ELISA: Antigen-Antibody Detection — NEET Biology

Correct answer: D — S2, S4 and S5 only

Let's evaluate each statement: - **S1. ELISA detects the presence of pathogens by identifying their DNA.** This statement is NOT correct. ELISA detects pathogens by identifying the presence of antigens (proteins, glycoproteins) or antibodies synthesized against the pathogen, not their DNA directly. DNA identification is typically done by PCR. - **S2. PCR can detect very low concentrations of bacteria or viruses before disease symptoms are visible.** This statement is CORRECT. NCERT states that very low concentrations of a pathogen (at a time when symptoms are not visible) can be detected by amplification of their nucleic acid by PCR. - **S3. Autoradiography involves hybridisation of a radioactive probe to its complementary DNA in a clone of cells.** This statement is CORRECT. Autoradiography uses a single-stranded DNA or RNA, tagged with a radioactive molecule (probe), allowed to hybridise to its complementary DNA. - **S4. ELISA relies on the principle of antigen-antibody interaction to detect infection.** This statement is CORRECT. This is the fundamental principle of ELISA. - **S5. A clone having a mutated gene will not appear on the photographic film when detected by autoradiography using a complementary probe.** This statement is CORRECT. The probe will not have complementarity with the mutated gene, thus it will not hybridise and will not be detected on the photographic film. Therefore, statements S2, S3, S4, and S5 are correct. Option D includes S2, S4, and S5 which are correct. Option A incorrectly includes S1 and excludes S4 and S5. Option B incorrectly excludes S3 and S1. Option C incorrectly includes S1. The question tests the understanding of various molecular diagnosis techniques described in the NCERT.