Recombinant DNA Technology
Recombinant DNA technology — the cutting, splicing and recombining of DNA across species boundaries — is the foundation of modern genetic engineering and biotechnology. The techniques developed since the 1970s have enabled the production of human insulin, growth hormone and clotting factors in bacterial fermenters; the engineering of crop plants with new traits; the construction of vaccines; and the gene therapy approaches covered later in this course. The conceptual core is simple: DNA from any source can be cut at predictable positions, joined to a carrier molecule, introduced into a host cell, and expressed there. The technical detail behind each step is what makes the technology work — and what the AQA specification expects you to know.
Spec mapping: This lesson sits in AQA 7402 Section 3.8.4 — Gene technologies allow the study and alteration of gene function. The specification expects candidates to describe how recombinant DNA can be produced using restriction endonucleases, reverse transcriptase, DNA ligase and vectors; how DNA fragments can be amplified, separated and analysed (covered in Lessons 5 and 6); and the in vivo introduction of recombinant DNA into host cells (transformation). (Refer to the official AQA specification document for exact wording.)
What Is Recombinant DNA?
Key Definition: Recombinant DNA is DNA that has been artificially created by combining DNA from two or more different sources (often from different species). The basic premise — that DNA is chemically the same molecule whether it comes from a bacterium, a plant or a human, and can therefore be cut and joined irrespective of origin — was established in the late 1970s and is the foundation of genetic engineering.
The basic process involves five steps:
- Isolating the gene of interest from one organism.
- Cutting the gene and a vector using restriction enzymes.
- Joining the gene into the vector using DNA ligase.
- Introducing the recombinant vector into a host cell (transformation).
- Selecting host cells that have taken up the recombinant DNA, and (where required) confirming expression of the encoded protein.
Mermaid: the recombinant DNA workflow
flowchart TD
A["Isolate gene of interest (mRNA → cDNA via reverse transcriptase, or genomic digest, or chemical synthesis)"] --> B["Cut gene + vector with same restriction enzyme → compatible sticky ends"]
B --> C["Mix; sticky ends anneal by complementary base pairing"]
C --> D["DNA ligase forms phosphodiester bonds → recombinant vector"]
D --> E["Transform host cells (heat shock, electroporation)"]
E --> F["Plate on selective medium (antibiotic) → only transformants survive"]
F --> G["Screen for recombinants (blue-white screen, PCR, hybridisation)"]
G --> H["Confirm expression (Western blot / functional assay)"]
Restriction Enzymes (Restriction Endonucleases)
Key Definition: Restriction enzymes are enzymes that cut DNA at specific base sequences called recognition sites. They were originally discovered in bacteria, where they function as a defence mechanism against bacteriophage DNA — the bacterium methylates its own DNA at the recognition sites so that the enzyme cannot cut its own genome, but invading phage DNA is unprotected and is cleaved.
Key features
- Each restriction enzyme recognises a specific palindromic sequence (typically 4–8 base pairs long). A palindromic sequence in molecular biology reads the same on both strands in the 5' to 3' direction.
- Restriction enzymes are named after the bacterium from which they were first isolated: EcoRI from Escherichia coli strain R, first identified (the "I" denotes the first enzyme from that organism); HindIII from Haemophilus influenzae strain d, third enzyme; BamHI from Bacillus amyloliquefaciens H, first enzyme.
- Hundreds of restriction enzymes are commercially available, providing a vast molecular toolkit. Type II restriction enzymes (the type used in cloning) cut precisely at the recognition site and do not require energy from ATP.
