Insertion of Recombinant DNA into the Host Cell/Organism

The key idea is that a recombinant DNA molecule is useless inside a test tube — it must be delivered into a living host cell, where it can replicate and express its gene. But a cell membrane does not normally let foreign DNA pass through. So the first step is to make the recipient cells competent — that is, capable of taking up DNA from their surroundings.

Once the cells are competent, they can take up the ligated (recombinant) DNA that is present in the surrounding medium. This process is called transformation. After transformation, only those cells that have actually taken up the recombinant DNA will show the new trait — but how do we separate them from the millions of cells that did not take up any DNA?

This is where a selectable marker comes in. A selectable marker is a gene that gives the host cell a clear, easily detectable advantage — usually resistance to an antibiotic. For example, if the recombinant DNA carries a gene for resistance to the antibiotic ampicillin, then after transformation, any E. coli cell that has taken up this DNA becomes ampicillin-resistant. The untransformed cells remain sensitive to ampicillin.

Now comes the selection step. The transformed cells are spread on an agar plate that contains ampicillin. Only the transformed (ampicillin-resistant) cells will survive and grow into colonies. The untransformed cells, lacking the resistance gene, die. In this way, the ampicillin resistance gene acts as a selectable marker — it allows us to select only those host cells that have successfully taken up the recombinant DNA.

Summary of the process:

• Competent cells are prepared to take up DNA. …