The genetic engineering toolkit

Higher tier only

One way to insert a desired gene into a host uses a vector. A vector is a DNA carrier; a small circular bacterial DNA molecule called a plasmid is one example.

The main stages are:

1. A suitable restriction enzyme recognises a specific DNA sequence and cuts the donor DNA on both sides of the desired gene.
2. The same suitable restriction enzyme cuts open the plasmid vector. In this example it makes staggered cuts, leaving short single-stranded overhangs called sticky ends. Not every restriction enzyme makes sticky ends.
3. The desired-gene fragment and plasmid have complementary sticky ends, so their exposed bases pair. This holds the pieces in the correct relative position, but it does not permanently seal them.
4. DNA ligase joins the DNA backbones at both junctions. The result is a recombinant plasmid containing DNA from more than one source.
5. The recombinant plasmid acts as the vector that carries the desired gene into a host cell. The new DNA can then be copied and, when expressed, can give the desired characteristic.

[DIAGRAM: asset_name: Biology 4.8, 4.10-4.11, 4.14 - Selective breeding and genetic engineering - diagram 01; asset_slug: biology_4_8_4_10_4_11_4_14_selective_breeding_and_genetic_engineering__diagram_01; recommended_method: image_gen; description: Four-stage monochrome Higher-tier process: donor DNA with the desired gene is cut by a suitable restriction enzyme to leave sticky ends; the same suitable restriction enzyme cuts a plasmid vector to create complementary sticky ends; the ends pair and DNA ligase seals both junctions; the closed recombinant plasmid vector moves into a bacterial host cell. Arrows run left to right and then into the host cell. Exclude promoters, antibiotic-resistance markers, tissue culture, gene guns and separate-science cloning detail.]