These are similar to the vectors described above but in addition have the required signals located before and after the ‘foreign’ gene which direct the host cell to translate the product of transcription into a protein. It is sometimes a difficult, expensive or time-consuming procedure to analyse for product from the ‘foreign’ gene and so, in addition to the selector genes described above, there are frequently reporter genes to indicate whether or not the signals are ‘switched on’ allowing the ‘foreign’ DNA to be expressed.
There are many reasons which are difficult to predict, why even a perfectly constructed gene may not be functional, such as the consequence of the exact site of insertion in the genome; hence the need for inbuilt controls.
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