๐ฌ Gene Transfer, Cloning & Selection
Study gene transfer methods for CUET Agriculture. Electroporation, microinjection, gene gun, Agrobacterium transfer and selection markers.
Overview: The rDNA Technology Process
Recombinant DNA technology follows a sequential workflow. This lesson covers the practical steps from DNA isolation through to obtaining the gene product.
Isolate DNA โ Cut โ Ligate โ Transfer to Host โ Select Transformants โ Express โ Purify
Step 1: Isolation of DNA
DNA must first be extracted from the source organism.
- Cell lysis โ break open cells:
- Bacteria: Lysozyme (digests peptidoglycan cell wall) + SDS detergent.
- Plants: Cellulase + Pectinase (digest cell wall).
- Fungi: Chitinase.
- RNA removal โ treat with RNase.
- Protein removal โ treat with Protease (proteinase K) or phenol-chloroform extraction.
- DNA purification โ Ethanol precipitation: chilled ethanol causes DNA to precipitate as fine threads that can be spooled (wound onto a glass rod).
Step 2: Cutting DNA
- Source DNA and vector DNA are both cut with the same restriction enzyme โ compatible ends.
- Produces sticky ends (preferred) or blunt ends.
- The insert fragments are ligated into the linearized vector.
Step 3: Ligation (Creating Recombinant DNA)
- Insert + linearized vector mixed with DNA ligase.
- Ligase seals nicks in the phosphodiester backbone โ recombinant DNA molecule (rDNA).
- Sticky end ligation is more efficient; blunt-end ligation may require linkers or adaptors.
Step 4: Gene Transfer into Host (Transformation)
Not all host cells will take up the recombinant DNA. Various methods are used:
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Overview: The rDNA Technology Process
Recombinant DNA technology follows a sequential workflow. This lesson covers the practical steps from DNA isolation through to obtaining the gene product.
Isolate DNA โ Cut โ Ligate โ Transfer to Host โ Select Transformants โ Express โ Purify
Step 1: Isolation of DNA
DNA must first be extracted from the source organism.
- Cell lysis โ break open cells:
- Bacteria: Lysozyme (digests peptidoglycan cell wall) + SDS detergent.
- Plants: Cellulase + Pectinase (digest cell wall).
- Fungi: Chitinase.
- RNA removal โ treat with RNase.
- Protein removal โ treat with Protease (proteinase K) or phenol-chloroform extraction.
- DNA purification โ Ethanol precipitation: chilled ethanol causes DNA to precipitate as fine threads that can be spooled (wound onto a glass rod).
Step 2: Cutting DNA
- Source DNA and vector DNA are both cut with the same restriction enzyme โ compatible ends.
- Produces sticky ends (preferred) or blunt ends.
- The insert fragments are ligated into the linearized vector.
Step 3: Ligation (Creating Recombinant DNA)
- Insert + linearized vector mixed with DNA ligase.
- Ligase seals nicks in the phosphodiester backbone โ recombinant DNA molecule (rDNA).
- Sticky end ligation is more efficient; blunt-end ligation may require linkers or adaptors.
Step 4: Gene Transfer into Host (Transformation)
Not all host cells will take up the recombinant DNA. Various methods are used:
A. Vector-Mediated (Indirect) Methods
| Method | Description |
|---|---|
| Transformation | Uptake of free DNA by competent cells. E. coli made competent by CaClโ treatment followed by 42ยฐC heat shock. |
| Conjugation | Transfer of DNA between bacteria through pili (direct physical contact). |
| Transduction | Transfer of DNA via bacteriophage. |
| Agrobacterium-mediated transformation | Ti plasmid of Agrobacterium tumefaciens naturally transfers T-DNA into plant cells. T-DNA integrates into plant nuclear genome. Most widely used for dicot plant transformation. |
B. Direct (Vectorless) Methods
| Method | Description |
|---|---|
| Electroporation | Brief electric pulses create temporary pores in cell membrane โ DNA enters. Used for bacteria, yeast, plant protoplasts, animal cells. |
| Microinjection | DNA injected directly into the nucleus using a fine glass micropipette under microscopy. Used for animal cells. |
| Gene gun (Biolistics / Particle bombardment) | DNA coated onto gold or tungsten microparticles, then shot into cells at high velocity. Used for monocot plants (wheat, rice, maize) and organelle transformation. |
| Liposome-mediated (Lipofection) | DNA enclosed in artificial lipid vesicles (liposomes) that fuse with cell membrane โ DNA delivered inside. |
| PEG (Polyethylene Glycol) | PEG facilitates DNA uptake by protoplasts. |
| Silicon carbide fibers | Tiny fibers create wounds in cell wall โ DNA enters through wounds. |
TIP
Agrobacterium works best for dicots (broad-leaved plants). The gene gun is preferred for monocots (cereals: rice, wheat, maize) because most monocots are not naturally susceptible to Agrobacterium infection.
