To isolate a particular gene for study, a gene library is created. This is a collection of bacteria
that house different cloned DNA fragments, one of which is of interest. The library may be of the
entire genome, or of cDNA, which is free of introns.
If you want to find which bacteria in a library contain a specific gene, you need a probe for that
gene. This is a radioisotope-labeled piece of DNA that will base-pair with the gene of interest.
This nucleic acid hybridization technique can be used with other procedures to select cells and
their DNA, which may be of interest to the researcher.
To screen for genes:
First, grow the bacterial colonies on suitable medium in a petri plate.
Place a nylon filter over the colonies and lift some cells off.
Place the filter in a solution to disrupt the cells but leave DNA sticking to the filter.
Add a radioactively-labeled probe DNA to the filter where it will bind to the DNA fragments of
complementary sequence.
Expose the filter to x-ray film to locate the gene of interest, which will be in the same
location as the cells in the petri plate.
