The Process of Genetic Engineering
Genetic engineering involves transferring a gene from one organism to another to create recombinant DNA. For ESAT, you must understand the process, the key enzymes involved, and be able to weigh the medical benefits against the potential risks.
Part of the ESAT Biology syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- The process involves isolating a target gene, using a vector (like a bacterial plasmid) to carry it, and inserting it into a host organism.
- Restriction enzymes act as 'molecular scissors' that cut DNA at specific recognition sites, often creating single-stranded 'sticky ends' which can base-pair with complementary ends.
- DNA ligase acts as 'molecular glue', catalysing the formation of phosphodiester bonds to permanently join the desired gene into the vector's DNA.
- A key application is producing human insulin in genetically modified bacteria, providing a safe and plentiful supply for treating diabetes.
- Medical benefits include mass production of drugs and hormones, and the potential for gene therapy to treat genetic disorders.
- Risks of medical applications include unknown long-term health effects, ethical objections to modifying human genetics, and the potential for genes to escape into the environment.
Definitions
- Genetic Engineering
- The direct manipulation of an organism's genome using biotechnology to introduce new characteristics.
- Recombinant DNA
- A DNA molecule formed by combining genetic material from different sources.
- Restriction Enzyme
- An enzyme that cleaves DNA at a specific nucleotide sequence (a recognition site).
- Ligase
- An enzyme that joins DNA fragments by catalysing the formation of a phosphodiester bond in the sugar-phosphate backbone.
- Vector
- A DNA molecule, such as a plasmid or virus, used to transfer foreign genetic material into a cell.
Worked example
A scientist aims to insert a gene for a human protein into a bacterial plasmid. The plasmid has a single recognition site for the restriction enzyme EcoRI. The human DNA containing the target gene has EcoRI sites flanking the gene (one just before its start, one just after its end). What are the essential enzymes required to create a stable, recombinant plasmid containing this gene?
- 1
Identify the goal:
to cut the target gene from the human DNA and insert it permanently into the bacterial plasmid.
- 2
The same restriction enzyme must be used to cut both the gene and the plasmid.
This creates complementary 'sticky ends' that can anneal (base pair) with each other.
- 3
Use the restriction enzyme EcoRI to excise the gene from the human DNA and to open the circular plasmid at its single recognition site.
- 4
After the gene's sticky ends anneal with the plasmid's sticky ends, the sugar-phosphate backbone must be sealed to make the join permanent.
- 5
Use DNA ligase to form phosphodiester bonds, joining the gene's DNA into the plasmid backbone, thus creating a stable recombinant DNA molecule.
Answer: EcoRI and DNA ligase
Common mistakes
- ×Forgetting the role of DNA ligase. Many students correctly identify that restriction enzymes are needed to cut the DNA, but fail to mention that ligase is essential to permanently join the gene into the vector.
- ×Failing to specify that the *same* restriction enzyme must be used on both the gene and the vector to ensure complementary sticky ends are produced.
- ×When asked for benefits AND risks, only providing one side of the argument. ESAT questions often require a balanced view, so ensure you address all parts of the prompt.
No-calculator tips
- ✓Use the mnemonic 'Cut and Paste': Restriction enzymes 'Cut' the DNA, and Ligase 'Pastes' it together. This helps to separate their functions clearly in your mind.
- ✓Visualise the process as a four-step sequence: 1. Isolate gene, 2. Cut gene and vector (with the same enzyme), 3. Mix to anneal sticky ends, 4. Add ligase to seal.
- ✓For benefit/risk questions, structure your thoughts. Think about effects on individuals (health), society (cost, ethics), and the environment (gene transfer).