Vectors constructed from bacteriophage X are about a thousand times more efficient than plasmid vectors in cloning large numbers of DNA fragments. For this reason, phage X vectors have been widely used to generate DNA libraries, comprehensive collections of DNA fragments representing the genome or expressed mRNAs of an organism. Two factors account for the greater efficiency of phage X as a cloning vector: infection of E. coli host cells by X virions occurs at about a thousandfold greater frequency than transformation by plasmids, and many more X clones than transformed colonies can be grown and detected on a single culture plate.
When a X virion infects an E. coli cell, it can undergo a cycle of lytic growth during which the phage DNA is replicated and assembled into more than 100 complete progeny phage, which are released when the infected cell lyses (see Figure 4-40). If a sample of X phage is placed on a lawn of E. coli growing on a petri plate, each virion will infect a single cell. The ensuing rounds of phage growth will give rise to a visible cleared region, called a plaque, where the cells have been lysed and phage particles released (see Figure 4-39).
(a) X Phage genome
^ Head Tail ^Replaceable region Lytic functions
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