In my biology textbook, it said that in transformation, the recombinant plasmid containing an antibiotic-resistance gene act as a 'selective marker'. That means that plasmid in transformed bacteria must have taken up the gene of interest and is able to grow in an antibiotic culture medium.

However, will there be a chance that the plasmid doesn't contain the gene of interest but only the resistance gene? How can those plasmids be identified and removed? And how can the resistance gene act as a 'selective marker'?

Thank you very much.

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    $\begingroup$ What research have you done to answer this question yourself? Have you even searched past questions on this site? This general topic comes up repeatedly. Please read the Help on asking a good question. $\endgroup$
    – David
    Dec 22, 2019 at 18:28

1 Answer 1


Suppose I pass out notes at your school. Some notes are coupons for a tanning salon. A few say "You are the chosen. Wear a green ribbon in your hair tomorrow. This note is your ticket to the awesome concert".

The next day you wonder who will be coming to the concert. You look for people with the green ribbon. That means they got the concert ticket.

Your plasmid has a gene of interest and a marker. The marker is antibiotic resistance. If you got it, then you fail to die when the antibiotics come. If you fail to die that means you have the gene of interest too. Antibiotic resistance is a marker for having got the plasmid, just like green ribbon is a marker for having got the concert ticket.

To get the ticket but not notification about the ribbon, or ribbon without ticket the note must have been torn or damaged. To get a plasmid with gene of interest but not antibiotic resistance means the plasmid must have been damaged. If the plasmids were good when you added them there is not much time for them to have been damaged by random events. It could definitely happen but odds of that happening increase with time, and in the short term odds are against a damaged plasmid or a torn note.


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