Example: EcoRI
EcoRI recognises and cuts the sequence:
5'-G | A A T T C-3'
3'-C T T A A | G-5'
The vertical lines show where the enzyme cuts. The resulting fragments have 5' overhangs ("AATT") that are single-stranded and complementary to other EcoRI-cut overhangs. Other enzymes used commonly include:
| Enzyme | Source | Recognition site | Overhang |
|---|
| EcoRI | E. coli | 5'-GAATTC-3' | 5' AATT (sticky) |
| BamHI | B. amyloliquefaciens | 5'-GGATCC-3' | 5' GATC (sticky) |
| HindIII | H. influenzae | 5'-AAGCTT-3' | 5' AGCT (sticky) |
| SmaI | Serratia marcescens | 5'-CCCGGG-3' | blunt (no overhang) |
| PstI | Providencia stuartii | 5'-CTGCAG-3' | 3' TGCA (sticky) |
Sticky ends vs blunt ends
- Sticky ends — the enzyme cuts the two strands at different positions within the recognition site, leaving short single-stranded overhangs (usually 2–4 bases). These overhangs can form hydrogen bonds with complementary sticky ends produced by the same restriction enzyme, facilitating the joining of DNA fragments. Sticky ends are the workhorse of cloning.
- Blunt ends — the enzyme cuts both strands at the same position, leaving no overhangs. Blunt ends can be joined but less efficiently because there are no complementary single-stranded regions to guide base pairing. Useful when no compatible sticky-end enzyme is available, or when the source DNA has been generated by mechanical shearing or PCR.
Exam Tip: Sticky ends are preferred in genetic engineering because they allow complementary base pairing between the gene insert and the vector, making ligation more efficient. Always specify that the same restriction enzyme must be used to cut both the gene and the vector to produce compatible sticky ends. Mixing two different enzymes is sometimes useful for "directional cloning" (so the gene can insert only one way round) but is more advanced material.
Where do restriction enzymes come from?
The discovery of restriction enzymes earned the 1978 Nobel Prize in Physiology or Medicine. Bacteria use them as a defence against bacteriophage (viral) DNA: the bacterium tags its own DNA with methyl groups at the recognition sites (a "restriction-modification system"), so the cognate enzyme cannot cut the host genome. Invading phage DNA lacks these protective marks and is cleaved into fragments that the bacterium can degrade. The biology is itself a useful synoptic anchor with epigenetic modification (covered in Lesson 1).
DNA Ligase
Key Definition: DNA ligase is an enzyme that catalyses the formation of phosphodiester bonds between the sugar-phosphate backbones of adjacent DNA fragments, permanently joining them together. The enzyme used in molecular biology is most commonly T4 DNA ligase (from bacteriophage T4), which can join both sticky and blunt ends.
- After the gene of interest and the vector have been cut with the same restriction enzyme, they are mixed together.
- Complementary sticky ends base-pair by hydrogen bonding — this is temporary and weak; the structure could dissociate again.
- DNA ligase seals the gaps in the sugar-phosphate backbone between the 3'-OH of one fragment and the 5'-phosphate of the next, forming a covalent phosphodiester bond and creating a continuous recombinant DNA molecule. ATP (or NAD+ depending on enzyme source) provides the energy.
- Ligase can also join blunt ends, though this is less efficient and may require higher enzyme concentrations and longer reaction times.
Exam Tip: The mechanism of ligation is often examined. Be specific: sticky-end annealing is non-covalent hydrogen bonding; ligase forms the covalent phosphodiester bonds that complete the recombinant molecule. Both steps are required.
Vectors
Key Definition: A vector is a carrier molecule used to transfer a gene of interest into a host cell. Vectors must be capable of replicating within the host cell so that the gene is propagated as the cells divide.
Plasmid Vectors
- Plasmids are small, circular, double-stranded DNA molecules found naturally in bacteria, typically encoding accessory functions (antibiotic resistance, virulence, metabolic genes) and replicating independently of the bacterial chromosome.
- Engineered cloning plasmids (pBR322, pUC18/19, pET vectors) are derived from natural plasmids with non-essential elements removed and useful features added.
- Key features of a typical cloning plasmid:
- Origin of replication (ori) — allows the plasmid to replicate autonomously in the host cell.
- Antibiotic resistance gene(s) — used as selectable markers to identify bacteria that have taken up the plasmid (e.g. ampicillin resistance via β-lactamase).
- Multiple cloning site (MCS, also called polylinker) — a short region containing recognition sites for several different restriction enzymes, providing flexibility in cloning.
