Genetically Modified Organisms

A genetically modified organism (GMO) — also called a transgenic organism — is one whose genome has been deliberately altered by genetic engineering. OCR A-Level Biology A specification 6.1.3 (i) requires you to understand the applications of GM bacteria, plants and animals, to know named examples, and to weigh the ethical, economic and environmental arguments surrounding their use.

Key Definitions:

• GMO — an organism whose DNA has been altered by genetic engineering, typically by inserting a gene from another species.
• Transgenic — containing DNA from a different species.
• Cisgenic — genetically modified using genes from the same species or a sexually compatible relative.

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GM Bacteria

Bacteria were the first organisms to be genetically modified, and they remain the most widely used. They reproduce rapidly, are easy to transform, and secrete proteins directly into the growth medium.

Insulin

Before 1982, insulin for diabetics was extracted from pig or cow pancreases — a laborious process that sometimes caused immune reactions because animal insulin differs slightly from human insulin. Genentech scientists cloned human insulin cDNA into E. coli (Lesson 2), producing identical human insulin in unlimited quantities. Today virtually all insulin used worldwide is recombinant.

Other Recombinant Proteins

GM bacteria also produce:

• Human growth hormone (somatotropin) for children with pituitary deficiency.
• Factor VIII for haemophiliacs (replacing blood-derived product contaminated with HIV and hepatitis in the 1980s).
• Interferons for cancer and multiple sclerosis.
• Erythropoietin (EPO) for anaemia.
• Enzymes such as chymosin (rennet) for vegetarian cheese-making, and amylases for detergents.

The advantages over extraction from animals include unlimited supply, purity, absence of pathogens and — critically — the ability to produce human proteins that animal sources cannot provide.

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GM Plants

Plants are harder to modify than bacteria because they have rigid cell walls, and genes must reach the nucleus of totipotent cells that can regenerate into a whole plant. The standard method uses Agrobacterium tumefaciens, a soil bacterium that naturally inserts part of its Ti plasmid into plant cells (causing crown gall disease). Scientists replace the tumour-causing genes with a gene of interest, and Agrobacterium does the insertion work for them.

Alternatives include:

• Gene guns (biolistics) — gold or tungsten microparticles coated with DNA are fired into plant tissue.
• Electroporation of protoplasts — plant cells with their cell walls removed.
• Viral vectors.

Golden Rice

Golden Rice contains genes for the synthesis of β-carotene (a precursor of vitamin A). Two genes — psy from daffodil (later maize) and crtI from the bacterium Erwinia — redirect carotenoid biosynthesis so that the endosperm accumulates β-carotene, giving the rice its golden colour.

The aim was humanitarian: hundreds of millions of children in Asia suffer vitamin A deficiency, which causes blindness and increases mortality from measles and diarrhoea. A bowl of Golden Rice provides a significant fraction of the daily requirement. Despite being developed in 1999, regulatory and political opposition delayed its approval for cultivation until 2021 (Philippines).

Bt Cotton and Bt Maize

Bt crops carry a gene from Bacillus thuringiensis encoding a protein (Bt toxin) that is lethal to certain insect larvae but harmless to mammals. Bt cotton, maize and brinjal are planted on tens of millions of hectares worldwide. They reduce insecticide use — Indian Bt cotton farmers cut pesticide applications by 40–80% — but raise concerns about resistance evolution in target pests and the effects on non-target insects such as monarch butterflies.

Herbicide-Tolerant Crops