Question

In E.coli transformation why the color alteration and antibiotic resistance are the two types of selectable markers on the plasmids?

Answer 1

Some plasmid vectors have blue-white as well as antibiotic selection markers. The difference between blue-white selection and antibiotic selction is that, the antibiotic selection can be applied to plasmids with or without the gene of interest wheras the blue-white selection is specific for your DNA insert cloned into the plasmid vector.

During the ligation reaction where you add in the restriction digested insert and vector (or)  the T-vector and any PCR product, there is a possibility that some of the plasmid remains as is without any ligation of the insert DNA, some may self-ligate (in the case of single enzyme digest and blunt-end ligation). During bacterial transformation, the bacteria does not differentiate between plasmids with and without the insert, so when you use an antibiotic selection, not all the colonies would have plasmids with the insert.

In the case of blue-white screening, the MCS (Multiple cloning site) region of plasmids is located in the beta-galactosidase gene region. This means that when the ligation is succesful, the DNA insert would be inbetween the beta-galactosidase, thereby disrupting its expression. As a result the enzyme woud not be produced and so it would not act on the substrate (X-gal) and you get white colonies in the case of those transformants with the insert.

When the ligation is unsuccesful and the plasmid is self-ligated, there is no insert here in the beta-galactosidase region, the enzyme would thus be synthesized and it would acton X-gal that would give rise to blue colonies.

Blue-white screening helps you pick those colonies that harbor your gene of interest, saving additional screening time/reactions compared to that with just the antibiotic resistance marker.