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01Nucleotides: Building Blocks of DNA & RNA

Nucleotides consist of: a nitrogenous base (purine: adenine, guanine — fused bicyclic; pyrimidine: cytosine, thymine [DNA only], uracil [RNA only] — monocyclic), a pentose sugar (2'-deoxyribose in DNA, ribose in RNA — 2'-OH of RNA makes it more reactive/less stable), and one to three phosphate groups. Nucleoside = base + sugar. Nucleotide = nucleoside + phosphate. Nucleotides join via 3'?5' phosphodiester bonds, creating the polynucleotide backbone with: a 5'-phosphate end and a 3'-hydroxyl end. The sequence of bases carries genetic information. ATP (adenosine triphosphate) is the universal energy currency; cAMP (cyclic adenosine monophosphate) is a second messenger.

02DNA Double Helix & Base Pairing

Watson and Crick (1953) deduced the double helix from X-ray crystallography (Franklin and Wilkins). DNA is a right-handed antiparallel double helix. The two strands are held by hydrogen bonds between specific base pairs: A–T (2 hydrogen bonds), G–C (3 hydrogen bonds — GC pairs contribute more to thermal stability). Chargaff's rules: [A] = [T], [G] = [C] in any double-stranded DNA. The helix has a major groove and minor groove — transcription factors and restriction enzymes bind primarily in the major groove (wider, more information accessible). B-DNA (physiological form): 10 bp/turn, pitch 34 Å, diameter 20 Å. A-DNA: 11 bp/turn, shorter/wider. Z-DNA: left-handed, forms in GC-rich regions.

03DNA Replication

DNA replication is semiconservative (Meselson-Stahl experiment, 1958): each daughter molecule retains one parental strand. Replication begins at origins of replication (single origin in bacteria; thousands in eukaryotes). Helicase unwinds the double helix; topoisomerases relieve torsional stress. Primase synthesizes short RNA primers (needed because DNA polymerases can only extend, not initiate). DNA polymerase III (bacteria) / Pol d, e (eukaryotes) extends in 5'?3' direction only — so one strand is synthesized continuously (leading strand) and one discontinuously as Okazaki fragments (lagging strand). DNA ligase joins Okazaki fragments after RNA primers are removed and replaced with DNA. Fidelity: one error per 10?–10¹° bp due to 3'?5' proofreading exonuclease.

04Transcription & the Central Dogma

The central dogma of molecular biology (Crick, 1958): DNA ? RNA ? Protein (with reverse transcription as a special case). Transcription: RNA polymerase reads the template (antisense) DNA strand 3'?5' and synthesizes mRNA 5'?3'. In bacteria: one RNA polymerase (with s factor for promoter recognition). In eukaryotes: RNA Pol I (rRNA), Pol II (mRNA, snRNA), Pol III (tRNA, 5S rRNA). Eukaryotic pre-mRNA processing: 5' capping (7-methylguanosine cap — protects from exonuclease, aids ribosome binding), 3' polyadenylation (poly-A tail of ~250 adenine nucleotides — stability, nuclear export), and splicing (removal of introns by spliceosome — snRNP complexes guided by 5' splice site, branch point, 3' splice site).

05Translation & the Genetic Code

The genetic code: 64 codons (4³) encode 20 amino acids + 3 stop codons (UAA, UAG, UGA). The code is: degenerate (most amino acids have multiple codons — synonymous codons), non-overlapping (each base belongs to only one codon), nearly universal (same code from bacteria to humans — exception: mitochondria), and comma-free (read in triplets without punctuation between codons). Translation at the ribosome (small subunit: mRNA and anticodon recognition; large subunit: peptidyl transferase activity — a ribozyme): initiation ? elongation (aminoacyl-tRNA ? A site; peptide bond formation; translocation) ? termination (release factors recognize stop codons). Antibiotics target translation: streptomycin (30S), erythromycin (50S), cycloheximide (eukaryotic 80S).

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Chapter 06 ??

