🧬 Recombinant DNA Technology — Tools
Learn rDNA technology for CUET Agriculture. Restriction enzymes, DNA ligase, vectors (pBR322, Ti plasmid) and gene cloning steps explained.
What is Biotechnology?
- Biotechnology = Use of living organisms or their products to modify or improve human health, food, and the environment.
- European Federation of Biotechnology (EFB) definition: "The integration of natural sciences and organisms, cells, parts thereof, and molecular analogues for products and services."
| Type | Examples |
|---|---|
| Traditional (Old) Biotechnology | Fermentation (bread, curd, cheese, wine, beer), selective breeding, composting |
| Modern Biotechnology | Recombinant DNA technology, gene cloning, genetic engineering, PCR, monoclonal antibodies |
NOTE
Traditional biotechnology has been practiced for thousands of years — making curd from milk is biotechnology! Modern biotechnology (1970s onwards) gave us tools to manipulate DNA directly and precisely.
Pioneers of Genetic Engineering
| Scientist | Contribution |
|---|---|
| Paul Berg | "Father of Genetic Engineering"; first recombinant DNA molecule (1972) by joining SV40 + λ phage + E. coli DNA. Nobel Prize 1980. |
| Stanley Cohen & Herbert Boyer | First to clone recombinant DNA in a living organism (1973); inserted Xenopus rRNA gene into E. coli plasmid. |
| Werner Arber, Daniel Nathans, Hamilton Smith | Discovery of restriction enzymes. Nobel Prize 1978. |
Principles of rDNA Technology
Five fundamental steps — a molecular "cut and paste" operation:
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What is Biotechnology?
- Biotechnology = Use of living organisms or their products to modify or improve human health, food, and the environment.
- European Federation of Biotechnology (EFB) definition: "The integration of natural sciences and organisms, cells, parts thereof, and molecular analogues for products and services."
| Type | Examples |
|---|---|
| Traditional (Old) Biotechnology | Fermentation (bread, curd, cheese, wine, beer), selective breeding, composting |
| Modern Biotechnology | Recombinant DNA technology, gene cloning, genetic engineering, PCR, monoclonal antibodies |
NOTE
Traditional biotechnology has been practiced for thousands of years — making curd from milk is biotechnology! Modern biotechnology (1970s onwards) gave us tools to manipulate DNA directly and precisely.
Pioneers of Genetic Engineering
| Scientist | Contribution |
|---|---|
| Paul Berg | "Father of Genetic Engineering"; first recombinant DNA molecule (1972) by joining SV40 + λ phage + E. coli DNA. Nobel Prize 1980. |
| Stanley Cohen & Herbert Boyer | First to clone recombinant DNA in a living organism (1973); inserted Xenopus rRNA gene into E. coli plasmid. |
| Werner Arber, Daniel Nathans, Hamilton Smith | Discovery of restriction enzymes. Nobel Prize 1978. |
Principles of rDNA Technology
Five fundamental steps — a molecular "cut and paste" operation:
- Cutting DNA at specific sites — restriction enzymes (molecular scissors).
- Joining DNA fragments — DNA ligase (molecular glue) → recombinant DNA.
- Inserting recombinant DNA into a vector (carrier).
- Transferring the vector into a host organism.
- Selecting transformed cells and obtaining the gene product.
Tools of Genetic Engineering
I. Restriction Endonucleases (Molecular Scissors)
- Cut DNA at specific palindromic recognition sequences (sequences that read the same on both strands in the 5'→3' direction).
- First discovered in E. coli: these enzymes naturally restrict bacteriophage growth by cutting foreign DNA. The bacterium's own DNA is protected by methylation.
- Named after the source organism.
