Grade 12 · Molecular, Cellular & Tissue Level · Lesson 3

Genetic Engineering & Biotechnology

How scientists cut, copy and read DNA — and use that power to make insulin, modify crops, and solve paternity and forensic cases. Run a virtual gel and interpret a real DNA profiling case yourself.

What is genetic engineering?

Genetic engineering is the deliberate manipulation of an organism's DNA using biotechnology — cutting out a gene from one organism and inserting it into another, so the second organism now expresses a new trait. This is different from selective breeding: instead of waiting generations for a desired combination of genes to appear naturally, scientists move a specific, known gene directly.

The molecular toolkit

ToolWhat it does
Restriction enzymeA bacterial enzyme that cuts DNA at a specific, short recognition sequence, producing fragments with matching "sticky ends"
Vector (e.g. plasmid)A small circular piece of DNA (often from bacteria) used to carry a foreign gene into a host cell
DNA ligaseAn enzyme that joins DNA fragments together, sealing the foreign gene into the vector to form recombinant DNA
PCR (polymerase chain reaction)A technique that rapidly makes millions of copies of a chosen DNA sequence from a tiny starting sample
Gel electrophoresisSeparates DNA fragments by size by pulling them through a gel using an electric current
Sticky ends: Most restriction enzymes cut DNA in a staggered pattern, leaving short single-stranded overhangs. If the SAME restriction enzyme is used to cut both the foreign gene and the plasmid vector, their sticky ends are complementary and will base-pair together — this is what lets DNA ligase join them into one continuous recombinant molecule.

Making recombinant DNA — the insulin example

Before genetic engineering, insulin for diabetics was extracted from the pancreases of cattle and pigs — expensive, limited in supply, and slightly different from human insulin. Today, genetically modified bacteria manufacture human insulin directly:

  1. The human gene for insulin is identified and cut out using a restriction enzyme.
  2. A bacterial plasmid is cut open with the same restriction enzyme, creating matching sticky ends.
  3. The human insulin gene is inserted into the plasmid and sealed in place with DNA ligase, forming recombinant DNA.
  4. The recombinant plasmid is inserted into a bacterium (commonly E. coli) — this step is called transformation.
  5. The bacterium is cultured in large fermentation tanks. As it reproduces, every descendant copies the plasmid too, and each cell reads the human gene and produces human insulin.
  6. The insulin is extracted, purified, and packaged for medical use.
Because the insulin produced this way is identical to human insulin, it causes far fewer allergic reactions than the older animal-extracted versions — and bacteria can be grown in unlimited, cheap, fast-multiplying batches.

Gel electrophoresis & DNA profiling

DNA is negatively charged, so when placed in a gel and exposed to an electric current, it migrates toward the positive electrode. The gel acts like a molecular sieve: smaller DNA fragments move through the gel matrix faster and travel farther, while larger fragments are held back closer to the loading well. This produces a pattern of bands, sorted by size.

DNA profiling (DNA "fingerprinting") exploits regions of DNA that vary a lot between individuals (short repeated sequences that differ in number from person to person). The typical process is:

  1. Collect a DNA sample (blood, cheek swab, hair root, or crime-scene evidence).
  2. Use PCR to amplify the variable regions of interest, since crime-scene or forensic samples are often too small to analyse directly.
  3. Cut the amplified DNA with restriction enzymes (or size-separate the amplified regions directly).
  4. Run the fragments through gel electrophoresis, producing a unique banding pattern for each individual.
  5. Compare banding patterns between samples — e.g. a child's pattern should show one band matching the mother and one matching the biological father.
Applications: paternity testing, forensic investigation (matching crime-scene DNA to a suspect), identifying disaster victims, and confirming family relationships in immigration cases.