Cellular Metabolism & Bioenergetics

ATP Synthesis, Krebs Cycle & Oxidative Phosphorylation

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???  Structural Diagram
Overview of cellular respiration: glycolysis ? Krebs cycle ? oxidative phosphorylation
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Overview of cellular respiration: glycolysis ? Krebs cycle ? oxidative phosphorylation

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Cellular Respiration & the Krebs Cycle

Cellular Respiration & the Krebs Cycle — Educational Video

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Audio Explanation — Cellular Metabolism & Bioenergetics
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01Pyruvate Dehydrogenase & Acetyl-CoA

The pyruvate dehydrogenase complex (PDC) bridges glycolysis and the citric acid cycle. Located in the mitochondrial matrix, the PDC catalyzes the oxidative decarboxylation of pyruvate: pyruvate + NAD? + CoA ? acetyl-CoA + CO2 + NADH. It is one of the most complex enzyme assemblies known, consisting of three enzymatic components (E1: pyruvate decarboxylase requiring TPP; E2: dihydrolipoyl transacetylase requiring lipoic acid and CoA; E3: dihydrolipoyl dehydrogenase requiring FAD and NAD?) and two regulatory enzymes (PDH kinase: inactivates PDC; PDH phosphatase: activates PDC). PDC is activated by pyruvate, CoA, NAD?, AMP; inhibited by acetyl-CoA, NADH, ATP — a master regulatory switch between glycolysis and the citric acid cycle.

02The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle (TCA cycle) is the central hub of aerobic metabolism, oxidizing acetyl-CoA to CO2 while capturing electrons as NADH and FADH2. Eight steps: Citrate synthase (acetyl-CoA + OAA ? citrate, irreversible), aconitase (citrate ? isocitrate), isocitrate dehydrogenase (isocitrate ? a-ketoglutarate + CO2 + NADH, regulated by ADP+), a-ketoglutarate dehydrogenase (a-KG ? succinyl-CoA + CO2 + NADH), succinyl-CoA synthetase (substrate-level phosphorylation ? 1 GTP), succinate dehydrogenase (succinate ? fumarate + FADH2, embedded in inner mitochondrial membrane, part of Complex II), fumarase, malate dehydrogenase (malate ? OAA + NADH). Per acetyl-CoA: 3 NADH, 1 FADH2, 1 GTP. Cycle intermediates serve as biosynthetic precursors (OAA ? glucose, a-KG ? glutamate, succinyl-CoA ? heme).

03Electron Transport Chain

The electron transport chain (ETC) in the inner mitochondrial membrane contains four complexes that transfer electrons from NADH/FADH2 to O2. Complex I (NADH:ubiquinone oxidoreductase): NADH ? ubiquinone (Q), pumps 4 H?/2e?. Complex II (succinate dehydrogenase): FADH2 ? Q, no proton pumping (explains lower ATP yield). Complex III (cytochrome bc1): QH2 ? cytochrome c, pumps 4 H?/2e? (via Q-cycle). Complex IV (cytochrome c oxidase): 4 cyt c ? O2 ? 2H2O, pumps 4 H?/2e?. Electrons flow down the redox potential gradient (NADH: -0.32 V ? O2: +0.82 V, ?E°' = 1.14 V, ?G°' = -220 kJ/mol). Cytochrome c shuttles between III and IV; coenzyme Q between I/II and III. Electron transport is coupled to proton pumping, creating the proton motive force.

04ATP Synthase & Chemiosmosis

Peter Mitchell's chemiosmotic theory (Nobel Prize 1978) explains how the proton gradient drives ATP synthesis. The proton motive force (?p = -59?pH + ??, in mV) consists of a chemical gradient (?pH ˜ -1 unit, pH 8 matrix vs. pH 7 intermembrane space) and electrical gradient (?? ˜ -180 mV, inside negative). ATP synthase (Complex V, F0F1-ATPase): F0 subunit embedded in inner membrane — protons flow through the c-ring (8–15 subunits) ? rotates the ? stalk ? drives conformational changes in F1 ß subunits (three states: Open/Loose/Tight per binding change mechanism of Boyer, Nobel Prize 1997) ? ADP + P? ? ATP. Each complete rotation (360°) produces ~3 ATP. Approximately 10–13 protons per ATP. Total yield: ~30 ATP per glucose (not the textbook 36–38, which overcounted).

