Explain the molecular diagnosis methods for the diagnosis of diseases.

(N/A) For effective treatment of a disease,early diagnosis and understanding its pathophysiology are very important.
Recombinant $DNA$ technology,$PCR$ (Polymerase Chain Reaction),and $ELISA$ (Enzyme-Linked Immunosorbent Assay) are some of the techniques that serve the purpose of early diagnosis.
Presence of a pathogen (bacteria,viruses,etc.) is normally suspected only when the pathogen has produced disease symptoms.
However,a very low concentration of bacteria or viruses can be detected by the amplification of their nucleic acid by $PCR$.
$PCR$ is now routinely used to detect $HIV$ in suspected $AIDS$ patients. It is also used to detect mutations in genes in suspected cancer patients. It is a powerful technique to identify many other genetic disorders.
In another method,a single-stranded $DNA$ or $RNA$ tagged with a radioactive molecule (probe) is allowed to hybridize to its complementary $DNA$ in a clone of cells,followed by detection using autoradiography.
The clone having the mutated gene will not appear on the photographic film because the probe will not have complementarity with the mutated gene.
$ELISA$ is based on the principle of antigen-antibody interaction.
Infection by a pathogen can be detected by the presence of antigens (proteins,glycoproteins,etc.) or by detecting the antibodies synthesized against the pathogen.