DNA molecules to a host cell for amplification is achieved in a process known astransform-
ation. Observations in the late 1920s, by Fred Griffith and later by Oswald Avery in the
early 1940s, suggested that bacteria could undergo rare natural transformation events.
The frequency of these events increased when bacterial cells were treated with cold
calcium chloride, which enhanced their competence, prior to a brief heatshock treatment
at 42 8 C. Alternative electroporation approaches are now commonly used for transformation.
These yield higher transformation frequencies and allow bacterial artificial chromosomes
(BACs), too large for conventional transformation, to be taken up successfully by bacterial
cells (Sheng et al. 1995). This general procedure formed the basis of clonal propagation, or
amplification, of DNA and initiated the development of DNA cloning vectors.
Figure 7.2.The joining of two linear DNA fragments, catalyzed by DNA ligase, creating phospho-
diester bonds between the 3^0 hydroxyl of one nucleotide and the 5^0 phosphate of another.
162 RECOMBINANT DNA, VECTOR DESIGN, AND CONSTRUCTION