Assertion (A): A single-stranded DNA or RNA (probe) tagged with a radioactive molecule is used to detect mutated genes.
Reason (R): The probe will not hybridize with the mutated gene, and the clone with the mutated gene will not appear on the photographic film.
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Both A and R are true, and R is the correct explanation of A.
Assertion (A): A single-stranded DNA or RNA (probe) tagged with a radioactive molecule is used to detect mutated genes. — True. A probe is a single-stranded DNA or RNA tagged with a radioactive molecule. In molecular diagnosis, it is used to find out whether a clone of cells carries a mutated gene.
Reason (R): The probe will not hybridize with the mutated gene, and the clone with the mutated gene will not appear on the photographic film. — True. A probe binds (hybridises) only to DNA with a complementary sequence. A mutated gene has a changed sequence, so the probe does not bind to it. After autoradiography, that clone gives no signal on the photographic film.
Why R explains A? A says that a radioactive probe can detect mutated genes. R tells how this works: the probe sticks only to the normal, complementary sequence, so the clone with the mutated gene is the one that does not show up on the film. This missing signal is what reveals the mutation. So both statements are correct, and R is the correct explanation of A.
NCERT Extract:
Molecular Diagnosis – “… A single stranded DNA or RNA, tagged with a radioactive molecule (probe) is allowed to hybridise to its complementary DNA in a clone of cells followed by detection using autoradiography. The clone having the mutated gene will hence not appear on the photographic film, because the probe will not have complementarity with the mutated gene.”