Molecular techniques: RFLP in sickle cell disease: 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 · Molecular techniques: RFLP in sickle cell disease

Sample item

In prenatal diagnosis, fetal DNA is cut with the restriction enzyme MstII and probed for the beta-globin gene. The sickle mutation (GAG to GTG) destroys an MstII site. What is seen in a fetus homozygous for HbS?

ANo band, because the gene is deletedBA single larger fragment because the restriction site is lostCAdditional small fragments because a new site is createdDTwo small fragments identical to normal

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

The answer and the reasoning

B. A single larger fragment because the restriction site is lost

Restriction enzymes recognise short palindromic sequences. The sickle point mutation abolishes one MstII site, so fragments that would be cut remain joined and a longer fragment appears on the Southern blot. A normal fetus shows the smaller fragments, and a carrier shows both patterns. Today PCR-based methods are more usual.

Option by option

A. No band, because the gene is deleted
Sickle cell disease is caused by a point mutation, and the gene is present.
B. A single larger fragment because the restriction site is lost (correct)
Correct. Loss of a cutting site yields one longer fragment.
C. Additional small fragments because a new site is created
The mutation removes a site; it does not create a new one.
D. Two small fragments identical to normal
This pattern is that of HbA, in which the site is present and cutting occurs.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, A single larger fragment because the restriction site is lost. Restriction enzymes recognise short palindromic sequences.

Why is "No band, because the gene is deleted" not the answer?

Sickle cell disease is caused by a point mutation, and the gene is present.

Why is "Additional small fragments because a new site is created" not the answer?

The mutation removes a site; it does not create a new one.

Why is "Two small fragments identical to normal" not the answer?

This pattern is that of HbA, in which the site is present and cutting occurs.

What concept does this question test?

It tests Molecular techniques: RFLP in sickle cell disease in Biochemistry. The sickle point mutation abolishes one MstII site, so fragments that would be cut remain joined and a longer fragment appears on the Southern blot. A normal fetus shows the smaller fragments, and a carrier shows both patterns. Today PCR-based methods are more usual.

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 Molecular techniques: RFLP in sickle cell disease?

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.