Molecular Diagnosis ( Diagnosis of Disease )

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Molecular Diagnosis ( Diagnosis of Disease )

• It is well known that an early diagnosis is very important for the effective treatment of the disease.
• Using conventional methods of diagnosis (serum and urine analysis, etc.) early detection is not possible.
• Recombinant DNA technology, Polymerase Chain Reaction (PCR) and Enzyme Linked Immunosorbent Assay (ELISA) are some of the techniques that serve the purpose of early diagnosis.
• The molecular probes are usually single stranded pieces of DNAs (sometimes RNAs ) labelled with radio isotopes such as 32p.
• Molecular probes are available for many genetic disorders such as Duchenne muscular dystrophy cystic fibrosis, Tay - Sachs disease.
• The analytical techniques used for the identification of a specific DNA, an RNA or a protein from thousands of each are collectively called blotting techniques.
• In Southern blotting extraction of DNA from the cells (say leucocytes) occurs.
• Latter on labelled DNA hybrid complexes are formed which can be identified on exposure to X - ray film.
• In North ern blotting RNA is identified by labelled DNA or RNA probe. In Western blotting, protein is identified with the help of labelled antibody probe.
• The radioactively labelled DNA probes are formed .
• Presence of a pathogen (bacteria, viruses, etc. ) is usually suspected only when the pathogen has produced a diseased symptom.
• By this time the number of pathogens is already very high in the body, but very low count of a bacteria or virus (when the symptoms of the disease are not yet visible) can be detected by multiplication of their nucleic acid by PCR.
• PCR can detect very low amounts of DNA.
• PCR is now usually used to detect HIV in suspected AIDS patients.
• It is also used to detect mutations in genes in suspected cancer patients.
• It is a good technique to identify many other genetic disorders.
• A single stranded DNA or RNA joined with a radioactive molecule (probe) is allowed to hybridize to its complementary DNA in a clone of cells.
• It is followed by detection using autoradiography.
• The clone having the mutated gene will not appear on the photographic film, because the probe will not have the complementarity with the mutated gene.
• ELISA is based on the principle of antigen - antibody interaction . It can detect very small amount of protein ( antibody or antigen ) with the help of enzyme ( e.g .. peroxidase or alkaline phosphatase).
• Infection by pathogen can be detected by the presence of antigens such as proteins, glycoproteins, etc. or by detecting the antibodies synthesised against the pathogen.

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