Applications: GMOs in agriculture and medicine

ExampleWhat was engineered, and why
Bt maize / Bt cottonA gene from the bacterium Bacillus thuringiensis is inserted, so the plant produces a protein toxic to specific insect pests — reducing the need for chemical pesticide spraying
Herbicide-resistant cropsEngineered to survive a specific weed-killer, so farmers can spray to remove weeds without harming the crop
Golden RiceEngineered to produce beta-carotene (a vitamin A precursor) in the grain, to help address vitamin A deficiency in regions where rice is a staple food
Recombinant human insulinBacteria engineered with the human insulin gene mass-produce insulin for diabetes treatment (see above)
Gene therapyA functional copy of a gene is introduced into a patient's cells to correct or compensate for a faulty gene (e.g. trials for cystic fibrosis and some immune disorders)

Advantages, disadvantages & ethical issues

CAPS expects you to be able to discuss and evaluate genetic engineering, not just describe it — so weigh both sides:

Points raised
AdvantagesHigher crop yields; less chemical pesticide use (Bt crops); improved nutrition (Golden Rice); cheaper, safer, unlimited-supply medical products (insulin); potential cures via gene therapy; solving crimes and exonerating the wrongly accused via DNA profiling
Disadvantages / concernsUnknown long-term health effects of GM foods are still debated; engineered genes could spread into wild populations via cross-pollination (e.g. herbicide resistance moving into weeds); reduced genetic diversity from monoculture farming; seed-patenting can disadvantage small farmers; ethical concerns over "designer" genetic modification in humans; privacy and misuse concerns over DNA profiling databases
Exam tip: "Discuss" or "evaluate" questions on genetic engineering want you to present points on BOTH sides and, where asked, a justified opinion — a one-sided answer loses marks even if every point you give is correct.

CRISPR-Cas9: precision gene editing

CRISPR-Cas9 is a newer, more precise genetic engineering tool compared to traditional restriction-enzyme methods. Cas9 is an enzyme that acts like programmable "molecular scissors": guided by a short piece of RNA that matches a specific target DNA sequence, Cas9 cuts the DNA at that exact location. Once cut, the cell's own repair machinery can be used to disable a gene, or to insert a new piece of DNA at that precise site.

CRISPR vs artificial selection: both involve humans deliberately choosing a trait to enhance or remove in an organism, for human benefit — but CRISPR directly edits the DNA sequence itself, while artificial selection works by selecting which existing organisms breed, over many generations, without directly touching their DNA.

Because CRISPR is fast, relatively cheap, and highly precise, it is being explored for gene therapy in humans, disease-resistant crops, and eliminating disease-carrying insect populations — but any new medical application (like a CRISPR-based treatment) must go through extensive clinical trials first, to confirm it is safe, effective, free of harmful "off-target" cuts elsewhere in the genome, and that its benefits outweigh its risks.

The genome

An organism's genome is its complete set of genetic material — all of the DNA/genes it possesses. Comparing genomes between individuals reveals that most DNA sequence is shared, but certain non-coding regions vary a great deal between individuals — these variable regions are exactly what DNA profiling (gel electrophoresis) exploits to distinguish one person's DNA from another's.

Gel Electrophoresis Simulator — a paternity case

A mother, her child, and an alleged father are DNA-profiled at one variable region. Run the gel and decide: could this man be the biological father?

Gel result

Click "Run gel" to load the samples and watch the DNA fragments separate by size.
0/8
Review the explanations above.
Answer in your exercise book. Show all reasoning for "discuss"/"explain" questions.
Question 1 · (3 marks)
Define genetic engineering, and name TWO enzymes used in the process, stating the function of each.
Question 2 · (5 marks)
Describe, in the correct order, the steps used to genetically engineer bacteria to produce human insulin.
Question 3 · (3 marks)
Explain why DNA fragments of different sizes separate during gel electrophoresis, and state which fragments (large or small) travel farthest.
Question 4 · (4 marks)
Discuss TWO advantages and TWO disadvantages of genetically modified crops.
Question 5 · Interpreting a DNA Profile (8 marks)
In a disputed paternity case, DNA profiling at one variable region produced the following fragment sizes (in base pairs, bp) for each person:
PersonFragment sizes (bp)
Mother600 and 200
Child600 and 450
Alleged Father 1450 and 300
Alleged Father 2350 and 200
(a) A child inherits one DNA fragment from each biological parent. Which of the child's TWO fragments must have come from the mother? Explain using the data. (2)
(b) The child's other fragment must have come from the biological father. Using the data, which of the two alleged fathers is more likely to be the biological father? Explain. (3)
(c) On a gel, would the 200 bp fragment travel closer to the well, or farther from the well, than the 600 bp fragment? Explain. (2)
(d) Explain why PCR is often performed on a DNA sample BEFORE running gel electrophoresis in real forensic or paternity cases. (1)