Reporter genes help select or identify host cells containing recombinant DNA

Even when a population of host cells is exposed to an appropriate vector, only a small proportion of the cells actually take up the vector. Furthermore, the process of making recombinant DNA is far from perfect. During a ligation reaction, DNA molecules can combine in various ways, many of which do not produce the desired recombinant molecule (see Figures 18.1 and 18.2). Methods have been developed to improve the chance that a desired combination will occur. A simple approach is to cut the vector with two different restriction enzymes that have sites near each other. This produces a molecule with incompatible sticky ends, which is much less likely to simply recircularize during the ligation reaction. The desired insert molecule is cut with the same two enzymes, so that theoretically a functional circular plasmid can be produced only if the vector and insert ligate with one another. Even so, it is often the case that only a small proportion of the ligation products have the desired recombinant sequence.

How can we identify or select the host cells that contain the desired sequence? One way is to use selectable markers, such as those for antibiotic resistance used in early experiments with recombinant DNA (see Figure 18.1). Selectable markers are one type of reporter gene, which is any gene whose expression is easily observed. Here are several types of reporter genes:

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