Probes

These are small nucleotide sequences useto to detect the presence of complementary sequences in nucelic acid samples. These are 15-30 base long this is achieved by permitting the probes to base pair with the sample nucleic acid and then identifying the samples that show base pairing with the probes. The detection of hybridzation is highly precise and extremely sensitive provided the probes are suitably labelled for an easy detection. The DNA and RNA both are used as a probes DNA probes are more convenient and preferable but double stranded DNA molecules can also be used RNA probes are ordinarily single stranded.
Preparation of probes Probes can be obtained by several ways some of the important ones are briefly described below.
(1) Highly purified mRNA can be used as a probe mRNA are naturally single stranded.
(2) Single strand RNA probes can be readily prepared by cloning the DNA seuence inserted into a special vector this vector has different and specific prokaryotic promoter on the two ends of the DNA insert.
(3) DNA segment isolated from the genome of an organism or cDNA molecules prepared by using mRNA can be cloned in E.coli different recombinant clones of this vector will yield probes complementary to one or the other strand of the cloned DNA segment.
(5) single strand cDNA probes can be prepared by limiting the copying of mRNA by reverse transcriptase to only one strand.

(6) PCR can be used to generate single stranded copies of DNA seuences this is called asymmetric PCR.