Question

What is called a ‘probe’?

Answer 1

Hint: A short sequence of nucleotide bases that will bind to specific regions of a target sequence of nucleotides. In a diagnostic test, the agent that is used to detect the presence of a molecule in the sample.

Complete answer:
A probe is defined as a single-stranded piece of DNA, labeled either with a radioisotope or with non- radioactive label, the nucleotide sequence which is complementary to the target DNA. Probes are labeled with radioisotopes, such as $^{32}p$ or tritium. These probes can be detected by the method of autoradiography, which involves placing the sample in direct contact with the photographic material, usually X-ray film. Alternatively, end-labeling probes with fluorescent tags can be used. The latter is visible under the UV- lamp. Probes can range in size from short as 10 nucleotide bases or a molecular weight of 3,300 to as long as 10,000 bases or more molecular weight of 3,300,000.

Additional Information: -The most common size ranges for most probes are between 14 and 40 bases. For statistical uniqueness, a minimum of 20 nucleotides bases is usually needed for a probe.
-The degree of homology between target sample and probe which results in stable hybridization.
-Short probes tend to hybridize nucleic acids at very high rates nearly in minutes whereas, longer probes may require reaction times of hours to achieve a stable hybridization.
-Long probes hybridized more stably than short probes at high temperatures and low salt concentration.

Note: -Probes get involved in the formation of heteroduplex with template DNA, and this is the key to the usefulness of molecular hybridization such as-
To search specific DNA sequences of the DNA library.
In southern and northern blot techniques, where probes are used to identify DNA or RNA fragments respectively
In the diagnosis of the genetic disorder, such as sickle cell anemia, thalassemia, cystic fibrosis, etc.