05Metabolic Integration & Regulation

Metabolism is tightly integrated across pathways and compartments. Key regulatory principles: allosteric regulation (AMP/ADP activate catabolic enzymes; ATP/NADH inhibit them — energy charge = ([ATP] + 0.5[ADP]) / ([ATP] + [ADP] + [AMP])), covalent modification (phosphorylation by PKA, dephosphorylation by phosphatases — second-to-second regulation), substrate availability, transcriptional control (hours timescale). AMPK (AMP-activated protein kinase): cellular energy sensor — activated by low energy (high AMP:ATP) ? activates catabolism (fatty acid oxidation, glycolysis), inhibits anabolism (fatty acid synthesis, gluconeogenesis). AMPK is activated by exercise and metformin (the most widely prescribed antidiabetic drug).

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Chapter 07 ??

Signal Transduction

How Cells Receive & Respond to Information

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Signal transduction pathways: receptor activation to cellular response
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Signal transduction pathways: receptor activation to cellular response

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Cell Signaling & Signal Transduction

Cell Signaling & Signal Transduction — Educational Video

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Audio Explanation — Signal Transduction
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01Principles of Cell Signaling

Cell signaling enables communication between cells and with the environment. Ligands (hormones, neurotransmitters, growth factors, cytokines) bind receptors with high specificity and affinity (Kd = 10?? to 10?¹¹ M). Signaling can be: endocrine (hormone travels in blood to distant target, seconds to hours), paracrine (acts on nearby cells), autocrine (cell signals itself), juxtacrine (direct cell-cell contact). Signal transduction cascades amplify the signal: one hormone binding can generate millions of second messenger molecules in milliseconds. Downstream responses include: changes in enzyme activity, gene expression, cell division, apoptosis, or differentiation.

02G-Protein Coupled Receptors (GPCRs)

GPCRs constitute the largest family of cell-surface receptors (~800 in humans) and are targets of ~34% of approved drugs. They have 7 transmembrane a-helices (7TM). Mechanism: ligand binding ? conformational change ? G-protein (heterotrimeric Gaß?) exchanges GDP for GTP on Ga ? Ga-GTP dissociates from Gß? ? both activate downstream effectors. Gs activates adenylyl cyclase ? ?cAMP ? PKA phosphorylates targets (CREB transcription factor, phosphorylase kinase, lipase). Gi inhibits adenylyl cyclase. Gq activates phospholipase C-ß ? IP3 (? ER Ca²? release) + DAG (? PKC activation). Ga has intrinsic GTPase activity (self-inactivating). Cholera toxin ADP-ribosylates Gas ? cannot hydrolyze GTP ? constitutive AC activation ? massive cAMP ? Cl? secretion ? secretory diarrhea.

03Receptor Tyrosine Kinases & the RAS Pathway

Receptor tyrosine kinases (RTKs) mediate responses to growth factors (EGF, insulin, PDGF, FGF). Structure: extracellular ligand-binding domain, single transmembrane helix, intracellular kinase domain. Activation: ligand binding ? receptor dimerization ? transautophosphorylation of tyrosine residues ? phosphotyrosines recruit SH2 domain-containing adaptor proteins (Grb2) ? Grb2-SOS recruits RAS ? SOS catalyzes GTP/GDP exchange on RAS ? RAS-GTP activates RAF kinase ? MEK ? ERK (MAP kinase) ? nuclear translocation ? phosphorylates transcription factors ? gene expression changes driving proliferation. RAS is mutated (constitutively active) in ~30% of all human cancers — one of the most common oncogenes.

