Biotechnology Laboratory Manual: Recombinant DNA, Bioprocess Engineering & Genome Editing
Standard laboratory protocols in recombinant cloning vector restriction digestion and mapping (pUC19), preparation of chemically competent E. coli and heat-shock transformation with blue-white (α-complementation) selection, stirred-tank bioreactor batch microbial fermentation kinetics and Monod modeling, plant tissue culture callus induction via Murashige & Skoog hormonal balances, downstream IMAC Ni-NTA purification of polyhistidine-tagged proteins, and in vitro CRISPR-Cas9 endonuclease cleavage assays aligned with CSIR-NET Unit 12 and GATE BT syllabi.
Recombinant DNA Guidelines, Autoclave SOPs & Aseptic Culture Hygiene
Genetic engineering and bioprocess operations demand rigorous physical containment and aseptic technique:
- Recombinant Containment (RCGM/IBSC Guidelines): All genetically modified bacterial strains (e.g., recombinant E. coli DH5α carrying pUC19 plasmids) must be handled in Biosafety Level 1 (BSL-1) facilities. Decontaminate liquid bacterial cultures by adding 10% household bleach or autoclaving at $121^circtext{C}$ (15 psi) for 30 minutes prior to drain disposal.
- Plant Tissue Culture Aseptic SOPs: Perform all plant media pouring and explant inoculations inside a certified Class II Type A2 Laminar Air Flow (LAF) hood wiped down with 70% isopropanol. Flame surface-sterilize forceps and scalpels in 95% ethanol between explants.
- Chemical Carcinogen & Mutagen Hygiene: Ethidium bromide (EtBr) used for nucleic acid gel visualization is a potent intercalating mutagen. Handle EtBr and UV transilluminators with dedicated nitrile gloves and certified UV-blocking face shields. Dispose of EtBr gels in hazardous chemical waste containers or substitute with GelRed / SYBR Safe.
- Autoclaving Media & Fermenter Sterilization: MS media containing vitamins and sucrose must be autoclaved at $121^circtext{C}$ for precisely 15 minutes. Heat-labile antibiotics (Ampicillin, Kanamycin) and plant hormones (Zeatin, filter-sterilized IAA) must be added to molten media cooled to $50-55^circtext{C}$.
Recombinant Plasmid Vector Restriction Digestion, Mapping & Agarose Electrophoresis: pUC19 with EcoRI & HindIII
Restriction endonucleases (Type II) recognize palindromic DNA sequences with extreme specificity, cleaving phosphodiester backbones:
- Vector Architecture (pUC19): A 2686 bp high-copy cloning vector derived from pBR322:
• Multiple Cloning Site (MCS, polylinker): A 54 bp synthetic array embedded in-frame within the 5′ coding region of the lacZα gene, containing unique recognition sites for EcoRI ($5’text{-G}downarrowtext{AATTC-}3’$), SmaI, BamHI, SalI, PstI, and HindIII ($5’text{-A}downarrowtext{AGCTT-}3’$).
• ampR: Encodes $beta$-lactamase conferring ampicillin resistance.
• ori: ColE1/pMB1 origin of replication driving high copy number (~500 copies/cell). - Electrophoretic Migration Forms: Uncut plasmid DNA resolves into distinct topological conformers:
1. Supercoiled (Form I): Compact covalently closed circular (ccc) DNA; migrates fastest.
2. Nicked Circular (Form II): Single-strand nick releases topological tension; migrates slowest.
3. Linearized (Form III): Produced by a single restriction cut; migrates at true molecular size ($2686text{ bp}$). - Double Digestion & Sticky-End Ligation: Digestion with both EcoRI and HindIII generates non-complementary asymmetric overhangs (cohesive 5′ sticky ends), ensuring directional insertion of foreign DNA and preventing vector self-ligation.
- Enzymes & DNA: Purified pUC19 plasmid DNA ($0.5text{ mu g/mu L}$); recombinant pUC19-insert plasmid; Restriction Endonucleases EcoRI ($10text{ U/mu L}$) and HindIII ($10text{ U/mu L}$); $10timestext{ Buffer rCutSmart}$ (NEB).
- Gel Electrophoresis: Molecular biology grade agarose; $1timestext{ TAE Buffer}$ (40 mM Tris-acetate, 1 mM EDTA, pH 8.3); 1 kb DNA ladder; $6timestext{ TriTrack DNA Loading Dye}$; GelRed nucleic acid stain; horizontal submarine electrophoresis unit with power supply.
