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Spec Mapping — OCR H420 Module 6.1.3 — Manipulating genomes, content statements covering recombinant DNA technology: restriction endonucleases and DNA ligase, the use of plasmid and viral vectors, marker genes for selection of transformed cells, and the introduction of recombinant DNA into bacterial, plant and animal hosts (refer to the official OCR H420 specification document for exact wording). This lesson sits at the heart of Module 6.1.3 — every downstream biotechnology lesson (GM crops, gene therapy, CRISPR, microbial production) presupposes the cloning workflow established here.
Genetic engineering (also called recombinant DNA technology) is the deliberate manipulation of genes to transfer them from one organism to another, producing transgenic or genetically modified (GM) organisms. OCR A-Level Biology A specification 6.1.3 requires you to understand the molecular toolkit — restriction enzymes, ligases, vectors, marker genes — and the standard cloning workflow used to create GM bacteria, plants and animals.
The intellectual debt of this lesson runs through three foundational papers. Paul Berg at Stanford in 1972 produced the first recombinant DNA molecule by splicing a fragment of bacteriophage λ DNA into the SV40 viral genome — work he later voluntarily paused because of biosafety concerns and which earned him a share of the 1980 Nobel Prize in Chemistry. Herbert Boyer at UCSF and Stanley Cohen at Stanford in 1973 showed that a recombinant plasmid carrying an antibiotic-resistance gene could be transformed into E. coli and replicated within the host — the experiment that effectively launched the biotechnology industry. The paraphrased account in modern textbooks emphasises that Boyer and Cohen's plasmid pSC101 was the first practical cloning vector and the conceptual ancestor of every modern engineering plasmid. The first biotechnology company, Genentech, was co-founded by Boyer in 1976; six years later the FDA approved Humulin, the first recombinant DNA drug.
Key Definitions:
- Recombinant DNA — DNA formed by joining DNA from two different sources.
- Restriction endonuclease — an enzyme that cuts DNA at a specific recognition sequence.
- Sticky ends — single-stranded overhangs produced by restriction enzymes that cut asymmetrically.
- DNA ligase — an enzyme that catalyses the formation of phosphodiester bonds between DNA fragments.
- Vector — a DNA molecule (typically a plasmid or virus) used to carry foreign DNA into a host cell.
- Transformation — the uptake of foreign DNA by a cell.
- Marker gene — a gene introduced alongside the gene of interest to identify successfully transformed cells.
Genetic engineering became possible in the 1970s with the discovery of two key enzymes:
Together these enzymes allow scientists to cut DNA at predictable positions and rejoin fragments from different sources.
The palindromic nature of the recognition site is what makes this work: the top-strand sequence reading 5'→3' is identical to the bottom-strand sequence reading 5'→3'. Cutting at the same offset on both strands therefore leaves identical single-stranded overhangs on the two products — and any DNA cut with the same enzyme produces overhangs that can anneal by Watson-Crick base-pairing.
Some restriction enzymes (like EcoRI) cut asymmetrically, leaving sticky ends — short single-stranded overhangs that can base-pair with complementary sticky ends from any other fragment cut with the same enzyme. Others (like SmaI) cut symmetrically, producing blunt ends. Sticky ends are preferred for cloning because they hybridise specifically and increase ligation efficiency.
| Enzyme | Source | Recognition sequence | Cut type |
|---|---|---|---|
| EcoRI | E. coli | GAATTC | Sticky (5' AATT) |
| BamHI | Bacillus amyloliquefaciens | GGATCC | Sticky (5' GATC) |
| HindIII | Haemophilus influenzae | AAGCTT | Sticky (5' AGCT) |
| SmaI | Serratia marcescens | CCCGGG | Blunt |
flowchart TD
A[Isolate gene of interest] --> B[Cut gene with restriction enzyme]
C[Cut plasmid vector with same enzyme] --> D[Mix gene + vector]
B --> D
D --> E[DNA ligase seals phosphodiester bonds]
E --> F[Recombinant plasmid]
F --> G[Transform into host bacterium]
G --> H[Select transformed cells using marker genes]
H --> I[Culture and express protein]
Three main methods:
Plasmids are the most common vectors: small, circular, double-stranded DNA molecules found naturally in bacteria. They replicate independently of the chromosome and can carry foreign DNA up to about 10 kb. Larger inserts require other vectors:
The vector is cut with the same restriction enzyme as the gene of interest so that their sticky ends are complementary. The gene and vector are mixed with DNA ligase, which catalyses the formation of phosphodiester bonds linking them into a recombinant plasmid.
