Biology · Biotechnology: Principles and Processes · NEET
Insertional inactivation is a trick to find bacteria that took up your recombinant plasmid. When foreign DNA is inserted inside a marker gene (like the tetracycline-resistance gene of pBR322 at its BamHI site), that gene stops working, so recombinant cells lose that one resistance while non-recombinants keep it. Memory hook: "insert INSIDE the gene = gene DIES = recombinant marked." In the newer blue-white method, insert kills the beta-galactosidase gene, so recombinant colonies stay WHITE.
Insert at the BamHI site sits inside the tetR gene, breaking it: the recombinant plasmid still resists ampicillin but loses tetracycline resistance. In the blue-white method, the insert breaks the beta-galactosidase (lacZ) gene, so recombinant colonies stay white instead of turning blue.
Your doubts, answered
Why is the recombinant colony WHITE, not blue?
The foreign DNA is inserted right inside the beta-galactosidase gene. This breaks the gene, so the cell cannot make the enzyme. With no enzyme, the chromogenic substrate is not cut, so no blue colour forms and the colony stays white. Non-recombinant cells have an intact gene, make the enzyme, cut the substrate and turn blue. So: WHITE = has insert = recombinant.
Why does pBR322 lose tetracycline resistance but keep ampicillin resistance?
pBR322 has two resistance genes: ampR (ampicillin) and tetR (tetracycline). The BamHI restriction site lies INSIDE the tetR gene. When you cut at BamHI and insert foreign DNA there, only the tetR gene is broken. The ampR gene is untouched, so the cell still resists ampicillin but is now sensitive to tetracycline. Which resistance is lost depends only on where the enzyme cuts.
What is the difference between a selectable marker and insertional inactivation?
A selectable marker is the gene itself (an antibiotic-resistance gene) that lets you keep only transformed cells. Insertional inactivation is the METHOD: you deliberately insert foreign DNA inside a second marker so that gene stops working. The marker tells you the cell took up a plasmid; insertional inactivation tells you the plasmid actually carries your insert (is recombinant).
Why did scientists move from antibiotic selection to blue-white selection?
The antibiotic (insertional inactivation) method needs plating cells on TWO plates with two different antibiotics and comparing them. This is slow and cumbersome. The blue-white method reads recombinants directly on a single plate by colour (white vs blue), so it is faster and easier. NCERT calls the antibiotic-based two-plate method cumbersome for this reason.
If the BamHI site is inside the tetR gene, which antibiotic do I use to select?
You use ampicillin. The recombinant plasmid still has an intact ampR gene, so recombinant cells grow on ampicillin. You then use tetracycline as the second test: recombinants die on tetracycline (tetR is broken), non-recombinants survive. The one that lives on ampicillin but dies on tetracycline is your recombinant.
⚠️ The NEET trap ✗ Blue colonies contain the DNA insert and are the recombinants. ✓ White colonies are the recombinants. The insert breaks the beta-galactosidase gene, so no blue colour forms; blue colonies are non-recombinants with an intact gene. 🧠 Insert BREAKS the colour gene, so recombinant = colourless = WHITE. Blue = boring = no insert.
Real NEET questions
2026
Insertion of a foreign DNA at BamHI site in an E. coli cloning vector pBR322 results in the loss of antibiotic resistance towards:
A · Ampicillin and tetracycline
B · Ampicillin
C · Tetracycline ✓
D · Gentamycin
Solution: In pBR322 the BamHI site lies within the tetracycline-resistance (tetR) gene. Inserting foreign DNA there insertionally inactivates tetR, so recombinant plasmids lose tetracycline resistance while still keeping ampicillin resistance. Gentamycin is not a marker on pBR322.
2025
In the represented plasmid an alien piece of DNA is inserted at the EcoRI site (inside the beta-galactosidase gene). Which strategy will be chosen to select the recombinant colonies?
A · White colour colonies will be selected ✓
B · Blue colour colonies grown on ampicillin
C · Using ampicillin and tetracycline containing medium plate
D · Blue colour colonies will be selected
Solution: When alien DNA is inserted inside the beta-galactosidase coding sequence, insertional inactivation stops enzyme production. Recombinant colonies cannot cleave the chromogenic substrate, so they stay colourless (white). Non-recombinants turn blue. Hence recombinants are the white colonies.
2021
Plasmid pBR322 has a PstI restriction site within gene ampR (ampicillin resistance). If this enzyme is used to insert a gene, the recombinant plasmid:
A · will lead to lysis of host cell
B · will produce a novel protein with dual ability
C · will not be able to confer ampicillin resistance to the host cell ✓
D · the transformed cells will resist ampicillin as well as produce the product
Solution: The PstI site lies inside the ampR gene. Inserting foreign DNA there causes insertional inactivation of ampR, so the recombinant plasmid cannot confer ampicillin resistance. This mirrors the NCERT BamHI-in-tetR example, just with the ampicillin gene instead.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is inserting foreign DNA inside a marker gene so that the gene stops working. The dead gene acts as a signal: cells whose marker is now broken are the ones carrying your insert (recombinants).
Which gene is inactivated in pBR322 by BamHI?
The tetracycline-resistance gene (tetR). The BamHI recognition site lies inside tetR, so an insert there breaks it. The ampR (ampicillin) gene stays working.
In blue-white selection, are recombinants blue or white?
White. The insert breaks the beta-galactosidase gene so no blue pigment is made. Non-recombinant colonies keep the enzyme and turn blue.
What is the chromogenic substrate for?
It is a colourless chemical that beta-galactosidase cuts to make a blue colour. If the enzyme is present (non-recombinant), the colony turns blue; if the gene is broken by an insert (recombinant), it stays white.
Why is blue-white selection considered better than antibiotic selection?
Antibiotic-based insertional inactivation needs two plates with two different antibiotics, which is cumbersome. Blue-white selection identifies recombinants on a single plate just by colour, so it is quicker and simpler.
