Genetics and Evolution NEET 2021: DNA Probe PYQ Answer

Genetics and Evolution NEET 2021: DNA Probe PYQ, what is the correct answer?

The Genetics and Evolution NEET 2021 DNA probe question has answer D. The mutated gene does not appear on the photographic film because the radiolabelled probe lacks complementarity with it. Autoradiography records radiation from a bound probe, not every DNA sequence present in the cells.

This NEET 2021 Biology question belongs to Genetics and Evolution, specifically DNA fingerprinting and autoradiography. It is a single-correct, hard-tier question with an expected solving time of 90 seconds.

A clone of cells is tested with a radiolabelled nucleic-acid probe designed to bind a complementary DNA sequence. The task is to decide how the cancer-causing mutated gene will appear after autoradiography.

The choices can be paraphrased as follows:

• No signal despite the probe having complementarity.
• A partial signal on the film.
• A complete and clear signal.
• No signal because probe-target complementarity is absent.

Step 1: What do the probe and autoradiography do separately?

The probe identifies a matching sequence through complementary base pairing, while autoradiography detects the radioactive label. These are separate functions. Radioactivity does not automatically reveal every gene, including an unlabelled mutated gene. You must first decide whether the labelled probe binds.

A probe is a radiolabelled single-stranded DNA or single-stranded RNA molecule, depending on the technique. It becomes associated with the target through hybridisation.

Hybridisation requires complementary base pairing between the probe and the target DNA. If the required complementarity is absent, the probe does not bind at that location.

Autoradiography detects or localises a radioactive isotope present in the biological sample. Here, it detects the radioactive probe that remains associated with the sample. It does not independently recognise whether a gene is normal or mutated.

Keep the roles separate:

• Probe: Finds a complementary nucleotide sequence.
• Hybridisation: Attaches the probe to that matching sequence.
• Autoradiography: Detects radioactivity from the probe that remains bound.

Step 2: Why does the mutated gene produce no film signal?

The probe is complementary to the corresponding non-mutated DNA sequence, but the mutated gene does not have the required complementarity with this probe. Without complementarity, the probe does not hybridise to the mutated gene. If no radiolabelled probe is bound at that location, autoradiography produces no corresponding appearance on the photographic film.

The complete causal sequence is:

1. The mutation changes the target DNA sequence.
2. The required complementarity with the probe is lost.
3. The probe cannot hybridise to the mutated gene.
4. No radiolabelled probe remains bound at that location.
5. There is no local radioactivity for the film to detect.
6. The mutated gene does not appear on the film.

Use this compact 90-second chain:

Mutation changes sequence → complementarity is lost → probe does not bind → no local radioactivity → no film signal.

Therefore, the mutated gene does not appear because the probe has no complementarity with it.

Final answer: D.

Why are the other three statements incorrect?

Only the statement combining absence on the film with absence of probe-target complementarity matches the official mechanism. The other statements either contradict hybridisation or assume a partial signal without support from the question.

• No appearance despite complementarity: This is internally inconsistent here. Successful complementary binding would retain the radioactive probe at the target and permit detection.
• Partial appearance: The question provides no basis for partial hybridisation or a weak signal. Do not infer an intermediate result merely because the gene is mutated.
• Complete and clear appearance: This would require successful binding of the radioactive probe to the target sequence. The mutated gene lacks the required complementarity.
• No appearance because complementarity is absent: This correctly links absent complementarity, failed hybridisation and no radioactive signal. It is option D.

Why is treating autoradiography as a gene photograph wrong?

The incorrect method starts with this thought: the gene is present in the cells, so it must appear on the film. That skips the required link between the DNA and the radioactive probe. Photographic film responds to radiation from the label, not to unlabelled DNA simply being present.

A radioactive probe gives a signal only where it has hybridised and remained bound. Use this order:

1. Check probe complementarity.
2. Decide whether hybridisation occurs.
3. Predict the autoradiographic signal.

The distinction is precise:

• Wrong: Mutated DNA is present, so the film must show it.
• Correct: The film shows radioactivity only where the labelled probe remains bound.

Do not infer a partial band merely because the target is described as mutated. A mutation does not automatically mean partial complementarity in the logic of this question.

Which related Genetics and Evolution questions test the same principle?

The same probe-first logic predicts all three results below. Full complementarity produces binding and a signal. Loss of the complementary sequence prevents binding and removes the signal. Autoradiography directly detects the radioactive label associated with a bound probe.

1. A radiolabelled single-stranded probe is fully complementary to a target DNA segment. After hybridisation and autoradiography, what is expected?

Answer: A signal appears at the target location because the probe binds there and its radioactivity is detected.

2. A mutation removes the sequence complementary to a particular probe. What happens to the autoradiographic signal at that locus?

Answer: The signal is absent because the probe cannot hybridise at that locus.

3. What does autoradiography directly detect in this experiment, the DNA sequence itself or the radioactive label?

Answer: It detects the radioactive label associated with the bound probe, not the unlabelled DNA sequence itself.

For another worked molecular genetics PYQ, read DNA Replication NEET 2022: Why the Answer Is 60 Cells. Before marking any autoradiography option, write the chain in four words: complementarity, binding, radioactivity, signal.