(b) : Gene bank or genomic library is a complete collection of cloned $DNA$ fragments which comprises the entire genome of an organism. Molecular probes are small $DNA$ segments that are used to detect the presence of complementary sequences in nucleic acid samples in genomic library. These are usually formed of $200-­500$ nucleotide sequences. These segments or probes are labelled either with radioactive or with nonradioactive compound. Probes with $DNA$ sequence complementary to the gene to be isolated are used. They bind with the desired gene, making it visible and help in isolating it from the library.