Recombinant DNA: blue-white screening: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

Try it

Biochemistry · Recombinant DNA: blue-white screening

Sample item

A researcher ligates a gene into the multiple cloning site within the lacZ-alpha region of an ampicillin-resistance plasmid and transforms E. coli that cannot make lacZ-alpha. Cells are plated on ampicillin, X-gal and IPTG. Which colonies contain the recombinant plasmid?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

B. White colonies that grow on ampicillin

Ampicillin kills cells that did not take up any plasmid, so only transformants form colonies. If the insert disrupts lacZ-alpha, no functional beta-galactosidase is made and X-gal is not cleaved, so the colony stays white. Blue colonies carry self-ligated plasmid with an intact lacZ-alpha.

Option by option

  • A. Blue colonies that grow on ampicillin

    Blue colonies on ampicillin hold plasmid without an insert, as lacZ-alpha is working and cleaves X-gal.

  • B. White colonies that grow on ampicillin (correct)

    Ampicillin resistance shows plasmid uptake and a white colour shows an interrupted lacZ-alpha, meaning an insert.

  • C. White colonies that do not grow on ampicillin

    Cells that cannot grow on ampicillin carry no plasmid, so colonies of this kind do not form.

  • D. Blue colonies that do not grow on ampicillin

    Cells that lack the plasmid cannot grow on ampicillin and would not give blue colonies.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, White colonies that grow on ampicillin. Ampicillin kills cells that did not take up any plasmid, so only transformants form colonies.

Why is "Blue colonies that grow on ampicillin" not the answer?

Blue colonies on ampicillin hold plasmid without an insert, as lacZ-alpha is working and cleaves X-gal.

Why is "White colonies that do not grow on ampicillin" not the answer?

Cells that cannot grow on ampicillin carry no plasmid, so colonies of this kind do not form.

Why is "Blue colonies that do not grow on ampicillin" not the answer?

Cells that lack the plasmid cannot grow on ampicillin and would not give blue colonies.

What concept does this question test?

It tests Recombinant DNA: blue-white screening in Biochemistry. If the insert disrupts lacZ-alpha, no functional beta-galactosidase is made and X-gal is not cleaved, so the colony stays white. Blue colonies carry self-ligated plasmid with an intact lacZ-alpha.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Recombinant DNA: blue-white screening?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.