Agrobacterium tumefaciens in Detail
- Causes crown gall disease in plants (tumor-like growths at stem-root junction).
- Contains Ti plasmid (tumor-inducing plasmid) with T-DNA (transferred DNA) region.
- In nature: T-DNA carries tumor-inducing genes โ integrates into plant genome โ causes uncontrolled cell division (crown gall).
- In biotechnology: tumor genes in T-DNA are replaced with the gene of interest (disarmed Ti plasmid). Agrobacterium is used as a gene delivery vehicle.
Step 5: Selection of Transformants
Not all host cells take up the vector. Selection identifies successfully transformed cells.
A. Antibiotic Resistance Screening
- Vector carries an antibiotic resistance gene (e.g., amp^R).
- Host cells grown on medium containing that antibiotic.
- Only transformed cells survive (have the vector โ have resistance).
- Non-transformed cells die.
B. Insertional Inactivation
How insertional inactivation works with pBR322
Foreign DNA is inserted into **one** of the resistance marker genes of pBR322, disrupting it.Example: Insert foreign DNA into the tet^R gene using BamHI:
- Transformed cells with intact vector (no insert): amp^R and tet^R โ grow on both ampicillin and tetracycline plates.
- Transformed cells with recombinant vector (insert in tet^R): amp^R but tet^S (tetracycline sensitive) โ grow on ampicillin plates but NOT on tetracycline plates.
- Replica plating: colonies from amp plate replicated onto tet plate. Colonies on amp only (not tet) = recombinant cells with insert.
C. Blue-White Screening (ฮฑ-complementation)
The most elegant and commonly used method in modern laboratories.
- Vector (e.g., pUC series) contains lacZ gene (encoding ฮฒ-galactosidase).
- Foreign DNA is inserted into the lacZ gene (via MCS), disrupting it.
- Cells plated on medium with X-gal (chromogenic substrate) + IPTG (inducer):
| Colony Color | lacZ Status | Meaning |
|---|---|---|
| Blue colonies | lacZ intact โ ฮฒ-galactosidase produced โ cleaves X-gal โ blue color | No insert (non-recombinant) |
| White colonies | lacZ disrupted โ no ฮฒ-galactosidase โ no blue color | Has insert (recombinant) โ |
IMPORTANT
Blue = Bad (no insert), White = Winner (has insert). Pick white colonies for further analysis.
D. PCR Screening and Colony Hybridization
- Colony PCR: Use primers flanking the insert site โ amplify; gel electrophoresis shows insert-containing colonies.
- Colony hybridization: Blot colonies onto nylon membrane โ hybridize with labeled probe complementary to insert.
Step 6: Expression and Obtaining the Gene Product
Expression Systems
For the transgene to produce a useful protein, it must be properly expressed in the host:
- Requires appropriate promoter (e.g., lac promoter, T7 promoter) upstream of the gene.
- In prokaryotes: mRNA is translated directly.
- In eukaryotes: requires eukaryotic promoter, poly-A signal, and sometimes intron-free cDNA.
Bioreactors
Large vessels for growing transformed organisms on industrial scale to produce gene products.
- Provide optimal conditions: temperature, pH, aeration, agitation, nutrient supply, waste removal.
- Stirred tank bioreactor โ most commonly used; has an agitator/impeller for mixing and aeration.
- Sparged tank bioreactor โ air is bubbled through the medium.
- Scales from a few liters (lab) to thousands of liters (industrial).
Downstream Processing
Purification of the gene product after bioreactor production:
- Separation โ centrifugation, filtration.
- Purification โ chromatography (ion exchange, affinity, gel filtration).