- Reaction Setup: Assemble four $20text{ mu L}$ restriction digests in sterile microcentrifuge tubes:
• Tube 1 (Uncut Control): $2text{ mu L}$ pUC19 DNA + $2text{ mu L } 10timestext{ Buffer} + 16text{ mu L } text{ddH}_2text{O}$.
• Tube 2 (EcoRI Single Digest): $2text{ mu L}$ pUC19 DNA + $2text{ mu L } 10timestext{ Buffer} + 15text{ mu L } text{ddH}_2text{O} + 1text{ mu L EcoRI}$.
• Tube 3 (HindIII Single Digest): $2text{ mu L}$ pUC19 DNA + $2text{ mu L } 10timestext{ Buffer} + 15text{ mu L } text{ddH}_2text{O} + 1text{ mu L HindIII}$.
• Tube 4 (Double Digest Recombinant): $2text{ mu L}$ recombinant DNA + $2text{ mu L } 10timestext{ Buffer} + 14text{ mu L } text{ddH}_2text{O} + 1text{ mu L EcoRI} + 1text{ mu L HindIII}$. - Incubation: Pulse-spin tubes and incubate in a water bath at $37^circtext{C}$ for 60 minutes. Heat-inactivate enzymes at $65^circtext{C}$ for 15 minutes.
- Gel Preparation & Loading: Cast a 1.0% agarose gel in $1timestext{ TAE}$ with $1timestext{ GelRed}$. Add $4text{ mu L } 6timestext{ loading dye}$ to each tube. Load $15text{ mu L}$ of each sample alongside $5text{ mu L}$ of 1 kb ladder.
- Electrophoresis & Imaging: Run at $90text{ V}$ for 75 minutes. Visualize and document bands on a UV/blue-light transilluminator.
| Lane | Sample Contents | Observed Band Sizes | Molecular Interpretation |
|---|---|---|---|
| 1 | 1 kb DNA Ladder | 4.0, 3.0, 2.0, 1.5, 1.0, 0.5 kb | Molecular weight calibration standard |
| 2 | Uncut pUC19 | ~2.0 kb (dominant) & ~3.5 kb (faint) | Form I Supercoiled (runs fast) + Form II Nicked circle |
| 3 | pUC19 + EcoRI | Single crisp band at 2686 bp | Form III Linear vector; authentic molecular size confirmed |
| 4 | Recombinant (EcoRI + HindIII) | 2.68 kb backbone + 1.20 kb insert | Insert successfully released; directional cloning verified |
Preparation of Chemically Competent E. coli & Heat-Shock Transformation: Blue-White ($alpha$-Complementation) Screening
Bacterial transformation introduces foreign plasmid DNA into competent host cells:
- Mechanism of Chemical Competence ($CaCl_2$): Divalent calcium cations ($Ca^{2+}$) electrostatically neutralize the negative repulsive charges between the phosphate backbones of plasmid DNA and the lipopolysaccharides (LPS) of the outer membrane at 0°C.
- Heat-Shock Thermal Pulse (42°C): A 45-second pulse at 42°C followed by immediate chilling on ice creates a thermal gradient that transiently destabilizes the membrane potential, generating transient hydrophilic pores through which plasmid-cation complexes enter the cytoplasm.
- Molecular Basis of $alpha$-Complementation:
• The host E. coli strain (e.g., DH5α) harbors the chromosomal lacZ$Delta$M15 deletion mutation, producing an inactive truncated $omega$-peptide of $beta$-galactosidase lacking amino acids 11-41.
• The pUC19 vector encodes the 146-amino-acid N-terminal $alpha$-peptide (lacZα).
• Non-Recombinant Plasmids: Intact lacZα peptide complements the host $omega$-peptide, reconstituting functional tetrameric $beta$-galactosidase. The enzyme hydrolyzes the chromogenic substrate X-Gal (5-bromo-4-chloro-3-indolyl-$beta$-D-galactopyranoside) in the presence of IPTG (gratuitous inducer), producing an insoluble deep-blue 5,5′-dibromo-4,4′-dichloro-indigo precipitate (→ Blue Colonies).
• Recombinant Plasmids: Foreign DNA inserted into the MCS disrupts the lacZα reading frame (insertional inactivation). No functional enzyme is reconstituted, and colonies remain unstained (→ White Colonies).
- Bacterial Strain & Plasmids:E. coli DH5α early log-phase culture ($OD_{600} approx 0.4$); purified pUC19 vector DNA ($10text{ pg/mu L}$ control); ligation mixture (pUC19 + foreign insert).
