(ii) oligonucleotide labelling involves the use of short random oligonucleotides, which are used as primers for copying the probe DNA in presence of labelled deoxyribonucleotides;
(iii) riboprobe preparation involves synthesis of labelled RNA, using DNA probe as template, in presence of a labelled ribonucleotide.
After hybridization with labelled probe, hybrids are detected by autoradiography 32p has the advantage over other radioisotopes, since it has high specific activity. However, in general, radioisotopes have some disadvantages.
They are difficult to handle and expensive to dispose off. Detection by autoradiography, while sensitive, may take a long time if there are few counts in the hybrid. Furthermore, radioisotopes have a short half lifes (e.g. 32p has a half life of 14.3 days) and therefore experiments should be completed preferably within one half life.