Nomenclature example — EcoRI:
- E = Genus (Escherichia)
- co = Species (coli)
- R = Strain (RY13)
- I = First enzyme isolated from this strain
Types of Restriction Enzymes
| Type | Cleavage Site | Use in rDNA |
|---|---|---|
| Type I | Cuts randomly, far from recognition site | Not used |
| Type II | Cuts within or near the recognition site | Most commonly used |
| Type III | Cuts 25-27 bp from recognition site | Rarely used |
Important Restriction Enzymes
| Enzyme | Source Organism | Recognition Sequence (5'→3') | Cut Type |
|---|---|---|---|
| EcoRI | Escherichia coli RY13 | G↓AATTC | Sticky (5' overhang) |
| HindIII | Haemophilus influenzae Rd | A↓AGCTT | Sticky (5' overhang) |
| BamHI | Bacillus amyloliquefaciens H | G↓GATCC | Sticky (5' overhang) |
| Hae III | Haemophilus aegyptius | GG↓CC | Blunt |
| EcoRV | Escherichia coli | GAT↓ATC | Blunt |
Sticky Ends vs. Blunt Ends
| Feature | Sticky Ends (Cohesive) | Blunt Ends |
|---|---|---|
| Structure | Single-stranded overhangs | No overhang; both strands cut at same position |
| Ligation efficiency | High — complementary overhangs base-pair first | Lower — require blunt-end ligase or linkers |
| Example enzymes | EcoRI, HindIII, BamHI | Hae III, EcoRV, SmaI |
IMPORTANT
When two DNA molecules from different sources are cut with the same restriction enzyme, they produce compatible sticky ends that can be joined by DNA ligase. This is the fundamental basis of rDNA technology.
II. Vectors (Cloning Vehicles)
A vector carries foreign DNA into a host cell. Without it, the foreign DNA would be degraded or lost.
Properties of an Ideal Vector
- Origin of replication (ori) — for autonomous replication in the host.
- Selectable marker (e.g., antibiotic resistance gene) — to identify transformed cells.
- Unique restriction sites (MCS) — for insertion of foreign DNA.
- Small size — easy to manipulate.
- High copy number — more gene product per cell.
Types of Vectors
| Vector | Host | Insert Size | Key Features |
|---|---|---|---|
| Plasmid (e.g., pBR322, pUC) | E. coli | Up to ~10 kb | Small, circular, extrachromosomal, self-replicating |
| Bacteriophage λ | E. coli | Up to ~23 kb | Linear DNA; middle portion replaced with insert |
| Cosmid | E. coli | ~35-45 kb | Plasmid + λ cos sites; packaged into phage heads |
| BAC (Bacterial Artificial Chromosome) | E. coli | Up to 300 kb | Based on F-plasmid; used in HGP (public consortium) |
| YAC (Yeast Artificial Chromosome) | Yeast | Up to 1000 kb (1 Mb) | Contains yeast centromere, telomeres, ori; largest inserts |
| Ti plasmid | Agrobacterium tumefaciens | Variable | Natural vector for dicot plant transformation; T-DNA integrates into plant genome |
| Shuttle vector | Multiple hosts | Variable | Replicates in two different hosts (e.g., E. coli + yeast) |
TIP
Insert size hierarchy: Plasmid (10 kb) < Lambda (23 kb) < Cosmid (45 kb) < BAC (300 kb) < YAC (1000 kb)
pBR322 — A Classic Cloning Vector
- Constructed by Bolivar and Rodriguez (1977). p = plasmid; BR = Bolivar & Rodriguez; 322 = serial number.
- Size: 4,361 bp.
- Selectable markers: amp^R (ampicillin resistance) + tet^R (tetracycline resistance).
- Contains unique restriction sites: BamHI and HindIII (within tet^R), PstI and SalI (within amp^R), EcoRI.
- Insertional inactivation: Inserting foreign DNA into a marker gene disrupts that gene → loss of resistance → used for selection.
Key Points to Remember
- Biotechnology: Old (fermentation, selective breeding) vs. Modern (rDNA, PCR, genetic engineering).
- Paul Berg (1972) — Father of Genetic Engineering; first rDNA molecule. Nobel 1980.
- Cohen & Boyer (1973) — first successful rDNA cloning in a living organism.
- Arber, Nathans, Smith — restriction enzymes. Nobel 1978.
- Restriction enzymes: Type II most used; cut at palindromic sequences.
- Nomenclature: EcoRI = E. coli strain R, first enzyme.
- Sticky ends (EcoRI, HindIII) preferred over blunt ends for cloning efficiency.
- Vector properties: ori + selectable marker + unique restriction sites + small size.
- pBR322: 4,361 bp; amp^R + tet^R markers; Bolivar & Rodriguez 1977.
- Insert capacity: BAC = 300 kb; YAC = 1,000 kb (largest).
- cDNA (no introns) preferred for prokaryotic expression systems.
- Most used prokaryotic host: E. coli K12; eukaryotic: Saccharomyces cerevisiae.