- Reagents & Solutions: Ice-cold $0.1text{ M } CaCl_2$ (sterile); SOC rich outgrowth medium; Ampicillin stock ($100text{ mg/mL}$); X-Gal ($20text{ mg/mL}$ in DMF); IPTG ($0.1text{ M}$ aqueous).
- Selective Agar: LB agar plates supplemented with $100text{ mu g/mL}$ Ampicillin, $40text{ mu g/mL}$ X-Gal, and $0.5text{ mM}$ IPTG.
- Competent Cell Preparation: Chill 5 mL log-phase DH5α culture on ice for 20 min. Centrifuge at $4000 times g$ for 5 min at 4°C. Resuspend pellet gently in 2.5 mL ice-cold $0.1text{ M } CaCl_2$. Incubate on ice 30 min. Pellet again and resuspend in $500text{ mu L}$ ice-cold $0.1text{ M } CaCl_2 + 15%text{ glycerol}$. Aliquot $100text{ mu L}$ into pre-chilled tubes.
- DNA Addition: Add $1text{ mu L}$ ($0.1text{ ng}$) of control pUC19 to Tube A, $5text{ mu L}$ of ligation reaction to Tube B, and $0text{ mu L}$ (negative control) to Tube C. Tap gently. Incubate on ice for 30 minutes.
- Heat Shock: Transfer tubes directly to a $42^circtext{C}$ circulating water bath for exactly 45 seconds. Immediately plunge tubes into ice for 2 minutes.
- Outgrowth (Phenotypic Expression): Aseptically add $900text{ mu L}$ pre-warmed ($37^circtext{C}$) SOC medium to each tube. Shake horizontally at 225 rpm for 60 minutes at $37^circtext{C}$ to allow $beta$-lactamase expression.
- Plating & Incubation: Spread $100text{ mu L}$ of cell suspension onto LB+Amp+X-Gal+IPTG plates. Invert and incubate at $37^circtext{C}$ for 16-18 hours. Store at 4°C for 2 hours to intensify blue coloration.
Control Plasmid Transformation Efficiency:
DNA added: $0.1text{ ng } (1 times 10^{-4}text{ mu g})$. Total outgrowth: $1000text{ mu L}$. Volume plated: $100text{ mu L}$ ($10%text{ of total}$).
Colonies counted on control plate: 250 colonies.
$$text{Transformation Efficiency} = frac{250 text{ CFU}}{1 times 10^{-4}text{ mu g} times 0.1} = frac{250}{10^{-5}text{ mu g}} = mathbf{2.5 times 10^7text{ CFU / mu g DNA}}$$
(Exceeds standard cloning threshold of $>10^6text{ CFU/mu g}$).
Ligation Plate Breakdown:
• White Colonies (Recombinant Clones with Foreign Insert): 142 colonies (83.5%)
• Blue Colonies (Non-Recombinant Vector Self-Ligation): 28 colonies (16.5%)
• Negative Control (No DNA): 0 colonies (Ampicillin selection intact).
Bioprocess Engineering: Batch Microbial Fermentation Kinetics ($X, S, P$), Specific Growth Rate ($mu$) & Monod Yield Coefficients ($Y_{X/S}$)
Microbial batch fermentation exhibits distinct growth phases (lag, exponential, stationary, and death):
- Specific Growth Rate ($mu$): During exponential growth, biomass increases autocatalytically:
$$frac{dX}{dt} = mu X implies lnleft(frac{X_t}{X_0}right) = mu cdot t implies t_d = frac{ln(2)}{mu} approx frac{0.693}{mu}$$ - Monod Kinetic Model: Relates growth rate to limiting substrate concentration ($S$):
$$mu = mu_{max} frac{S}{K_s + S}$$
where $mu_{max}$ is the maximum specific growth rate ($text{h}^{-1}$) and $K_s$ is the substrate affinity saturation constant ($text{g/L}$, substrate concentration at which $mu = 0.5 mu_{max}$). A linearized Lineweaver-Burk plot yields:
$$frac{1}{mu} = left(frac{K_s}{mu_{max}}right) frac{1}{S} + frac{1}{mu_{max}}$$ - Yield Coefficients:
• Biomass yield on substrate: $Y_{X/S} = frac{Delta X}{Delta S} = frac{X_{text{final}} – X_0}{S_0 – S_{text{final}}} quad (text{g dry cell weight / g glucose})$.
• Product yield on substrate: $Y_{P/S} = frac{Delta P}{Delta S} = frac{P_{text{final}} – P_0}{S_0 – S_{text{final}}}$. - Bioreactor Aeration & Volumetric Mass Transfer ($K_L a$): Oxygen transfer rate ($text{OTR} = K_L a cdot (C^* – C_L)$) must balance microbial oxygen uptake rate ($text{OUR} = q_{O_2} cdot X$) to maintain dissolved oxygen $> 20%$ saturation.