04Second Messengers: cAMP, Ca²?, & Lipid Signals

Second messengers are small, rapidly diffusible molecules that relay and amplify signals: cAMP (generated by adenylyl cyclase, degraded by phosphodiesterase PDE — target of caffeine inhibition and sildenafil/Viagra) ? activates PKA. cGMP (analogous, activated by nitric oxide ? soluble guanylyl cyclase, or natriuretic peptides ? receptor guanylyl cyclase) ? activates PKG ? vascular smooth muscle relaxation. Ca²? (released from ER via IP3 receptors or ryanodine receptors; entered from extracellular via VGCC or SOCE) binds calmodulin (CaM) ? CaM-kinase II (CaMKII) ? multiple targets. Diacylglycerol (DAG) + Ca²? activate PKC. Phosphoinositide-3-kinase (PI3K) ? PIP3 ? PDK1 ? AKT/PKB ? cell survival, growth, metabolism.

05Nuclear Receptors & Transcriptional Regulation

Nuclear receptors (steroid hormone receptors, thyroid hormone receptor, retinoic acid receptor, PPAR) bind lipid-soluble ligands directly in the cytoplasm or nucleus. They are ligand-activated transcription factors with: ligand-binding domain (LBD), DNA-binding domain (DBD, zinc finger motifs), and activation function domains. Unliganded steroid receptors are bound to heat shock proteins (Hsp90) masking the nuclear localization signal. Ligand binding ? Hsp90 dissociation ? nuclear import ? dimerization ? binding to hormone response elements (HREs) in DNA ? coactivator recruitment ? chromatin remodeling ? transcription. Glucocorticoid receptor: cortisol ? anti-inflammatory gene expression. Estrogen receptor (ERa): mutations in breast cancer, target of tamoxifen (competitive antagonist) and aromatase inhibitors.

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Chapter 08 ??

Vitamins, Minerals & Cofactors

Essential Micronutrients in Metabolism

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NADH structure — derived from niacin (vitamin B3), essential electron carrier
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NADH structure — derived from niacin (vitamin B3), essential electron carrier

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Vitamins and Cofactors in Biochemistry

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01Fat-Soluble Vitamins (A, D, E, K)

Vitamin A (retinol): visual cycle (11-cis-retinal + opsin ? rhodopsin; light isomerizes to all-trans-retinal ? signal cascade ? vision), gene expression regulation (retinoic acid binds RAR nuclear receptor). Deficiency: night blindness, xerophthalmia. Vitamin D (calciferol): synthesized in skin from 7-dehydrocholesterol via UV light ? 25-hydroxylation in liver ? 1,25-(OH)2D3 (calcitriol) in kidney; nuclear receptor activation ? intestinal Ca²? absorption. Deficiency: rickets (children), osteomalacia (adults). Vitamin E (a-tocopherol): lipid-soluble antioxidant, protects polyunsaturated fatty acids from lipid peroxidation chain reactions. Vitamin K (phylloquinone): cofactor for ?-carboxylation of glutamate in coagulation factors (II, VII, IX, X) and proteins C, S; warfarin inhibits vitamin K reductase ? anticoagulation.

02Water-Soluble Vitamins: The B Complex

B1 (thiamine ? TPP): pyruvate dehydrogenase, a-ketoglutarate dehydrogenase; deficiency ? Wernicke-Korsakoff (alcoholics). B2 (riboflavin ? FAD/FMN): electron carriers in ETC (Complex I, II). B3 (niacin ? NAD?/NADP?): electron carriers, >400 enzymatic reactions; deficiency ? pellagra (4 Ds: dermatitis, diarrhea, dementia, death). B5 (pantothenic acid ? CoA): acyl group carrier (fatty acid metabolism, TCA cycle); universally available in food, deficiency extremely rare. B6 (pyridoxal phosphate PLP): transamination, decarboxylation, racemization of amino acids; deficiency ? convulsions (GABA synthesis requires PLP). B7 (biotin): CO2 carrier for carboxylases (pyruvate carboxylase, acetyl-CoA carboxylase); avidin in raw egg whites binds biotin ? deficiency. B9 (folate ? THF): one-carbon transfers in purine synthesis, thymidylate synthesis, methionine cycle; deficiency ? megaloblastic anemia, neural tube defects. B12 (cobalamin): methionine synthase (regenerates THF), methylmalonyl-CoA mutase; deficiency ? megaloblastic anemia, subacute combined degeneration.

