Lesson 9 of 12 · 6 min
Molecular diagnosis
NCERT §10.2.3
At the hospital lab, a technician shows Kavya a PCR machine and a stack of plates coated with antibodies. 'Blood tests tell us late,' she says. 'These tell us early.'
The lesson in notes
In short
Effective treatment needs early diagnosis and an understanding of the disease's pathophysiology. Conventional methods such as serum and urine analysis cannot detect disease early.
Early diagnosis is possible with three techniques: recombinant DNA technology, PCR (Polymerase Chain Reaction) and ELISA (Enzyme Linked Immuno-sorbent Assay).
A pathogen is usually suspected only after it causes symptoms, by which time its concentration in the body is already very high.
PCR amplifies the nucleic acid of a bacterium or virus, so it can be detected at a very low concentration, before symptoms appear.
PCR is now a routine test for HIV when AIDS is suspected, and it finds gene mutations when cancer is suspected. It also identifies many other genetic disorders.
A probe is single-stranded DNA or RNA carrying a radioactive tag. The probe is left to pair (hybridise) with complementary DNA in a clone of cells and is then located by autoradiography.
A clone carrying a mutated gene does not show on the photographic film, because the probe is not complementary to the mutated gene.
ELISA works on antigen-antibody interaction. Infection is detected either from the pathogen's antigens (proteins, glycoproteins and so on) or from the antibodies made against the pathogen.