A level Biology exam revision resources written by A level Examiners


In genetic engineering genes are taken from one organism and inserted into another. Genes that code for useful substances, such as hormones, enzymes and antibiotics, are often transferred into microorganisms, which then produce large quantities of these substances.

The following points give the detail of the process of transferring a gene from one organism to another.

  • The gene is cut out from the DNA of the donor organism using a restriction endonuclease enzyme. This cuts out the relevant section of the organism's DNA, leaving sticky ends - which consist of a single strand with a few base pairs - that will enable the gene to be inserted into a small circular piece of bacterial DNA called a plasmid. Plasmids are often used as vectors to take the selected gene into bacterial cells. Plasmids occur naturally in bacterial cells and replicate independently of the main bacterial DNA.
  • The same restriction endonuclease that cut out from the DNA of the donor organism is used to cut open the plasmid. This leaves complementary sticky ends to which the selected gene can be attached.
  • The sticky ends of the gene and the open plasmid are joined together by ligase which is another enzyme.
  • The plasmids are then introduced into the target organism using a technique such as heat shock, and transformed cells are selected and cloned.
  • Genetic markers in the plasmids, such as genes that confer antibiotic resistance, enable genetic engineers to identify bacteria that have successfully taken up the selected gene.
  • Transformed bacteria are cultured on a large scale in industrial fermenters and the useful product is then extracted..   Exam Advice

The process of gene transfer is illustrated in the diagram below. In this diagram the specific restriction endonuclease used is called EcoRI.

gene transfer

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