03Vitamin C & Antioxidant Biochemistry

Vitamin C (ascorbic acid) is a water-soluble antioxidant and enzyme cofactor. Its role in collagen synthesis: prolyl hydroxylase and lysyl hydroxylase require Fe²? and ascorbate; these enzymes hydroxylate proline and lysine residues in procollagen, enabling triple helix formation and cross-linking. Deficiency causes scurvy: impaired collagen ? vascular fragility, poor wound healing, bleeding gums, loosening teeth. Ascorbate reduces Fe³? ? Fe²? in intestine, enhancing non-heme iron absorption. As an antioxidant, it scavenges reactive oxygen species (ROS: superoxide O2•?, hydrogen peroxide H2O2, hydroxyl radical •OH) and regenerates oxidized vitamin E. High-dose vitamin C as anticancer therapy remains controversial.

04Essential Minerals & Their Biochemical Roles

Macrominerals: Ca²? (structural in bone, second messenger, muscle contraction), Mg²? (cofactor for all kinases via ATP-Mg²? complex; 300+ enzymes), Na?/K? (membrane potential, osmolarity), Cl? (gastric HCl), P (ATP, nucleic acids, phosphoproteins), S (in Cys, Met, CoA, glutathione). Trace minerals: Fe (hemoglobin, cytochromes, iron-sulfur clusters; Fe³? absorbed as Fe²? or bound to heme; ferritin storage, transferrin transport), Zn²? (100+ enzymes including carbonic anhydrase, carboxypeptidase; zinc finger DNA-binding domains), Cu (ceruloplasmin, cytochrome c oxidase, superoxide dismutase), I? (thyroid hormones T3/T4; deficiency ? goiter, hypothyroidism), Se (selenocysteine in glutathione peroxidase — ROS defense), Mn (arginase in urea cycle, mitochondrial SOD).

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Chapter 09 ??

DNA Repair, Mutation & Cancer

Genome Integrity & Its Failure

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DNA damage and repair: maintaining genome integrity is essential for life
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DNA damage and repair: maintaining genome integrity is essential for life

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DNA Repair and Mutation

DNA Repair and Mutation — Educational Video

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01Types of DNA Damage

DNA is constantly damaged — an estimated 70,000 lesions per cell per day. Types: Oxidative damage (ROS ? 8-oxoguanine, the most common oxidative lesion, which mispairs with A; •OH ? single-strand breaks, double-strand breaks), Alkylation (alkylating agents ? O6-methylguanine, which mispairs with T; N7-methylguanine), UV radiation ? cyclobutane pyrimidine dimers (CPD) and 6-4 photoproducts (T-T covalent linkage blocking replication), Deamination (C ? U, spontaneous, ~100/cell/day), Depurination (hydrolysis of N-glycosidic bond ? abasic/AP site, ~10,000/cell/day), Double-strand breaks (DSBs, most dangerous — caused by ionizing radiation, replication errors, chemotherapy), Cross-links (nitrogen mustard chemotherapy, cisplatin — intrastrand and interstrand cross-links block replication).

