Recombinant DNA technology and genetic engineering, the polymerase chain reaction, gel electrophoresis and DNA profiling, and the applications and ethics of gene technology.

What this dot point is asking

CCEA wants you to describe recombinant DNA technology and genetic engineering, explain the polymerase chain reaction, describe gel electrophoresis and DNA profiling, and discuss the applications and ethics of gene technology.

Recombinant DNA technology

The gene of interest can be obtained by cutting it out with a restriction enzyme, by making complementary DNA (cDNA) from messenger RNA using reverse transcriptase, or by synthesising it. Using the same restriction enzyme on both the gene and the plasmid produces matching sticky ends, so the base pairs are complementary and ligase can seal them. Marker genes (for antibiotic resistance or fluorescence) let scientists identify which host cells took up the recombinant plasmid. This technology is used to make products such as human insulin, human growth hormone, and to produce genetically modified crops.

PCR and gel electrophoresis

Each PCR cycle doubles the amount of DNA, so the quantity grows exponentially as $2^n$ after $n$ cycles, turning a trace sample into millions of copies in a couple of hours. DNA is negatively charged because of its phosphate groups, so it moves towards the positive electrode (anode) during electrophoresis; the gel acts as a molecular sieve so small fragments move fastest.

DNA profiling and ethics

DNA profiling uses electrophoresis of repeated, variable regions of DNA (short tandem repeats) to produce a banding pattern unique to an individual (except identical twins). It is used in forensic identification, paternity testing and conservation (checking population diversity). Gene technology raises ethical and safety concerns: the welfare of modified organisms, the escape of engineered genes into wild populations, ownership and patents on genes and seeds, equal access to expensive medical benefits, and public unease about modifying human embryos.

Examples in context

Example 1. Solving a cold case with DNA profiling. A blood stain from an old crime scene contains only a tiny amount of degraded DNA. Forensic scientists use PCR to amplify specific short tandem repeat regions until there is enough DNA to run on a gel. The resulting banding pattern is compared with a suspect's profile and with the national database. A match at enough loci gives odds of billions to one against a chance match. This shows PCR and electrophoresis working together in a real application.

Example 2. Golden Rice and the GM debate. Golden Rice is genetically engineered to make beta-carotene (a precursor of vitamin A) by inserting genes from other organisms, aiming to reduce vitamin A deficiency and childhood blindness. Supporters point to the public-health benefit; critics raise concerns about corporate control of seeds, unknown ecological effects, and whether the dose delivered is enough. This is a clear case for discussing the benefits and ethical concerns of gene technology in an exam answer.

Try this

Q1. State the role of restriction enzymes and DNA ligase in genetic engineering. [2 marks]

• Cue. Restriction enzymes cut DNA at specific sequences; ligase joins the gene into a vector.

Q2. Explain how gel electrophoresis separates DNA fragments. [2 marks]

• Cue. An electric field moves negatively charged DNA through the gel; smaller fragments travel further.

Q3. Starting with 5 copies of a DNA sequence, calculate how many copies are present after 4 PCR cycles. [2 marks]

• Cue. Each cycle doubles the DNA, so $5 \times 2^4 = 5 \times 16 = 80$ copies.