Q.What is molecular diagnosis? Briefly explain how techniques like PCR and ELISA allow the early detection of diseases such as HIV or cancer, well before overt symptoms appear.

[!TLDR] Molecular diagnosis detects disease at a very early stage using PCR (amplifying trace pathogen DNA/RNA) or ELISA (detecting a disease-specific protein via antibody binding), before symptoms appear. …

Traditional diagnostic methods often rely on detecting either a fairly substantial quantity of a pathogen present in a patient's body, or a measurable, developed antibody response mounted by the patient's own immune system -- both of which may take time to build up to detectable levels after an infection or disease process has actually begun, meaning a disease might not be reliably detectable until well after it has already progressed and symptoms have started to appear.

Molecular diagnosis techniques based on recombinant DNA technology can detect disease far earlier because they are sensitive enough to identify even very small amounts of a pathogen's genetic material or a disease-associated protein. PCR is one such technique: because PCR can exponentially amplify even a single starting copy of a target DNA or RNA sequence into millions of detectable copies, it can reveal the presence of a pathogen -- for example, HIV -- in a patient's blood sample at a stage when the actual quantity of virus present is still far too low to be detected by older, less sensitive methods. …

Introduce the general problem (traditional methods need a larger, later-developing signal) before explainin …

Do not describe PCR and ELISA as detecting the same kind of target -- PCR detects genetic material (DNA/RNA), while ELISA detects a specific …