Biology · Ch 10 — Biotechnology and Its Applications
Molecular Diagnosis
Molecular Diagnosis
Why Early Diagnosis Matters
For any disease to be treated effectively, two things are critical: catching it early and understanding its pathophysiology — how the disease alters normal body function. Conventional diagnostic methods like serum and urine analysis cannot detect a disease in its earliest stages. By the time a pathogen produces visible symptoms, its concentration in the body is already very high. This delay can be costly.
Modern molecular techniques have changed this. Recombinant DNA technology, Polymerase Chain Reaction (PCR), and Enzyme Linked Immuno-sorbent Assay (ELISA) now make early diagnosis possible.
PCR — Amplifying the Invisible
PCR can detect a bacterium or virus even when its concentration is extremely low — before any symptom appears. How? It amplifies the pathogen's nucleic acid (DNA or RNA) millions of times, making it detectable. This is why PCR is now routinely used to detect HIV in suspected AIDS patients. It is also used to detect mutations in genes in suspected cancer patients, and to identify many other genetic disorders.
PCR works because it can copy a specific DNA sequence exponentially. Even a single molecule of pathogen DNA can be amplified enough to be seen on a gel.
DNA Probes and Autoradiography
Another powerful method uses a probe — a single-stranded DNA or RNA molecule tagged with a radioactive label. This probe is allowed to hybridise (bind) to its complementary DNA in a clone of cells. Detection is done by autoradiography, where a photographic film is exposed to the radioactive signal.
If the gene in the cell is mutated, the probe will not find its complementary sequence, and that clone will not appear on the film. This allows scientists to identify cells carrying a mutated gene.