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Genetically modified organisms and recombinant DNAEdexcel International A Level Biology: Revision notes

Section 1

GM organisms and recombinant DNA

A genetically modified (GM) organism contains a gene from another species. DNA made by joining DNA from two different sources is recombinant DNA. Because the genetic code is universal, a human gene placed in a bacterium, plant or animal can be transcribed and translated to make the human protein.

GM organisms are used to produce drugs such as human insulin, antibodies, clotting factors and vaccine antigens.

Key termsGM organismrecombinant DNAtransgene

Section 2

Making recombinant DNA

  1. Restriction endonucleases cut DNA at specific recognition sequences. Many make a staggered cut, leaving short single-stranded sticky ends (for example EcoRI cuts at GAATTC). The gene is cut from human DNA in this way.
  2. The same enzyme cuts the plasmid (the vector), giving complementary sticky ends.
  3. Gene and plasmid are mixed and the sticky ends anneal by complementary base pairing (hydrogen bonds).
  4. DNA ligase forms phosphodiester bonds between the sugar-phosphate backbones, sealing the gaps. This makes a recombinant plasmid.

The gene also needs a promoter (where RNA polymerase binds) so that it is transcribed in the host.

Key termsrestriction endonucleasesticky endDNA ligasevectorpromoter
Common mistake

Restriction endonucleases cut and DNA ligase joins. Do not say that ligase 'sticks' sticky ends together: the sticky ends join by base pairing, and ligase seals the backbone.

Section 3

Inserting recombinant DNA into cells

  • Bacteria: cells are made permeable using calcium ions and a heat shock, or an electric pulse (electroporation), so they take up plasmids. Only a few cells do so, so a marker gene (antibiotic resistance or a gene for a fluorescent protein) is used to select the transformed cells.
  • Plants: the gene is put into the Ti plasmid of Agrobacterium tumefaciens, which transfers its T-DNA into plant cells. Whole plants are then regenerated from the transformed cells.
  • Animals: the DNA is microinjected into a fertilised egg, so that every cell of the animal that develops contains the gene and it is inherited.
Key termstransformedmarker geneelectroporation

Section 4

Drugs from GM microorganisms

Bacteria such as E. coli are used to make human insulin. They grow quickly in fermenters, so large amounts are made cheaply and continuously. The insulin is identical to human insulin, so there is little immune reaction, and there is no reliance on animal pancreases.

Limitation: bacteria are prokaryotes and cannot add sugar groups or fold some large proteins correctly. Yeast, a eukaryote, can be used to make products such as the hepatitis B vaccine antigen.

Key termsfermenterprokaryote

Section 5

Drugs from GM plants and animals

GM animals: goats and sheep have been made to secrete human proteins, such as antithrombin, in their milk. The transgene is joined to a promoter that works only in mammary gland cells. The drug is harvested without killing the animal and the gene is inherited, so a herd can be bred. Eukaryotic cells process the protein correctly. Drawbacks: slow, costly and ethical concerns.

GM plants: crops such as tobacco and maize have been modified to make human antibodies or vaccine antigens. They are cheap to grow on a large scale and are not contaminated by human pathogens, but some modifications may differ from human ones.

Key termstransgenic animalantibody

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Exam questions on Genetically modified organisms and recombinant DNA

  1. A biotechnology company makes human insulin using the bacterium Escherichia coli. The insulin gene is cut from human DNA using a restriction endonuclease. The same enzyme is used to cut bacterial plasmids. The fragments are mixed and joined, and the resulting recombinant plasmids are put into bacteria that are then grown in large fermenters.
    Suggest two advantages of using genetically modified bacteria to make insulin rather than extracting insulin from pig pancreases.2 marks
  2. A research group produces goats that secrete human antithrombin (a protein that prevents unwanted blood clotting) in their milk. The antithrombin gene is joined to a promoter that is active only in mammary gland cells, and the recombinant DNA is injected into fertilised goat eggs, which are then implanted into surrogate mothers.
    Suggest two advantages of making antithrombin in the milk of GM goats.2 marks
  3. Researchers want a tobacco plant to make a human antibody for use as a drug. They cut the antibody gene from human DNA and the T-DNA region of the Ti plasmid of Agrobacterium tumefaciens with the same restriction endonuclease. The T-DNA also carries a gene for resistance to the antibiotic kanamycin. Leaf discs are infected with the recombinant Agrobacterium and grown on a medium containing kanamycin, and plants are regenerated from the discs that survive.
    Describe how the antibody gene is inserted into the Ti plasmid.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).