Final answer:
Library screening is employed to identify a gene expressed at high temperatures by isolating the gene of interest from a broad genomic library using methods like DNA hybridization and Southern Blotting for further study.
Step-by-step explanation:
The purpose of library screening to clone a gene expressed in an organism at high temperature is to identify and isolate the specific gene of interest from a broader genomic library. Initially, a 'shotgun' approach is taken where all genes from the organism's genome are cloned into the library. Following this, individual gene clones are identified using methods such as expression analysis of the gene product or DNA hybridization with a complementary probe to locate the desired gene. After isolating a positive clone (plaque), it undergoes purification, restriction enzyme digestion, and gel electrophoresis. Southern Blotting is then used to transfer DNA fragments onto a nylon filter, which is probed with a tagged probe to find the smallest fragment containing the gene. This fragment can be subcloned into a suitable vector for further study, such as expression analysis or establishment of causation for a trait or disease.