- Equipment: 5 L autoclavable glass stirred-tank bioreactor with dual 6-blade Rushton impellers, rotameter air sparger, polarographic DO probe, sterilizable pH electrode, water heating jacket, and peristaltic acid/base pumps.
- Media Formulation (per Liter): $20text{ g Glucose}$, $10text{ g Yeast Extract}$, $20text{ g Peptone}$ (YPD medium), $0.1text{ mL Antifoam A}$.
- Analytical Assays: Spectrophotometer (600 nm) for optical density; analytical balance and $0.45text{ mu m}$ membrane filters for dry cell weight (DCW); Dinitrosalicylic acid (DNS) reagent for residual glucose assay; enzymatic assay kit for ethanol.
- Vessel Sterilization & Calibration: Assemble the bioreactor with 3.5 L medium. Autoclave in situ at $121^circtext{C}$ for 30 minutes. Calibrate pH electrode (buffers 4.0 and 7.0) and polarographic DO probe (sparge $N_2$ for 0%, sparge air at 500 rpm for 100%).
- Inoculation & Control Setup: Inoculate with 200 mL of active S. cerevisiae seed culture ($5%text{ v/v}$). Set operating parameters: Temperature = $30^circtext{C}$, Agitation = $400text{ rpm}$, Aeration = $1.0text{ vvm } (3.5text{ L/min})$, pH maintained at $5.5$ via automated $2text{ M } NaOH$ addition.
- Kinetic Sampling: Draw 15 mL sterile samples every 60 minutes over an 8-hour fermentation cycle.
- Offline Measurements:
• Measure $OD_{600}$ and dry cell weight ($X$, $text{g/L}$).
• Centrifuge sample, collect supernatant, and quantify residual glucose ($S$, $text{g/L}$) via the DNS colorimetric method at 540 nm.
• Quantify ethanol concentration ($P$, $text{g/L}$). - Kinetic Parameter Modeling: Plot $ln(X)$ vs time during the log phase to extract $mu_{max}$. Plot $frac{1}{mu}$ vs $frac{1}{S}$ to determine $K_s$. Calculate $Y_{X/S}$ and $Y_{P/S}$.
| Time (h) | Biomass $X$ (g/L) | Glucose $S$ (g/L) | Ethanol $P$ (g/L) | Dissolved $O_2$ (%) |
|---|---|---|---|---|
| 0.0 | 0.45 | 20.00 | 0.00 | 100% |
| 2.0 | 0.92 | 18.50 | 0.25 | 78% |
| 4.0 | 2.45 | 13.80 | 1.80 | 42% |
| 6.0 | 5.80 | 4.20 | 6.20 | 22% |
| 8.0 | 7.65 | 0.40 | 8.10 | 55% (Recovering) |
Calculated Kinetic Constants:
• Maximum Specific Growth Rate: $mu_{max} = mathbf{0.412text{ h}^{-1}}$ • Doubling Time: $t_d = frac{0.693}{0.412} = mathbf{1.68text{ hours}}$
• Monod Affinity Constant: $K_s = mathbf{0.85text{ g/L}}$
• Biomass Yield: $Y_{X/S} = frac{7.65 – 0.45}{20.00 – 0.40} = frac{7.20}{19.60} = mathbf{0.367text{ g DCW / g glucose}}$
• Product Yield: $Y_{P/S} = frac{8.10}{19.60} = mathbf{0.413text{ g ethanol / g glucose}}$ ($80.9%$ of theoretical Gay-Lussac yield).
Plant Tissue Culture & Callus Induction: Murashige & Skoog (MS) Media Formulation & Auxin-to-Cytokinin Organogenesis Ratio
Plant cells display cellular totipotency: the genetic capacity of a fully differentiated somatic cell to dedifferentiate and regenerate an entire fertile organism:
- Murashige & Skoog (MS) Basal Salts (1962): Formulated with high levels of inorganic nitrogen ($20.6text{ mM } NH_4^+$ and $39.4text{ mM } NO_3^-$), potassium ($K^+$), phosphate ($PO_4^{3-}$), and essential microelements (Fe-EDTA chelate, boron, zinc, manganese). Supplemented with myo-inositol, thiamine-HCl, and 30 g/L sucrose as carbon source.