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Biotechnology (definition) | Use of living organisms or their products to modify/improve human health, food, and environment |
| Traditional vs Modern Biotech | Traditional: fermentation (bread, curd, cheese, wine, beer), selective breeding, composting Modern: rDNA technology, gene cloning, genetic engineering, PCR, monoclonal antibodies |
| Paul Berg | Father of Genetic Engineering; first rDNA molecule (1972) — joined SV40 + λ phage + E. coli DNA; Nobel Prize 1980 |
| Stanley Cohen & Herbert Boyer | First to clone rDNA in a living organism (1973); inserted Xenopus rRNA gene into E. coli plasmid |
| Werner Arber, Daniel Nathans, Hamilton Smith | Discovery of restriction enzymes; Nobel Prize 1978 |
| Five steps of rDNA Technology | Cutting (restriction enzymes) → Joining (DNA ligase) → Inserting into vector → Transferring to host → Selecting transformants |
| Restriction endonucleases | Cut DNA at specific palindromic recognition sequences; bacterium's own DNA protected by methylation |
| EcoRI nomenclature | E = Escherichia (genus), co = coli (species), R = strain RY13, I = first enzyme isolated |
| Type II restriction enzymes | Cut within or near recognition site; most commonly used in rDNA technology |
| Type I restriction enzymes | Cut randomly, far from recognition site; not used in rDNA |
| Type III restriction enzymes | Cut 25-27 bp from recognition site; rarely used |
| EcoRI | Source: E. coli RY13; recognition: G↓AATTC; produces sticky ends (5' overhang) |
| HindIII | Source: Haemophilus influenzae Rd; recognition: A↓AGCTT; sticky ends |
| BamHI | Source: Bacillus amyloliquefaciens H; recognition: G↓GATCC; sticky ends |
| Hae III | Source: Haemophilus aegyptius; recognition: GG↓CC; produces blunt ends |
| Sticky ends vs Blunt ends | Sticky: single-stranded overhangs, high ligation efficiency Blunt: no overhang, lower ligation efficiency, needs linkers |
| Basis of rDNA technology | Two DNA molecules cut with the same restriction enzyme produce compatible sticky ends joined by DNA ligase |
| Vector (definition) | Carries foreign DNA into a host cell; without it, foreign DNA would be degraded or lost |
| Ideal vector properties | Origin of replication (ori), selectable marker (antibiotic resistance), unique restriction sites (MCS), small size, high copy number |
| Plasmid (e.g., pBR322, pUC) | Host: E. coli; insert up to ~10 kb; small, circular, extrachromosomal |
| Bacteriophage λ | Host: E. coli; insert up to ~23 kb; middle portion replaced with insert |
| Cosmid | Host: E. coli; insert ~35-45 kb; plasmid + λ cos sites |
| BAC | Host: E. coli; insert up to 300 kb; based on F-plasmid; used in Human Genome Project |
| YAC | Host: Yeast; insert up to 1000 kb (1 Mb); largest inserts; has centromere, telomeres, ori |
| Ti plasmid | From Agrobacterium tumefaciens; natural vector for dicot plant transformation; T-DNA integrates into plant genome |
| Insert size hierarchy | Plasmid (10 kb) < Lambda (23 kb) < Cosmid (45 kb) < BAC (300 kb) < YAC (1000 kb) |
| pBR322 | Constructed by Bolivar and Rodriguez (1977); size: 4,361 bp; markers: amp^R + tet^R; uses insertional inactivation for selection |
| Passenger DNA types | Genomic DNA (exons + introns), cDNA (only exons, made by reverse transcriptase), Synthetic DNA (chemically synthesized) |
| cDNA | Synthesized from mRNA using reverse transcriptase; contains only exons; preferred for prokaryotic expression (prokaryotes can't splice introns) |
| Most used prokaryotic host | E. coli strain K12 — fast growth, well-studied, easy to transform |
| Most used eukaryotic host | Saccharomyces cerevisiae (yeast) — provides post-translational modifications (glycosylation) |
| DNA ligase | Joins DNA fragments by forming phosphodiester bonds ("molecular glue") |
| Reverse transcriptase | Synthesizes cDNA from mRNA (RNA → DNA) |
| Lysozyme / Cellulase / Chitinase | Digest cell walls of bacteria / plants / fungi respectively |
| Terminal transferase | Adds nucleotides to 3' end of DNA |
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