Discuss briefly how a probe is used in molecular diagnostics.

Probe :

Probe is a short, single stranded DNA or RNA of approximately $15$ - $20$ nucleotides which are either radioactive or fluorescent labelled.

Application of a probe in diagnostics:

• Molecular diagnostics is done to identify a particular sequence of nucleotides in a mixture of nucleotides.
• First probe is allowed to hybridize with its complementary DNA in a clone of cells.
• Then the presence of the hybridized probe is detected by autoradiography.
• The cell with mutated gene will not be observed on the photographic film because the probe will not have complementarity with the mutated gene.
• Thus the mutation can be detected.
• The commonly used marker is ${}^3$${}^2$P which is incorporated into the phosphodiester bond in the probe DNA.