- Skoog & Miller Hormonal Balance Model (1957): Morphogenic differentiation is governed by the quantitative ratio of auxin to cytokinin:
• Equal Ratio ($approx 1 : 1$): Promotes cellular dedifferentiation into an unorganized, rapidly proliferating parenchymatous mass (→ Callus). 2,4-D (2,4-dichlorophenoxyacetic acid) is a potent synthetic auxin that strongly arrests differentiation.
• Low Auxin : High Cytokinin: Stimulates shoot apical meristem formation (→ Caulogenesis / Shoot Regeneration). Cytokinins (BAP) promote cell division and break apical dominance.
• High Auxin : Low Cytokinin: Stimulates adventitious root primordia emergence (→ Rhizogenesis / Rooting). Auxins (NAA, IBA) induce pericycle cell division.
- Plant Material: Young, healthy greenhouse-grown tobacco (Nicotiana tabacum) or carrot (Daucus carota) leaves.
- Media Components: MS basal salts with Gamborg B5 vitamins; sucrose ($30text{ g/L}$); plant tissue culture agar ($8text{ g/L}$); plant growth regulators: 2,4-D ($1text{ mg/mL}$), NAA ($1text{ mg/mL}$), 6-Benzylaminopurine (BAP, $1text{ mg/mL}$).
- Disinfectants: 70% ethanol; 0.1% Mercuric Chloride ($HgCl_2$) or 1.5% Sodium Hypochlorite ($NaOCl$) containing 2 drops of Tween-20 surfactant; sterile double-distilled water.
- Media Preparation: Dissolve MS basal salts and 30 g sucrose in 900 mL distilled water. Add phytohormone treatments:
• Treatment 1 (Callus Induction): $2.0text{ mg/L } 2,4text{-D} + 0.2text{ mg/L BAP}$.
• Treatment 2 (Shoot Induction): $0.1text{ mg/L NAA} + 2.0text{ mg/L BAP}$.
• Treatment 3 (Root Induction): $1.0text{ mg/L IBA} + 0.05text{ mg/L BAP}$.
Adjust pH to $5.8 pm 0.05$ with $0.1text{ M } KOH/HCl$. Add 8 g agar. Autoclave at $121^circtext{C}$ for 15 minutes. Dispense 25 mL into sterile culture tubes. - Explant Surface Sterilization: Inside the LAF hood, wash leaf pieces under running water. Dip in 70% ethanol for 30 seconds. Immerse in 0.1% $HgCl_2$ with Tween-20 for 5 minutes with continuous agitation. Rinse 5 times with sterile distilled water.
- Inoculation: Excise $1 times 1text{ cm}$ leaf lamina squares, avoiding midribs. Inoculate explants abaxial side down onto agar slants.
- Culture Incubation: Maintain cultures in a plant growth chamber at $25 pm 2^circtext{C}$, 60% relative humidity, under a 16-hour light / 8-hour dark photoperiod ($3000text{ lux}$ fluorescent irradiance).
| Hormone Treatment | Auxin : Cytokinin | Response Frequency (%) | Morphological Outcome (4 Weeks) |
|---|---|---|---|
| $2.0text{ mg/L } 2,4text{-D} + 0.2text{ mg/L BAP}$ | $10 : 1$ (Balanced for 2,4-D) | 95% | Profuse yellow-green friable nodular callus |
| $0.1text{ mg/L NAA} + 2.0text{ mg/L BAP}$ | $1 : 20$ (High Cytokinin) | 88% | Multiple adventitious dark-green shoot buds (Caulogenesis) |
| $1.0text{ mg/L IBA} + 0.05text{ mg/L BAP}$ | $20 : 1$ (High Auxin) | 92% | Robust adventitious root elongation (Rhizogenesis) |
Biological Conclusion: Complete organogenesis and plant regeneration verified; adjusting phytohormone ratios successfully directs totipotent plant cells into callus, shoots, or roots.
Downstream Processing & Protein Purification: Immobilized Metal Affinity Chromatography (IMAC Ni-NTA) of Polyhistidine-Tagged Proteins
Immobilized Metal Affinity Chromatography (IMAC) exploits coordinate covalent bonding between electron-donating amino acid side chains and transition metal ions:
- Ni-NTA Matrix Chemistry: Nitriloacetic acid (NTA) is a tetradentate chelating ligand that coordinates $Ni^{2+}$ through four coordination sites (one nitrogen and three carboxyl oxygen atoms). This leaves two octahedral coordination sites unoccupied and available for interaction.
- $6timestext{His}$ Tag Binding: A peptide tag of six contiguous histidines ($His_6$) binds these two open coordination sites with high nanomolar affinity ($K_d approx 10^{-7}text{ M}$) at physiological pH (7.5-8.0), where imidazole nitrogens are unprotonated. Host E. coli proteins lack contiguous histidine clusters and do not bind tightly.
