Q.What is meant by using transgenic animals as 'bioreactors'? Give an example of a pharmaceutically useful protein produced in the milk of a transgenic animal.

[!TLDR] Using transgenic animals as 'bioreactors' means engineering them to secrete a valuable therapeutic protein in their milk; example: a therapeutic blood-clotting factor produced in transgenic sheep/cattle milk. …

The term 'bioreactor', in an industrial or laboratory sense, normally refers to a controlled vessel or system used to grow living cells or organisms in order to produce a desired biological product. When applied to a transgenic animal, the same underlying idea is used, but the animal's own body -- specifically, its mammary gland and milk-production system -- serves as the production system instead of a laboratory fermenter.

This is achieved by genetically engineering the animal to carry the gene for a desired, valuable protein (most often a human therapeutic protein) linked to specific genetic control (regulatory) sequences that direct that gene to be switched on, and its protein product to be expressed and secreted, specifically within the animal's mammary gland tissue during milk production, rather than throughout the animal's whole body. As a result, the transgenic animal produces significant quantities of the desired protein directly within its own milk, which can then be collected and the target protein extracted and purified from it through standard purification techniques -- effectively using the animal's normal milk-production process as a large-scale, ongoing production system for the protein. …

Explain the 'bioreactor' analogy explicitly (the animal's body substituting for a laboratory production vessel) …

Do not say the protein is produced throughout the whole animal's body indiscriminately -- the defining feature of this approach is that expression is deliberately DIRECTED specifi …