Genetic engineering is the deliberate act of taking a gene that codes for a useful characteristic — say, "make human insulin" — cutting it out of human DNA, and inserting it into a completely different organism (usually bacteria) so that they start making the protein for us, in bulk, in a factory.

Genetically Modified Micro-organisms

This is the classic route for making medicinal proteins like human insulin and human blood clotting factors. The process, in order:

Genetically Modified Plants

A similar idea, but the gene is first put into a plasmid, transferred into a bacterium, and that bacterium is then used to infect plant cells — it acts as a vector carrying the gene into the plant's DNA. Alternatively, a "gene gun" fires DNA-coated metal pellets directly into plant cells. Once the gene reaches the plant cell nucleus, the cell is grown into a full adult plant, and every cell of that plant now carries a copy of the gene. Examples: human insulin and a cholera vaccine produced in modified plants — the plant can literally be eaten to deliver the drug.

Genetically Modified Animals

The gene is injected directly into the nucleus of a zygote (fertilised egg), which is then implanted into a surrogate animal's uterus and develops into an adult. Every cell of the resulting animal carries the gene. The protein is then purified from a convenient source — often the animal's milk. Example: human blood clotting proteins from the milk of genetically modified animals.

This is the "master process" that sits behind everything in this chapter. Producing a transgenic organism always follows the same five stages. Examiners love asking you to describe this process in order, so let's build it properly.

Stage 1 — Identification

Find the gene that codes for the characteristic you want — e.g. pest resistance in crops, or the human insulin gene.

Stage 2 — Isolation

There are three ways to actually get hold of (isolate) the gene:

Stage 3 — Multiplication

The isolated gene is copied millions of times using the polymerase chain reaction (PCR), carried out in a machine called a thermocycler. PCR uses free nucleotides, DNA polymerase, and DNA primers to build many identical copies.

Stage 4 — Transfer (via a vector)

The gene is inserted into another organism's DNA using a vector — a "delivery vehicle." Common vectors: DNA plasmids, viruses, or liposomes. Once an organism has taken up the vector, it is said to be transformed.

Stage 5 — Identification of transformed cells (marker genes)

Not every cell successfully takes up the gene — transformation is inefficient. So scientists attach a marker gene alongside the desired gene. Any cell that takes up the desired gene will also take up the marker gene, so you can test for the marker to find the cells that worked.

Once identified, the transformed cells are cloned so that all resulting cells carry the desired gene (e.g. grown up in a fermenter for bacteria).

Restriction Endonucleases — The Molecular Scissors

Restriction endonucleases (restriction enzymes) are enzymes that cut DNA at a very specific, predictable base sequence called a restriction site. For example, the enzyme HindIII always cuts at the sequence AAGCTT — nowhere else. Crucially, they cut the two DNA strands unevenly, leaving short, single-stranded overhangs called sticky ends.

DNA Ligase — The Molecular Glue

Once the desired gene's sticky ends have paired up with the vector's complementary sticky ends, the enzyme DNA ligase catalyses the formation of phosphodiester bonds in the sugar-phosphate backbone, permanently joining the two DNA fragments together. If this is done to a plasmid, the result is called a recombinant plasmid.

Once you've built your recombinant DNA, you need to physically get it inside a new cell — and ideally into that cell's nucleus so it can be transcribed. There are several delivery methods, each with pros and cons.

Plasmid Vectors

Plasmids are small circular loops of double-stranded DNA found in bacteria. The desired gene is spliced in using restriction enzymes + DNA ligase (as above), creating a recombinant plasmid. Getting the plasmid into a bacterial cell is called transformation, and can be done by:

• Bathing plasmids and bacteria in ice-cold calcium chloride solution, then briefly heating to 40°C — makes the bacterial membrane temporarily permeable.
• Electroporation — giving the bacteria a small electric shock to make the membrane porous.

Viral Vectors

Viruses naturally reproduce by injecting their DNA into host cells — which makes them excellent natural vectors. Different viruses can infect animal, plant, or bacterial cells, so viral vectors can transform many cell types. Crucially, viruses are very good at reaching the nucleus of the cell (unlike plasmids, which often fail to get past the nuclear membrane in eukaryotic cells). Viral vectors are used in gene therapy — e.g. delivering non-mutated genes to treat cystic fibrosis.

Liposome Vectors

Liposomes are tiny spherical vesicles surrounded by a phospholipid bilayer. Because their outer layer is chemically identical to a cell's own membrane, they can fuse directly with the cell surface membrane, releasing their DNA cargo inside. Also used in gene therapy.

Gene Guns

DNA fragments coat tiny metal pellets (gold or tungsten), which are fired at high speed directly into cells. Cells that survive the impact can incorporate the new DNA into their genome. Useful for plant cells, which have a tough cell wall that other vectors struggle to cross.

Microinjection

A fine glass micropipette physically injects DNA directly into a cell. Commonly used to insert DNA into animal zygotes to create transgenic animals.

Not every gene in a cell is "switched on" at once — a gene is only active (expressed) if it's being transcribed into mRNA and translated into protein. A microarray is a tool that can scan thousands of genes at once and tell you exactly which ones are active — hugely useful in medicine (spotting harmful mutations), forensics, and biotechnology.

How a Microarray Experiment Works — Step by Step

Technologies like microarrays and DNA sequencing generate a staggering amount of data — genome sequences, gene expression patterns, protein amino acid sequences. No human could manually process that. Bioinformatics is the interdisciplinary science that combines biology, computer science, and statistics to collect, organise, store, and analyse all this biological data.

Applications of Bioinformatics

• Studying and comparing gene function across different species.
• Comparing DNA sequences to establish evolutionary relationships.
• Rapidly identifying new drug candidates by scanning molecule libraries.
• Simulating the safety/effects of drug candidates on cells using computer models before real testing.
• Developing personalised medical treatments from microarray/sequencing data.
• Developing new genetic tests for identifying genetic disease.
• Identifying desired genes for recombinant DNA work, and finding the best insertion point in a GMO's genome — useful for developing new gene therapies.

GMOs offer huge medical and agricultural potential, but they raise real, ongoing scientific and ethical concerns — especially when GMOs are food crops. Exam questions often ask you to "discuss" or "evaluate," so you need arguments on both sides.