Genetic Engineering

Genetic engineering is the process of modifying the genome of an organism by introducing a gene from a different organism. The resulting organism is called a genetically modified organism (GMO). In this lesson we cover the process, the tools involved, and key examples.

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What Is Genetic Engineering?

Genetic engineering involves taking a gene that codes for a desired characteristic from one organism and inserting it into the genome of a different organism. The receiving organism then expresses the new gene, producing the desired protein.

Unlike selective breeding, which works within a single species, genetic engineering can transfer genes between completely different species — for example, from a bacterium to a plant, or from a human to a bacterium.

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The Steps of Genetic Engineering

You must know these steps in the correct order for your exam:

Step 1: Identify the Desired Gene

Scientists identify the gene that codes for the desired characteristic. For example, the gene for human insulin, or the gene from Bacillus thuringiensis that produces a natural insecticide.

Step 2: Cut Out the Gene Using Restriction Enzymes

Restriction enzymes (also called restriction endonucleases) act as molecular scissors. They cut DNA at specific recognition sequences (short sequences of bases).

• Restriction enzymes cut the donor DNA on either side of the desired gene, releasing it.
• Some restriction enzymes make staggered cuts, producing sticky ends — short, single-stranded overhangs of DNA that can bind to complementary sticky ends.

Step 3: Prepare the Vector

A vector is a carrier used to transfer the gene into the target organism. The most common vectors are:

Vector	Description
Bacterial plasmid	A small, circular ring of DNA found in bacteria. The plasmid is cut open with the same restriction enzyme used to cut out the gene, creating complementary sticky ends.
Virus	Some viruses can be modified to carry the desired gene and inject it into the target cell.

Step 4: Insert the Gene into the Vector Using Ligase

DNA ligase is an enzyme that joins pieces of DNA together. It seals the desired gene into the cut plasmid by forming bonds between the sticky ends.

The result is a recombinant plasmid — a plasmid containing DNA from two different organisms.

Step 5: Insert the Vector into the Target Organism

The recombinant plasmid is inserted into the target organism (e.g., a bacterium, plant cell or animal cell). Methods include:

• Heat shock or electrical pulse — makes bacterial cell membranes temporarily permeable so plasmids can enter
• Microinjection — using a very fine needle to inject DNA directly into a cell
• Gene gun (biolistics) — tiny gold or tungsten particles coated with DNA are fired into plant cells

Step 6: The Target Organism Expresses the New Gene

The target organism reads the new gene and produces the protein it codes for. If the organism reproduces, it passes the gene to its offspring.

Summary Diagram

graph TD
    A["Identify desired gene in donor organism"] --> B["Restriction enzyme cuts out gene"]
    C["Plasmid extracted from bacterium"] --> D["Same restriction enzyme cuts plasmid open"]
    B --> E["Gene and plasmid have complementary sticky ends"]
    D --> E
    E --> F["DNA ligase joins gene into plasmid"]
    F --> G["Recombinant plasmid formed"]
    G --> H["Plasmid inserted into target organism"]
    H --> I["Target organism expresses new gene"]

Exam tip: Learn the order of the steps and the names of the enzymes. The most commonly tested points are: restriction enzymes cut DNA, ligase joins DNA, and a plasmid is the vector. If you mix up the enzyme names, you will lose marks.

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Key Enzymes in Genetic Engineering

Enzyme	Function	Analogy
Restriction enzyme	Cuts DNA at specific recognition sequences	Molecular scissors
DNA ligase	Joins pieces of DNA together	Molecular glue

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Important Terminology

Term	Definition
Genetically modified organism (GMO)	An organism whose genome has been altered by genetic engineering
Vector	A carrier used to transfer DNA into a target organism (e.g., plasmid, virus)
Plasmid	A small, circular ring of DNA found in bacteria; commonly used as a vector
Recombinant DNA	DNA that contains genetic material from two different organisms
Sticky ends	Short, single-stranded overhangs of DNA produced by restriction enzymes
Transgenic	An organism containing a gene from a different species

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Examples of Genetic Engineering

Example 1: Human Insulin Production

Before genetic engineering, insulin for diabetic patients was extracted from the pancreases of pigs or cattle. This was expensive, required large numbers of animals, and sometimes caused allergic reactions because the insulin was slightly different from human insulin.

The genetic engineering approach:

1. The human insulin gene is identified and cut out using restriction enzymes.
2. A bacterial plasmid is cut open with the same restriction enzyme.
3. The insulin gene is inserted into the plasmid using DNA ligase.
4. The recombinant plasmid is inserted into Escherichia coli (E. coli) bacteria.
5. The bacteria are grown in large fermenters (bioreactors) under controlled conditions.
6. The bacteria read the human gene and produce human insulin.
7. The insulin is extracted, purified and used to treat diabetes.

Advantages over animal insulin:

• It is identical to human insulin, so fewer allergic reactions
• It can be produced in large quantities cheaply
• It does not require animal slaughter
• It is acceptable to people with religious or ethical objections to animal products

Example 2: Bt Crops (Insect-Resistant Plants)

Bt crops contain a gene from the soil bacterium Bacillus thuringiensis:

• This gene codes for a protein (Bt toxin) that is toxic to certain insect pests (particularly caterpillars and beetle larvae).
• When insects eat the plant, the Bt toxin kills them.
• The plant is therefore naturally pest-resistant and requires less chemical pesticide.

Examples of Bt crops: Bt cotton, Bt maize, Bt soybean.

Example 3: Golden Rice

Golden Rice has been genetically engineered to produce beta-carotene (a precursor of vitamin A) in the edible part of the grain:

• Normal rice grains are white and contain no beta-carotene.
• Genes from daffodils and the bacterium Erwinia uredovora were inserted into rice.
• The rice grains turn golden-yellow due to the beta-carotene.
• Golden Rice was developed to combat vitamin A deficiency, which affects millions of people in developing countries and can cause blindness and increased mortality in children.