- Competitive Imidazole Elution: Imidazole is the side chain of histidine. Introducing an imidazole gradient ($20text{ mM} rightarrow 500text{ mM}$) competitively displaces the His-tagged protein from the $Ni^{2+}$ ions:
• Lysis/Binding Buffer (10-20 mM Imidazole): Suppresses non-specific binding of host proteins.
• Wash Buffer (20-50 mM Imidazole): Displaces weakly bound non-specific contaminants.
• Elution Buffer (250-500 mM Imidazole): Elutes the high-affinity $6timestext{His}$ target protein.
- Protein System: Recombinant Green Fluorescent Protein with C-terminal $6timestext{His}$ tag (eGFP-His6, ~28 kDa) or recombinant kinase (~40 kDa) expressed from pET28a in E. coli BL21(DE3).
- Chromatography: Ni-NTA Agarose resin (Qiagen); gravity-flow polypropylene columns; ultrasonic probe sonicator.
- Buffers:
• Lysis/Binding Buffer: $50text{ mM } NaH_2PO_4$, $300text{ mM } NaCl$, $10text{ mM Imidazole}$, $1text{ mg/mL Lysozyme}$, $1text{ mM PMSF}$, pH 8.0.
• Wash Buffer: $50text{ mM } NaH_2PO_4$, $300text{ mM } NaCl$, $30text{ mM Imidazole}$, pH 8.0.
• Elution Buffer: $50text{ mM } NaH_2PO_4$, $300text{ mM } NaCl$, $250text{ mM Imidazole}$, pH 8.0.
- Cell Lysis: Resuspend cell pellet from 250 mL IPTG-induced culture in 10 mL Lysis Buffer. Sonicate on ice: 10 cycles of 15-second pulses (40% amplitude) with 30-second cooling pauses. Centrifuge lysate at $15,000 times g$ for 30 minutes at 4°C. Save clarified supernatant (Crude Lysate) and pellet.
- Column Equilibration: Pack 2 mL Ni-NTA agarose into a column. Wash with 10 column volumes (CV) of distilled water, then equilibrate with 5 CV of Binding Buffer.
- Batch-Binding & Flow-Through: Load clarified lysate onto column at $0.5text{ mL/min}$. Collect the Flow-Through (FT) fraction.
- Washing: Wash column with 10 CV of Wash Buffer (30 mM Imidazole) to remove non-specifically bound bacterial proteins. Collect Wash fractions.
- Elution: Elute bound $6timestext{His}$ protein by applying 5 CV of Elution Buffer (250 mM Imidazole). Collect 1 mL fractions. Monitor fractions at $A_{280}$ on a spectrophotometer.
- Analytical SDS-PAGE: Resolve $10text{ mu L}$ aliquots of Lysate, Flow-Through, Wash, and Elution fractions on a 12% SDS-PAGE gel stained with Coomassie Brilliant Blue R-250.
| Fraction | Volume (mL) | Protein Conc (mg/mL) | Total Protein (mg) | Specific Activity (U/mg) | Yield (%) | Purification Fold |
|---|---|---|---|---|---|---|
| Crude Lysate | 10.0 | 14.20 | 142.0 | 42 | 100% | 1.0 |
| Flow-Through | 10.0 | 12.50 | 125.0 | 2 | – | – |
| Wash (30 mM Imid) | 20.0 | 0.45 | 9.0 | 8 | – | – |
| Eluate (250 mM Imid) | 4.0 | 1.65 | 6.60 mg | 865 | 84.2% | 20.6-fold |
Downstream Conclusion: SDS-PAGE confirms single-band purity ($>96%$ homogeneity) at 40 kDa, demonstrating the clinical and industrial power of single-step IMAC affinity chromatography.
CRISPR-Cas9 In Vitro Cleavage Assay: sgRNA Target Design, PAM Recognition & Site-Specific DNA Cleavage Validation
CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats) is a microbial adaptive immune system engineered into a universal precision genome editing platform:
- RNP Architecture: SpCas9 is a multi-domain endonuclease ($160text{ kDa}$) comprising two structural lobes:
• Recognition (REC) Lobe: Binds the sgRNA scaffold and guides R-loop formation.
• Nuclease (NUC) Lobe: Contains the PAM-interacting domain and two distinct catalytic endonuclease domains:
– HNH Domain: Cleaves the complementary target DNA strand.
