Click a term to reveal its definition. Work through all ten before moving on.
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To move a gene from one organism to another, scientists need two molecular tools — one to cut DNA at precisely the right place, and one to seal the new gene in permanently.
Imagine DNA as a very long roll of ticker tape with coded instructions on it. Restriction enzymes are a special pair of scissors that only cut the tape at one specific pattern of letters — they recognise a sequence like GAATTC and cut only there. Ligase is the sellotape — it sticks the cut ends back together, permanently and precisely.
Restriction endonucleases are enzymes that recognise a specific, short base sequence in a DNA strand — called a recognition site. Every time the enzyme encounters that exact sequence, it cuts the DNA molecule. Different restriction enzymes recognise different sequences.
This means scientists can choose which enzyme to use depending on where in the DNA they need to cut. For example, an enzyme recognising the sequence GAATTC will only cut at every point where GAATTC occurs — nowhere else.
Some restriction enzymes make a staggered cut — instead of cutting straight across both strands, they cut at slightly offset positions. This leaves short, single-stranded overhangs called sticky ends.
These sticky ends are important because they are complementary to any other sticky end produced by the same restriction enzyme. This means the donor gene and the opened plasmid (both cut with the same enzyme) have matching sticky ends that will naturally hydrogen-bond together before ligase seals them.
DNA ligase forms permanent covalent bonds (phosphodiester bonds) between the sugar-phosphate backbone of joined DNA fragments. It acts after the sticky ends have come together via hydrogen bonding.
If sticky ends are the temporary handshake holding two fragments together, ligase is the permanent weld that makes the join last. Without ligase, the gene would simply fall out again.
The same restriction enzyme must be used to cut both the donor DNA and the plasmid. This ensures matching sticky ends — which means the gene can be inserted precisely and joins easily.
Once the target gene has been cut from donor DNA and has sticky ends, it must be carried into the host bacterium. The carrier molecule is called a vector.
Think of the gene as a parcel. On its own, a parcel cannot enter a building — it needs delivery. A plasmid is like a delivery van: it enters the bacterium carrying the gene inside. A bacteriophage is like a syringe: it attaches to the cell wall and physically injects the DNA directly into the bacterium.
Bacteria naturally contain small, circular pieces of DNA called plasmids, separate from their main chromosome. Scientists cut open a plasmid with a restriction enzyme, insert the desired gene (sticky ends bond → ligase seals), and mix the recombinant plasmid with bacteria. Some bacteria take up the plasmid — this process is called transformation.
A bacteriophage is a virus that infects bacteria. It attaches to the cell wall and injects its DNA. If the foreign gene has been incorporated into the phage DNA, it is injected too and becomes part of the bacterium's genome. Phages were used as vectors in early genetic engineering but plasmids are more common today.
The DNA that results from joining the donor gene to the plasmid is called recombinant DNA — DNA made by combining genetic material from two different organisms.
Drag the steps to show how a transgenic bacterium is produced using a plasmid vector.
People with Type 1 diabetes cannot produce sufficient insulin — the hormone that regulates blood glucose. Before genetic engineering, insulin had to be extracted from the pancreases of pigs and cattle. This animal insulin is slightly different in structure to human insulin and caused immune reactions in some patients.
The human insulin gene is isolated from human DNA and inserted into E. coli bacteria using a plasmid vector. These bacteria are then grown in enormous industrial containers called fermenters, where conditions (temperature, pH, oxygen, nutrients) are carefully controlled. Because bacteria reproduce every 20 minutes, billions of insulin-producing bacteria are generated rapidly.
Each GM bacterium is like a worker handed a new instruction manual (the insulin gene). The fermenter is the factory building. One worker becomes billions within hours, all producing the same product — pure human insulin.
1. Chemically identical to human insulin — no immune side effects. 2. Produced in unlimited, consistent quantities. 3. No animal slaughter required. 4. Faster and cleaner than extraction from animal tissue.
Unlike bacteria (a single cell), a plant contains billions of cells. To produce a GM plant, every cell must carry the new gene. This means inserting the gene into a single cell, then growing a whole plant from that cell using micropropagation.
This soil bacterium naturally inserts its plasmid into plant cells. Scientists replace the bacteria's harmful genes with the desired gene. Leaf discs from the target plant are floated on a solution containing the modified bacteria — some cells take up the recombinant plasmid. These cells are then cultivated on nutrient medium and grown into whole plants.
Limitation: Agrobacterium cannot infect cereal crops (monocots like wheat, rice, maize).
For cereals and other plants where Agrobacterium fails, a gene gun fires tiny pellets of gold coated with the target DNA at high velocity directly into plant tissue. Some pellets penetrate cells and the DNA is incorporated into the genome. Modified cells are then grown into whole plants by micropropagation.
In both methods, once a single cell has taken up the new gene, micropropagation (growing tissue in nutrient medium) allows scientists to produce thousands of identical GM plants from that one cell.
A transgenic organism is one that has had genetic material from a different species transferred into its genome. The prefix trans- means "across".
Trans (across) + genic (genes) = genes moved across species. A bacterium containing the human insulin gene is transgenic. Golden rice (genes from daffodils + a bacterium) is transgenic. A sheep producing a human protein in its milk is transgenic.
Selective breeding stays within the same species — it is not transgenic. Transgenic requires the gene to cross the species boundary. If a student says "all GM organisms are transgenic", remind them that not all genetic modification crosses species (e.g. knocking out a gene within the same species is not transgenic).
Click a term on the left, then click its matching description on the right.
Use this table as a final checklist. For each objective, can you explain it in your own words?
| Ref. | Topic | Key points to know |
|---|---|---|
| 5.12 | Restriction & Ligase Enzymes | Restriction enzymes cut DNA at specific recognition sequences, producing sticky ends. Ligase joins DNA fragments by forming permanent covalent bonds. The same restriction enzyme is used for both donor DNA and vector to create matching sticky ends. |
| 5.13 | Vectors | Vectors (plasmids or bacteriophages) carry recombinant DNA into host cells. Plasmids are small circular bacterial DNA. Bacteriophages inject DNA into bacteria. Transformation = uptake of a plasmid by a bacterium. |
| 5.14 | GM Insulin | Human insulin gene inserted into E. coli via plasmid. Bacteria grown in fermenters produce human-identical insulin in industrial quantities. Advantages: no side effects, unlimited supply, no animal slaughter. |
| 5.15 | GM Plants | Agrobacterium tumefaciens used as vector for most plants. Gene gun fires DNA-coated gold pellets for cereals. Micropropagation grows whole plants from modified cells. Examples: golden rice (Vitamin A), Bt cotton, herbicide-resistant crops. |
| 5.16 | Transgenic | Transgenic = genetic material transferred FROM one species TO a different species. Not the same as selective breeding (same species). All GM organisms with cross-species genes are transgenic. |