Radioactive Probe and DNA Hybridisation for Diagnosis
Biology · Biotechnology and Its Applications · NEET
A radioactive probe is a short single-stranded piece of DNA (or RNA) that carries a radioactive label. In molecular diagnosis, this probe is allowed to base-pair (hybridise) with the DNA of a person's cells. If the gene is normal, the probe binds and shows a bright spot on a photographic film (autoradiography). If the gene is mutated, the probe cannot bind because there is no complementarity, so no spot appears. Memory hook: "Probe binds only its perfect match — no match, no spot, so the mutation shows up as silence."
A single-stranded radioactive probe hybridises only with the complementary normal gene, giving a dark spot on the film by autoradiography. A mutated gene lacks complementarity, so the probe does not bind and no spot appears — that blank marks the mutation.
Your doubts, answered
Why does the mutated gene NOT appear on the photographic film?
The probe is made complementary to the normal gene. In a mutated gene the base sequence is changed, so the probe has no complementarity and cannot hybridise (base-pair) there. With no probe bound, there is no radioactivity in that region, so no dark spot forms on the film. The absence of a spot is the signal that a mutation is present.
Is the radioactive probe single-stranded or double-stranded?
It is single-stranded. Only a single strand can pair with the complementary single strand of the patient's DNA. If the probe were double-stranded, its two strands would already be paired with each other and could not hybridise with the target DNA.
Does the probe stick to the normal gene or the mutated gene?
It sticks to the NORMAL gene, because the probe sequence is complementary to the normal sequence. So a bright spot means the normal gene is present; a missing spot points to the mutated gene. Students often flip this — remember the probe rewards the match, not the mutation.
What does 'hybridise' mean here in simple words?
Hybridise means two single strands of nucleic acid join by complementary base pairing (A with T, G with C). The probe strand hybridises to its matching strand in the sample DNA, forming a double strand only where the sequences match.
How is the bound probe actually seen?
By autoradiography. The probe carries a radioactive label. When the sample is pressed against a photographic film, the radiation exposes the film only where the probe has bound. This makes a visible dark spot marking the location of the matching (normal) DNA.
⚠️ The NEET trap ✗ The radioactive probe binds to the mutated gene and lights it up on the film. ✓ The probe binds only to the NORMAL (complementary) sequence. A mutated gene has no complementarity, so the probe does NOT bind and NO spot appears — the mutation is detected by the absence of signal. 🧠 Probe = matchmaker for the normal gene. No match, no glow. The mutation shows itself by staying dark.
Real NEET questions
NEET
Now a days it is possible to detect the mutated gene causing cancer by allowing radioactive probe to hybridise its complementary DNA in a clone of cells, followed by its detection using autoradiography because:
A · mutated gene does not appear on a photographic film as the probe has no complementarity with it ✓
B · mutated gene does not appear on photographic film as the probe has complementarity with it
C · mutated gene partially appears on a photographic film
D · mutated gene completely and clearly appears on a photographic film
Solution: The radioactive probe is designed to be complementary to the NORMAL gene sequence. Where the gene is mutated, the base sequence differs, so the probe has no complementarity and cannot hybridise (base-pair) there. With no probe bound, no radioactivity is recorded and that region does not appear on the autoradiograph. Option B reverses the logic; C and D wrongly assume the probe pairs with the mutation.
Solved Biotechnology and Its Applications NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Why is a radioactive probe used in molecular diagnosis?
To find a specific gene or mutation very early, even before symptoms appear. The probe pairs only with its matching DNA, and its radioactive label lets us see exactly where the match occurs on a film.
Can the probe be RNA instead of DNA?
Yes. A probe can be a single-stranded DNA or RNA sequence. What matters is that it is complementary to the target and carries a label (radioactive or fluorescent).
Is this the same as ELISA?
No. ELISA detects antigens or antibodies (proteins) of a pathogen using an enzyme reaction. A radioactive probe detects DNA/RNA sequences by hybridisation. Probe = nucleic acid based; ELISA = protein/antibody based.
Where is this used in NEET?
It is part of 'Molecular Diagnosis' in Biotechnology and Its Applications, alongside PCR and ELISA. NEET tests the logic that absence of a signal indicates a mutated (non-complementary) gene.