- Promoter (in expression vectors) — drives transcription of the cloned gene. Often inducible (lac, T7, tet promoters) so expression can be turned on at the desired time.
- Plasmids can typically carry inserts of up to ~10 kilobases (kb).
Bacteriophage Vectors
- Bacteriophages (phages) are viruses that infect bacteria. The classical cloning phage is lambda (λ).
- Part of the phage DNA (non-essential genes encoding lysogeny functions) can be replaced with the gene of interest.
- The recombinant phage is packaged into viral particles in vitro and used to infect bacteria, introducing the foreign DNA.
- Phage vectors can carry larger DNA inserts (up to ~25 kb for lambda phage).
- Infection efficiency is typically much higher than plasmid transformation, making phages useful for constructing genomic libraries.
Other vectors
- Yeast artificial chromosomes (YACs) — can carry very large inserts (up to ~1,000 kb); used in genome mapping projects such as the Human Genome Project.
- Bacterial artificial chromosomes (BACs) — F-plasmid-derived; carry ~100–300 kb inserts; widely used in genome sequencing.
- Cosmids — hybrid vectors combining features of plasmids and phage; can carry ~45 kb inserts.
- Viral vectors for mammalian cells — modified adenoviruses, retroviruses, lentiviruses, adeno-associated viruses used to deliver genes into mammalian cells (covered in Lesson 7 on gene therapy).
- Ti plasmid of Agrobacterium tumefaciens — used to transform plant cells; transfers a T-DNA segment that integrates into the plant chromosome. The basis of most genetically modified crops.
Obtaining the Gene of Interest
There are several ways to obtain the gene for cloning. The choice depends on the source organism, the size of the gene, and whether or not it needs to be free of introns for downstream expression.
1. Reverse transcriptase and cDNA
- mRNA is isolated from cells that actively express the gene of interest (e.g., mRNA for insulin is isolated from pancreatic β-cells; mRNA for haemoglobin from reticulocytes).
- The enzyme reverse transcriptase (originally isolated from retroviruses, where it converts the viral RNA genome to DNA inside the host cell) uses the mRNA as a template to synthesise a complementary DNA (cDNA) strand. A short oligo-dT primer anneals to the poly-A tail of mRNA and reverse transcriptase extends from it.
- The mRNA strand is then degraded (using RNase H or alkaline conditions).
- DNA polymerase synthesises the second DNA strand, producing double-stranded cDNA.
- cDNA contains only exons (because it is made from processed, intron-spliced mRNA) and therefore lacks introns. This is essential when expressing eukaryotic genes in prokaryotes: bacterial host cells lack the spliceosome and cannot remove eukaryotic introns. Without cDNA, the bacterium would translate the intron sequences as if they were coding, producing nonsense.
2. Chemical synthesis
- Short genes or specific DNA sequences can be synthesised chemically using automated DNA synthesisers.
- Codons can be optimised for the host organism — different organisms have different preferred codons for the same amino acid, and codon optimisation can substantially increase expression levels.
- Useful for small genes (typically under ~5 kb), when the amino acid sequence of the protein is known but the gene sequence is not, or when an entirely artificial sequence is desired.
3. Restriction enzyme digestion of genomic DNA
- The entire genome of an organism is cut with a restriction enzyme, and the fragment containing the gene of interest is identified and isolated (e.g., by screening a gene library with a labelled probe).
- This approach is most useful when working with prokaryotic organisms (no introns) or when the regulatory regions surrounding the gene are needed in addition to the coding sequence.
4. PCR amplification
- If the gene sequence is known, primers can be designed to flank the gene and the gene can be amplified directly from genomic DNA by PCR (covered in Lesson 5). This is now the workhorse approach for cloning known genes from small biological samples.
Transformation
Key Definition: Transformation is the process by which a host cell takes up foreign DNA (e.g., a recombinant plasmid) from its surroundings. The term is biological: it was used long before recombinant DNA technology, originally to describe the heritable change observed in pneumococci by Griffith in 1928 — Avery, MacLeod and McCarty later showed in 1944 that the "transforming principle" was DNA, which established DNA as the genetic material.
Methods of transformation