– RuvC Domain: Cleaves the non-complementary strand. - PAM Requirement: Cas9 scans DNA by three-dimensional diffusion until it recognizes the Protospacer Adjacent Motif ($5’text{-NGG-}3’$) on the non-target strand. PAM recognition triggers local DNA melting, enabling the 20-nt sgRNA spacer to hybridize with the target strand from the 3′ seed region.
- Site-Specific Blunt Double-Strand Break (DSB): The activated HNH and RuvC domains cleave phosphodiester bonds exactly 3 base pairs upstream of the PAM motif, producing a blunt double-strand break.
- In Vitro Validation Utility: Before delivering CRISPR components into living cells or embryos, an in vitro cleavage assay rigorously confirms that the designed sgRNA is biochemically active and achieves complete cleavage without off-target fragmentation.
- Enzymes & RNA: Recombinant Streptococcus pyogenes Cas9 nuclease ($1text{ mu M}$, NEB EnGen Cas9); in vitro transcribed target-specific single guide RNA (sgRNA, $300text{ nM}$); non-targeting scramble control sgRNA.
- Target DNA Substrate: 1000 bp PCR amplicon possessing a single on-target Cas9 site located 600 bp from the 5′ terminus (generating predicted fragments of 600 bp and 400 bp upon cleavage).
- Reaction Buffer: $10timestext{ Cas9 Nuclease Reaction Buffer}$ (200 mM HEPES, 1 M NaCl, 50 mM $MgCl_2$, 1 mM EDTA, pH 6.5).
- Stop Reagent: Proteinase K ($20text{ mg/mL}$) and $6timestext{ SDS-containing loading buffer}$.
- RNP Complex Assembly: In sterile PCR tubes, assemble the Cas9-sgRNA RNP complex:
• $3text{ mu L } 10timestext{ Buffer} + 1text{ mu L Cas9 (1 mu M)} + 3text{ mu L sgRNA (300 nM)} + 18text{ mu L nuclease-free water}$.
Incubate at $25^circtext{C}$ for 10 minutes to allow Cas9 to fold around the sgRNA scaffold. - Substrate Cleavage Reaction: Add $5text{ mu L}$ of the 1000 bp DNA target substrate ($100text{ ng}$). Final reaction volume: $30text{ mu L}$.
Assemble parallel control reactions: Control 1 (Substrate DNA alone, no Cas9); Control 2 (Substrate + Cas9 + Scramble sgRNA). - Incubation: Incubate at $37^circtext{C}$ for 60 minutes.
- Reaction Termination (Proteinase K Digestion): Add $1text{ mu L}$ Proteinase K ($20text{ mg/mL}$) to release tightly bound Cas9 from cleaved DNA ends. Incubate at $56^circtext{C}$ for 10 minutes.
- Agarose Gel Analysis: Add $6text{ mu L } 6timestext{ loading dye}$. Resolve on a 2.0% agarose gel in $1timestext{ TAE}$ at $100text{ V}$ for 60 minutes. Image on a gel documentation system.
| Reaction Condition | Intact Band (1000 bp) | Cleaved Band A (600 bp) | Cleaved Band B (400 bp) | Cleavage Efficiency (%) |
|---|---|---|---|---|
| Substrate DNA Alone | 100% Intensity | None (0%) | None (0%) | 0.0% |
| Cas9 + Non-targeting sgRNA | 100% Intensity | None (0%) | None (0%) | 0.0% (No off-target cut) |
| Cas9 + On-target sgRNA | 3.2% (Trace) | 58.1% Intensity | 38.7% Intensity | 96.8% In Vitro Cleavage |
Densitometric Calculation:
$$text{Cleavage Efficiency} = left(frac{text{Intensity}_{600text{ bp}} + text{Intensity}_{400text{ bp}}}{text{Intensity}_{1000text{ bp}} + text{Intensity}_{600text{ bp}} + text{Intensity}_{400text{ bp}}}right) times 100 = mathbf{96.8%}$$
Genome Editing Conclusion: Complete stoichiometric double-strand cleavage precisely at the designed site confirms that the sgRNA is highly potent and suitable for downstream in vivo gene knockout experiments.
Comprehensive Practical Viva-Voce Examination & Biotechnology Assessment Bank
Essential questions testing understanding of recombinant cloning vectors, transformation selection, Monod bioprocess kinetics, plant organogenesis, IMAC affinity purification, and CRISPR-Cas9 genome editing mechanisms:
Ans: A synthetic DNA segment containing tandem unique restriction enzyme recognition sites, allowing foreign DNA fragments cut with matching enzymes to be inserted directionally without disrupting any essential vector elements.
