A library can have as many as hundreds or thousands of cloned fragments, which are used to find the recombinant DNA molecule having the gene of interest to a investigator. Such selection is achieved by using a particular probe that will identify and indicate only the preferred clone.
a) A clone that contains DNA encoding the protein actin can be identified by creating a probe from the known amino acid sequence. On the other hand, the gene, for actin cloned in a different species should be used, as probe to identify the homologous gene in Drosophila. When an expression vector is used, it is feasible to find out a clone coding, for actin by screening with actin antibodies
b) A clone encoding specific tRNA is identified by:
The free RNA can be radioactively labeled and are used as probes. By hybridizing the specific tRNA molecule as a probe, the probe can detect a clone coding, for the probe.