02DNA Repair Mechanisms

Base excision repair (BER): removes small, non-helix-distorting lesions. DNA glycosylase removes damaged base ? AP endonuclease ? lyase/Pol ß ? DNA ligase. Repairs 8-oxoG, uracil, methylated bases. Nucleotide excision repair (NER): removes bulky helix-distorting lesions (CPD, 6-4 PP). XPC (lesion recognition) ? TFIIH helicase unwinds ? XPA, XPG, XPF/ERCC1 cut ~30 nt on both sides ? DNA Pol fills gap. NER defects cause xeroderma pigmentosum (UV-induced skin cancer, neurodegeneration). Mismatch repair (MMR): corrects replication errors — MutS (mismatch recognition), MutL/MutH (strand discrimination, excision) ? re-synthesis. MMR defects cause Lynch syndrome (hereditary non-polyposis colorectal cancer, HNPCC) and ~15% of sporadic colorectal cancers (microsatellite instability).

03Double-Strand Break Repair

DSBs are repaired by two main pathways. Homologous recombination (HR): template-directed, high-fidelity, active in S/G2 phases when sister chromatid is available. MRN complex (MRE11-RAD50-NBS1) detects DSB ? activates ATM kinase ? H2AX phosphorylation (?H2AX, a DSB marker) ? 5' end resection ? RPA coats ssDNA ? RAD51 (recombinase) forms filament ? strand invasion of homologous template ? DNA synthesis ? resolution. BRCA1/BRCA2 are critical for HR; mutations cause hereditary breast and ovarian cancer by forcing error-prone NHEJ. Non-homologous end joining (NHEJ): direct ligation of broken ends, active in all phases, error-prone (small insertions/deletions). Ku70/Ku80 ? DNA-PKcs ? Artemis ? XRCC4-LigIV-XLF complex.

04Oncogenes, Tumor Suppressors & Cancer Biochemistry

Cancer is fundamentally a disease of abnormal gene expression. Proto-oncogenes (normal growth-promoting genes) become oncogenes by: point mutation (RAS G12V — constitutively GTP-bound), gene amplification (HER2/ERBB2 in breast cancer), chromosomal translocation (BCR-ABL in CML ? constitutively active tyrosine kinase, target of imatinib/Gleevec). Tumor suppressor genes require loss of both alleles (two-hit hypothesis, Knudson 1971): TP53 (the 'guardian of the genome' — activates p21/CDK inhibitor, BAX apoptosis; mutated in >50% of cancers), RB (retinoblastoma protein — blocks E2F transcription factors preventing S-phase entry; phosphorylation by cyclin D-CDK4/6 releases E2F), PTEN (phosphatase opposing PI3K ? AKT pathway), APC (Wnt pathway).

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Chapter 10 ???

Gene Expression & Regulation

Epigenetics, Transcription Factors & RNA Processing

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The lac operon: a classic model of prokaryotic gene regulation
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The lac operon: a classic model of prokaryotic gene regulation

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Gene Expression & Regulation

Gene Expression & Regulation — Educational Video

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01Prokaryotic Gene Regulation: The Operon

Bacterial genes for related functions are organized into operons — polycistronic units transcribed as a single mRNA. The lac operon (Jacob and Monod, Nobel 1965) controls lactose metabolism: In absence of lactose: lac repressor (tetrameric protein) binds operator (O1) ? blocks RNA Pol ? genes OFF. In presence of lactose: allolactose (inducer, isomer of lactose made by ß-galactosidase) binds repressor ? conformational change ? repressor releases operator ? RNA Pol transcribes lacZ (ß-galactosidase), lacY (permease), lacA (transacetylase). Catabolite repression (positive control): when glucose is present, cAMP is low ? CAP (catabolite activator protein) without cAMP does not bind ? reduced transcription even with lactose present. This AND gate logic ensures lac operon is maximally expressed only when glucose is absent AND lactose is present.

02Eukaryotic Transcription Regulation

Eukaryotic gene regulation is vastly more complex: cis-regulatory elements (promoters, enhancers, silencers, insulators) and trans-acting transcription factors. Core promoter contains TATA box (TATAAAA, ~30 bp upstream of TSS), recognized by TBP (TATA-binding protein) component of TFIID. Enhancers can act from >1 Mb away via DNA looping — activators bound to enhancers contact Mediator complex which contacts RNA Pol II. Combinatorial control: different combinations of transcription factors (activators, coactivators, repressors, corepressors) at composite enhancers drive tissue-specific gene expression from a common genome. Pioneer transcription factors (FOXA1, GATA) open closed chromatin to allow other TFs to bind.