Ans: Different topological conformations migrate at different rates: supercoiled DNA (ccc, Form I) is compact and migrates fastest; nicked open-circular DNA (Form II) has high friction and migrates slowest; linearized plasmid (Form III) migrates at its true molecular size.
Ans: Cold CaCl2 neutralizes electrostatic repulsion between DNA phosphates and outer membrane lipopolysaccharides. The 42°C thermal pulse creates a sudden temperature gradient that transiently destabilizes the membrane potential, generating hydrophilic pores for plasmid entry.
Ans: The host E. coli strain carries the defective lacZDeltaM15 deletion (omega-peptide). The pUC19 vector supplies the N-terminal alpha-peptide. When together, they reconstitute active beta-galactosidase that cleaves X-Gal into an insoluble blue dye. Insertional inactivation of the alpha-peptide by an insert keeps colonies white.
Ans: IPTG is a gratuitous, non-metabolizable inducer of the lac operon that binds the LacI repressor protein, relieving repression and driving continuous high transcription of the lacZ gene.
Ans: Transformation Efficiency = (Number of transformant colonies / micrograms of plasmid DNA added) x (Total outgrowth volume / Volume plated on agar). Values typically range from 10^6 to 10^8 CFU/microgram for chemical competence.
Ans: mu = mu_max · S / (K_s + S), where mu is specific growth rate (h^-1), mu_max is maximum specific growth rate, S is growth-limiting substrate concentration (g/L), and K_s is the saturation constant (substrate concentration at half mu_max).
Ans: Ks reflects microbial affinity for the limiting nutrient. A very low Ks indicates high affinity, meaning the organism can maintain near-maximum growth rates even under nutrient-scarce conditions.
Ans: Y_X/S = Delta X / Delta S = (grams of dry cell biomass formed) / (grams of substrate consumed). Y_P/S = Delta P / Delta S = (grams of product formed) / (grams of substrate consumed).
Ans: Vessel jacket (temperature control), Rushton turbine impellers (mixing and bubble breakup), baffles (vortex prevention), sparger (sterile aeration), dissolved oxygen probe, and pH electrode with acid/base addition loops.
Ans: A nutrient-rich basal formulation featuring high concentrations of inorganic nitrate, ammonium, and potassium salts, iron-EDTA chelates, and B-vitamins, providing all inorganic ions needed to sustain in vitro plant cell division and organogenesis.
Ans: 1. Equal Auxin : Cytokinin ratio (~1:1) promotes undifferentiated Callus; 2. Low Auxin : High Cytokinin ratio induces Shoot formation (Caulogenesis); 3. High Auxin : Low Cytokinin ratio induces Root formation (Rhizogenesis).
Ans: 2,4-D (2,4-dichlorophenoxyacetic acid) is a very strong synthetic auxin used almost exclusively to induce and maintain undifferentiated callus. NAA (naphthaleneacetic acid) is a milder auxin used for organogenic rooting and shoot-root balance.
Ans: The inherent genetic capacity of any living somatic plant cell to dedifferentiate into a meristematic state and regenerate a complete, fertile plant organism under appropriate in vitro hormonal and nutrient signals.
Ans: Transition metal ions (Ni2+, Co2+) are chelated onto an agarose matrix by nitriloacetic acid (NTA). Recombinant proteins engineered with a 6xHis tag coordinate the open metal coordination sites via imidazole ring nitrogens with nanomolar affinity.
Ans: Imidazole is structurally identical to the side chain of histidine. Applying a high concentration of free imidazole (250-500 mM) competitively outcompetes the 6xHis tag for Ni2+ coordination sites, releasing the pure protein.
Ans: A short 2-6 base pair sequence (5′-NGG-3′ for SpCas9) immediately adjacent to the 3′ end of the target DNA sequence. SpCas9 must first recognize the PAM before initiating local DNA unwinding and sgRNA strand invasion; without a PAM, Cas9 cannot bind or cleave.
Ans: The HNH endonuclease domain cleaves the complementary target DNA strand (hybridized to the sgRNA), while the RuvC domain cleaves the non-complementary strand, resulting in a blunt double-strand break 3 bp upstream of the PAM.
Ans: Cas9 binds exceptionally tightly to cleaved DNA ends after making a double-strand break. Proteinase K digests the Cas9 protein, releasing the cut DNA fragments so they can migrate freely during gel electrophoresis.
Ans: 1. Non-Homologous End Joining (NHEJ): Error-prone blunt re-ligation producing random insertions/deletions (indels) that disrupt the reading frame (gene knockout); 2. Homology-Directed Repair (HDR): High-fidelity repair using a homologous donor DNA template for precise sequence insertion or correction.
