Q.A plasmid without a selectable marker was chosen as vector for cloning a gene. How does this affect the experiment?

A selectable marker in a plasmid vector allows researchers to distinguish transformed cells (those that have taken up the recombinant plasmid) from non-transformed cells. Without it, every bacterial colony that grows on the culture plate could be either a transformed cell carrying the recombinant DNA or simply a non-transformed cell that survived because it never needed to take up the plasmid in the first place.

The immediate consequence is that you cannot identify which colonies actually contain your gene of interest. Transformation efficiency is never 100%—most cells in the population remain untransformed. Normally, a selectable marker like an antibiotic resistance gene lets you grow cells on antibiotic-containing medium, so only transformed cells survive and form colonies. Without this selection pressure, both transformed and non-transformed cells grow indiscriminately. …

The entire logic of molecular cloning rests on a simple problem: when you mix plasmid DNA with bacterial cells and attempt transformation, only a tiny fraction of cells actually take up the plasmid. Most cells remain untransformed. Without a way to tell these two populations apart, you're searching for a needle in a haystack—except you can't see the needle.

Selectable markers exist precisely to solve this problem. They are genes on the plasmid that confer a trait the host cell didn't have before, most commonly antibiotic resistance. When you plate all the cells (transformed and untransformed alike) on a medium containing that antibiotic, only the cells carrying the plasmid survive. The untransformed cells, lacking resistance, die. What grows on your plate is therefore enriched—ideally composed entirely—of transformants.

Now imagine running the same experiment with a plasmid that lacks any selectable marker. You perform the transformation, mix your cells with the plasmid and the foreign gene you want to clone, and then plate them. Every cell grows, because there's no selective pressure. You have no phenotypic difference to exploit. The colonies on your plate are an unselected mix, the vast majority of which never took up the plasmid at all.

Method 2 -- quantify the screening burden with and without a marker

Put a rough number on the problem instead of describing it qualitatively. Bacterial transformation efficiency is typically low -- often well under 1% of cells in a transformation mixture actually take up a plasmid.

• With a selectable marker (e.g. an antibiotic-resistance gene): plate everything on medium containing the antibiotic. Every surviving colony, by construction, took up the plasmid -- you might screen 10-50 colonies to confirm a correct insert, an entirely manageable number. …