ABSTRACT
The resulting mix of different recombinant DNA molecules is used to transform bacteria to produce very many different clones-a genomic DNA library. In such cases the recombinant DNAs are expressed to give the desired protein but with a short peptide or protein tag attached that is known as an affinity tag because it is attached to assist purification of the recombinant protein by affinity chromatography. PCR uses reaction cycles consisting of consecutive steps that require different temperatures, but alternate methods use constant temperatures; the people describe isothermal amplification approaches to amplifying DNA in vitro at the end of this chapter. DNA sequencing technologies that use single unamplified DNA templates-sometimes called single-molecule sequencing or third-generation sequencing-avoid the biases introduced by PCR, and have the potential for producing very long sequences at low cost. Many alternative massively-parallel sequencing technologies are available or under development by a variety of companies.
