This document discusses cDNA and genomic libraries. It defines cDNA as complementary DNA synthesized from mRNA, and genomic libraries as collections of DNA fragments representing an organism's entire genome. It describes how cDNA libraries are constructed by synthesizing cDNA from purified mRNA, and genomic libraries by partially digesting genomic DNA and inserting fragments into cloning vectors. It also explains how these libraries are screened using probes to identify clones containing genes of interest or encoding specific proteins.