03Epigenetics: Chromatin Remodeling & DNA Methylation

Epigenetics: heritable changes in gene expression without DNA sequence changes. Histone modifications: H3K4me3 (active promoters), H3K27ac (active enhancers), H3K27me3 (Polycomb silencing), H3K9me3 (heterochromatin), H4K16ac (active chromatin, inhibits 30-nm fiber). Histone acetyltransferases (HATs) add acetyl groups ? open chromatin; HDACs remove them ? closed chromatin. Writers/readers/erasers of histone code. DNA methylation: methyltransferases DNMT1 (maintenance), DNMT3A/3B (de novo) methylate cytosine in CpG dinucleotides ? 5-methylcytosine ? recruits MBD proteins and HDACs ? gene silencing. CpG islands (CpG-rich regions in promoters) are normally unmethylated ? active transcription; hypermethylation silences tumor suppressor genes in cancer (e.g., BRCA1, p16/CDKN2A promoter hypermethylation).

04Non-Coding RNAs: miRNA, siRNA & lncRNA

The human genome is pervasively transcribed, producing vast amounts of non-coding RNA (ncRNA). MicroRNAs (miRNAs): ~22-nt RNAs; primary miRNA transcribed by Pol II ? Drosha processing in nucleus ? pre-miRNA exported ? Dicer cleavage ? miRNA:miRNA* duplex ? RISC complex (Argonaute) ? complementary base-pairing with 3'UTR of target mRNA ? translational repression or mRNA degradation. >2000 human miRNAs regulate ~60% of protein-coding genes. siRNA: exogenous double-stranded RNA ? Dicer ? RISC ? perfect complementarity ? mRNA cleavage (RNAi). Long non-coding RNAs (lncRNAs, >200 nt): XIST (X-chromosome inactivation — coats X chromosome in cis ? Polycomb recruitment ? silencing of one X in females), HOTAIR (epigenetic scaffolding), NEAT1 (paraspeckle organization).

05CRISPR-Cas9 & Genome Editing

CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats) has revolutionized molecular biology and medicine. In bacteria, CRISPR is an adaptive immune system: viral sequences stored in genome as spacers between repeats ? transcribed as crRNA ? guides Cas9 to matching viral DNA ? Cas9 cuts. The guide RNA (gRNA = crRNA + tracrRNA) directs Cas9 (a dual nuclease domain HNH + RuvC) to any ~20-nt DNA sequence adjacent to a PAM (5'-NGG-3' for SpCas9) ? creates a blunt DSB. Cells repair via NHEJ (insertions/deletions ? gene knockout) or HDR (precise edits using donor template). Applications: gene therapy (sickle cell disease — CTX001 edits BCL11A enhancer ? fetal hemoglobin reactivation), base editing, prime editing, diagnostic detection (SHERLOCK, DETECTR). Doudna and Charpentier, Nobel Prize 2020.

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Chapter 11 ???

Biotechnology & Applied Biochemistry

Tools, Techniques & Therapeutic Applications

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PCR thermocycler — one of the most transformative tools in molecular biology
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PCR thermocycler — one of the most transformative tools in molecular biology

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Biotechnology & Biochemical Techniques

Biotechnology & Biochemical Techniques — Educational Video

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01PCR & DNA Amplification Techniques

The polymerase chain reaction (PCR, Kary Mullis, Nobel Prize 1993) amplifies specific DNA sequences exponentially. Three steps in each cycle: denaturation (94–98°C — double helix melts), annealing (50–65°C — primers bind flanking sequences), extension (72°C — Taq polymerase extends from primers in 5'?3' direction). 30 cycles ? 2³° ˜ 10? copies from a single molecule. Key variant: RT-PCR (reverse transcriptase converts mRNA ? cDNA ? PCR) for gene expression analysis. Real-time/quantitative PCR (qPCR): fluorescent dye (SYBR Green) or probe (TaqMan) enables real-time monitoring — quantitates starting template. Applications: diagnostics (COVID-19 testing), forensics, cloning, mutagenesis, paternity testing, ancient DNA analysis.