- Concentration โ dialysis, ultrafiltration.
- Formulation โ preparing the product for its intended use.
- Quality control โ ensuring safety and efficacy.
Key Points to Remember
- DNA isolation: lysing enzymes (lysozyme for bacteria, cellulase for plants, chitinase for fungi) โ RNase โ protease โ ethanol precipitation.
- Ligation: same restriction enzyme on both insert and vector โ compatible ends โ DNA ligase seals.
- Competent E. coli: CaClโ treatment + 42ยฐC heat shock.
- Agrobacterium Ti plasmid โ for dicots (T-DNA integrates into plant genome; crown gall disease).
- Gene gun (biolistics) โ DNA-coated gold/tungsten particles โ for monocots (rice, wheat, maize).
- Selection: antibiotic resistance โ antibiotic plates; insertional inactivation โ replica plating; blue-white screening โ X-gal/IPTG (blue = no insert, white = insert).
- Bioreactor: industrial fermentation for gene product; stirred tank most common.
- Downstream processing: separation โ purification (chromatography) โ concentration โ formulation โ QC.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| rDNA workflow | Isolate DNA โ Cut โ Ligate โ Transfer to Host โ Select Transformants โ Express โ Purify |
| DNA isolation โ cell lysis | Bacteria: lysozyme (peptidoglycan) Plants: cellulase + pectinase Fungi: chitinase |
| RNA removal | Treat with RNase |
| Protein removal | Protease (proteinase K) or phenol-chloroform extraction |
| DNA purification | Ethanol precipitation โ chilled ethanol causes DNA to precipitate as fine threads (spooled on glass rod) |
| Cutting DNA | Source DNA and vector cut with the same restriction enzyme โ compatible ends |
| Ligation | Insert + linearized vector sealed by DNA ligase โ recombinant DNA molecule |
| Transformation (CaClโ method) | E. coli made competent by CaClโ treatment followed by 42ยฐC heat shock โ uptake of free DNA |
| Conjugation | DNA transfer between bacteria through pili (direct contact) |
| Transduction | DNA transfer via bacteriophage |
| Agrobacterium-mediated transformation | Ti plasmid of A. tumefaciens transfers T-DNA into plant nuclear genome; most widely used for dicot transformation |
| Crown gall disease | Caused by Agrobacterium tumefaciens; tumor-like growths; in biotech, tumor genes in T-DNA replaced with gene of interest (disarmed Ti plasmid) |
| Electroporation | Brief electric pulses create temporary membrane pores โ DNA enters; works on bacteria, yeast, protoplasts, animal cells |
| Microinjection | DNA injected into nucleus using fine glass micropipette; used for animal cells |
| Gene gun (Biolistics) | DNA coated onto gold or tungsten microparticles, shot at high velocity; used for monocots (wheat, rice, maize) and organelle transformation |
| Lipofection | DNA enclosed in liposomes (artificial lipid vesicles) that fuse with cell membrane |
| PEG method | Polyethylene glycol facilitates DNA uptake by protoplasts |
| Agrobacterium vs Gene gun | Agrobacterium โ best for dicots Gene gun โ best for monocots (cereals) |
| Antibiotic resistance screening | Vector carries antibiotic resistance gene; only transformed cells survive on antibiotic medium |
| Insertional inactivation (pBR322) | Foreign DNA inserted into one marker gene (e.g., tet^R) disrupts it; replica plating identifies recombinants (grow on ampicillin but NOT tetracycline) |
| Blue-white screening | Vector has lacZ gene; insert disrupts it Blue colonies = no insert (ฮฒ-galactosidase cleaves X-gal) White colonies = has insert (lacZ disrupted) |
| X-gal and IPTG | X-gal = chromogenic substrate; IPTG = inducer of lacZ; used together for blue-white screening |
| Colony PCR | Primers flanking insert site โ amplification โ gel electrophoresis confirms insert |
| Expression systems | Require appropriate promoter (lac, T7); prokaryotes translate mRNA directly; eukaryotes need eukaryotic promoter + poly-A signal |
| Stirred tank bioreactor | Most commonly used bioreactor; has agitator/impeller for mixing and aeration |
| Downstream processing steps | Separation (centrifugation) โ Purification (chromatography) โ Concentration (dialysis) โ Formulation โ Quality control |
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