Gene Cloning: cDNA and Genomic Cloning and Analysis of Cloned DNA Sequences

In order to clone a DNA sequence that codes for a required gene product, the gene has to be removed from the organism and cloned in a vector molecule. There are two ways by which gene libraries are made, referred to as complementary (cDNA) and genomic DNA libraries. Cloning of DNA fragments obtained from cDNA and genomic DNA have been discussed in terms of cDNA and genomic DNA cloning. This chapter summarizes the principal steps involved in cDNA cloning. The most commonly used procedure for cloning cDNAs involves the addition of complementary homopolymeric tracts to double-stranded cDNA and to the plasmid vector. Genomic libraries and clones are necessary in addition to cDNA clones because cDNA clones are generated from processed mRNA, which lack introns and other sequences surrounding the gene. Immunological detection of clones synthesizing a foreign protein has also been successful where the inserted gene sequence is expressed.