02Gel Electrophoresis & DNA Sequencing

Agarose gel electrophoresis separates DNA fragments by size: DNA (negatively charged) migrates toward anode; smaller fragments move faster through gel pores. Ethidium bromide (intercalates into DNA, fluorescent under UV) or SYBR-safe for visualization. Southern blotting: DNA separated on gel ? transferred to nitrocellulose membrane ? hybridized with labeled probe ? detect specific sequences (classic diagnostic for sickle cell disease via restriction fragment length polymorphisms). Sanger sequencing (chain termination, 1977): DNA synthesis with ddNTPs (dideoxynucleotides lacking 3'-OH) incorporated randomly ? fragments of all lengths ? separated by capillary electrophoresis ? read sequence from size. Next-generation sequencing (NGS): massively parallel — Illumina sequencing by synthesis ? billions of reads simultaneously ? whole human genome in <24 hours for <$1000.

03Protein Techniques: Electrophoresis & Mass Spectrometry

SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis): SDS denatures proteins and imparts uniform negative charge ? separation by molecular weight. Western blot (immunoblot): SDS-PAGE ? transfer to membrane ? block non-specific binding ? primary antibody (specific to protein) ? secondary antibody (enzyme-conjugated) ? chemiluminescent or colorimetric detection. ELISA (enzyme-linked immunosorbent assay): antibody-based quantification in solution — sandwich ELISA is highly sensitive. Mass spectrometry: ionization (ESI or MALDI) ? mass analyzer (TOF, quadrupole, Orbitrap) ? detector ? m/z spectrum ? protein identification (database search against proteome) and quantification. Proteomics: global protein profiling; structural mass spec identifies posttranslational modifications.

04Recombinant DNA, Cloning & Expression Systems

Recombinant DNA technology: restriction endonucleases (cut DNA at specific palindromic sequences; EcoRI recognizes GAATTC, cuts between G and A) + DNA ligase join foreign DNA into vectors (plasmids for bacteria; baculovirus for insect cells; AAV for gene therapy). Transformation introduces recombinant plasmid into E. coli; selection (antibiotic resistance), screening (blue/white, colony PCR). Expression systems: E. coli (high yield, no glycosylation; inclusion bodies for insoluble proteins — refolding required); yeast (P. pastoris — secretion, some glycosylation); CHO (Chinese hamster ovary) cells — gold standard for therapeutic proteins requiring complex glycosylation (antibodies, erythropoietin, insulin). Transgenic animals (e.g., knockout mice — homologous recombination to replace gene with null allele) are essential research tools.

05Drug Discovery & Biochemical Pharmacology

Modern drug discovery uses biochemical knowledge at every stage. Target identification: genomics/proteomics reveal disease-relevant proteins. Target validation: knockouts, RNAi, patient genetics. High-throughput screening (HTS): robotic testing of >106 compounds against purified targets. Hit-to-lead optimization: structure-activity relationships (SAR), medicinal chemistry, ADMET (absorption, distribution, metabolism, excretion, toxicity). Structure-based drug design: X-ray crystallography of target–ligand complexes guides optimization. Successful examples: imatinib (Gleevec) — ABL kinase inhibitor in CML, discovered via crystal structure; HIV protease inhibitors (indinavir, saquinavir) — structure-based design revolutionized AIDS treatment; statins (HMG-CoA reductase inhibitors — competitive, mechanism-based: lovastatin resembles transition state of mevalonate pathway); monoclonal antibodies (trastuzumab/Herceptin targets HER2; rituximab targets CD20).

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Madhu Rayala