Restriction enzymes, ligase and vectors

Use of biological resources · Selective breeding and genetic modification · note 2 of 4

Spec 5.12, 5.13, 5.16Download all Selective breeding and genetic modification notes (PDF)Download all notes (PDF)5 pages

Restriction enzymes, ligase and vectorsSpec 5.12, 5.13, 5.16

In short

Genetic engineering transfers a gene from one species into a different species, making a transgenic organism. A restriction enzyme cuts the gene out of DNA, leaving sticky ends. The same enzyme cuts open a vector, such as a plasmid or virus, and ligase joins the gene into it, making recombinant DNA that the vector inserts into cells.

Genetic engineering modifies the genome of an organism to introduce desirable characteristics. A gene is usually taken from one species and put into a different species. This is what transgenic means: the transfer of genetic material from one species to a different species.

The gene for the desired characteristic is cut out of the chromosome of the donor organism using a restriction enzyme. Each restriction enzyme cuts DNA at a specific sequence of bases.
The cut leaves short, unpaired strands of DNA at each end. These are called sticky ends.
The vector is cut open using the same restriction enzyme, so it has matching sticky ends. A vector is a carrier that takes up the DNA, such as a bacterial plasmid or a virus.
The gene and the vector are mixed. The sticky ends join by complementary base pairing, and the enzyme ligase joins the DNA strands together. This makes recombinant DNA, for example a recombinant plasmid.
The vector inserts the recombinant DNA into the cells of the target organism. The cells now make the protein coded for by the new gene.

Restriction enzyme: An enzyme that cuts DNA at a specific base sequence.
Ligase: An enzyme that joins pieces of DNA together.
Vector: A plasmid or virus that takes up a piece of DNA and inserts this recombinant DNA into other cells.
Transgenic: Describes an organism with genetic material transferred from one species to a different species.

Flow diagram: a restriction enzyme cuts the insulin gene out of human DNA and cuts open a plasmid, leaving matching sticky ends; the sticky ends pair up, ligase joins the DNA to make a recombinant plasmid, and the plasmid is put into a bacterium which makes human insulin. — The same restriction enzyme gives matching sticky ends; ligase joins them to make a recombinant plasmid.

Exam tip:

Use the same enzyme to cut both the gene and the vector. That gives matching sticky ends. Ligase then does the joining.

Written and checked against the Edexcel IGCSE Science Double Award (4SD0) specification · Updated October 2026

Frequently asked questions

What is selective breeding in plants?

Selective breeding in plants is when humans choose parent plants with a desired characteristic, such as disease resistance, larger grains or higher yield, and breed them together. The offspring that show the characteristic most strongly are bred again, and this is repeated over many generations until the characteristic is strongly shown in the variety.

What are the disadvantages of selective breeding?

Selective breeding reduces genetic variation, because only a few individuals are used for breeding. If a new disease or a change in the environment appears, all individuals may be affected. Breeding closely related individuals, called inbreeding, can also make harmful inherited conditions more common in domesticated animals.

What is genetic engineering in agriculture?

Genetic engineering in agriculture is transferring a gene for a useful characteristic from one species into a crop plant, making a genetically modified plant. For example, a gene from Bacillus thuringiensis makes crops produce Bt toxin, which kills insect pests. This gives higher yields and less insecticide use, though genes may spread to wild plants.

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