Q.Discuss briefly how a probe is used in molecular diagnostics.

How a Probe is Used in Molecular Diagnostics

A probe is a short, single-stranded DNA or RNA molecule that carries a radioactive or fluorescent label. Its purpose is to detect the presence of a complementary sequence in a sample, making it a powerful tool for identifying pathogens or genetic abnormalities.

The technique relies on hybridization — the probe binds specifically to its complementary nucleotide sequence if present in the sample DNA or RNA. Because the probe is labeled, this binding event becomes visible through autoradiography (for radioactive labels) or fluorescence detection. If the probe finds its match, a signal appears; if not, no signal is produced.

In molecular diagnostics, probes are used to:

• Detect infectious agents like bacteria or viruses by targeting their unique genetic sequences
• Identify mutations or variations in human genes linked to diseases …

Molecular diagnostics relies on the principle that every organism carries unique nucleic acid sequences—a genetic fingerprint. The challenge is to find and identify these sequences in a complex mixture of biological material. This is where a probe becomes indispensable.

A probe is essentially a short stretch of single-stranded DNA or RNA, typically synthesised in the laboratory, that has been designed to match a specific target sequence. The key to its function lies in complementary base pairing: adenine pairs with thymine (or uracil in RNA), and guanine pairs with cytosine. When the probe encounters its complementary sequence in a sample, it binds tightly to it—a process called hybridisation. This molecular handshake is highly specific; the probe will only stick to the exact sequence it was designed for, ignoring everything else.

The real power of a probe comes from the label attached to it. This label is usually a radioactive isotope, a fluorescent dye, or an enzyme that produces a coloured product. Without this label, hybridisation would occur invisibly. The label acts as a beacon, making the probe-target complex detectable. Once the probe has bound to its target, the label emits a signal—radioactivity that exposes photographic film, fluorescence visible under UV light, or a colour change—revealing the presence and often the location of the target sequence.

Method — 3-step working of a probe, applied to diagnostics

1. Design: make a short ssDNA/ssRNA sequence complementary to the pathogen's/mutant gene's own sequence, and label it.
2. Hybridise: mix the probe with the patient's sample; it binds ONLY if the matching sequence is present. …