Bacterial cells are made competent (able to take up DNA) by treatment with calcium chloride at low temperature, followed by a brief heat shock. This makes the plasma membrane temporarily permeable. An alternative is electroporation, in which a pulse of electricity opens transient pores.
Only a small fraction of bacteria (often <1%) take up the plasmid. A further subset take up the original unmodified vector without an insert. To identify cells that contain the recombinant plasmid, scientists use marker genes.
Early vectors carried antibiotic resistance genes (e.g. ampR for ampicillin, tetR for tetracycline). Cells transformed with the plasmid grow on plates containing the antibiotic; untransformed cells die. This simple selection is powerful but raises concerns about antibiotic resistance spreading to wild bacteria.
Some plasmids carry two resistance genes, one of which contains the restriction site used for insertion. If a gene is successfully inserted, it disrupts that resistance gene (e.g. tetR). Cells are transferred by replica plating to plates with different antibiotics. Cells that grow on ampicillin but not tetracycline must contain the recombinant plasmid.
Modern vectors often carry green fluorescent protein (GFP) from the jellyfish Aequorea victoria. Transformed cells glow green under UV light. Insertional inactivation of GFP means recombinant cells lose fluorescence — a visual test without the use of antibiotics.
The gusA gene from E. coli encodes β-glucuronidase, which converts a colourless substrate into a blue product. GUS staining is widely used in plant transformation to confirm expression.
Many cloning vectors (pUC18/19, pBluescript) include a fragment of the E. coli lacZ gene encoding the α-peptide of β-galactosidase. The multiple cloning site sits inside this fragment. A successful insert disrupts the lacZα sequence; the bacterium can no longer complement a host strain's lacZΔM15 mutation and so cannot cleave X-gal (a colourless chromogenic galactoside) into the blue product 5,5'-dibromo-4,4'-dichloro-indigo. Blue colonies therefore lack inserts; white colonies contain recombinants. This is the canonical screen taught at A-Level.
A modern cloning plasmid carries (1) an origin of replication so it propagates inside the host; (2) a selectable marker such as the ampicillin-resistance gene bla / ampR; (3) a screenable marker such as lacZα for blue-white screening; and (4) a multiple cloning site (MCS) containing dense, unique restriction sites engineered into the lacZα reading frame so that any insert disrupts the marker.
Inserting a gene into a host is only half the battle — it must also be expressed. This requires an appropriate promoter (recognised by host RNA polymerase), a ribosome binding site (Shine–Dalgarno sequence in prokaryotes) and, for secretion, a signal peptide. Genes from eukaryotes cloned into E. coli usually require their introns removed (hence the use of cDNA) and a bacterial promoter attached.
Cloning vectors used for therapeutic protein production typically place the gene of interest under an inducible promoter — a regulatory element that is repressed under normal growth conditions but switched on by adding a chemical inducer once the culture has reached high biomass. The classical example is the lac promoter repressed by LacI; expression is induced by adding IPTG (isopropyl-β-D-thiogalactopyranoside), a non-metabolisable lactose mimetic. The advantage is that expression is uncoupled from growth: cells grow rapidly without burdening themselves with foreign-protein production, and only switch to expression when commanded. Stronger T7-based systems (pET vectors) deliver dramatic induction ratios and are the workhorse of modern protein biochemistry. The tac promoter (a hybrid of trp -35 and lac -10 sequences) gives stronger expression than lac alone while remaining IPTG-inducible.
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