Phages, Plasmids, and Phagemids

DNA Libraries Contained in a Phage Can be Screened with Radioactive DNA, Where the Screening Process Does Not Depend on Transcription and Translation of the Gene of Interest 

The tool used for screening a DNA library contained in a virus is a radioactive DNA probe. This DNA probe is designed, by the researcher, so that its sequence closely or exactly matches the sequence expected in the gene of interest. The DNA probe consists of single-stranded DNA. It may be only 25 deox5mucleotides long, or it can be several himdred nucleotides long. 

The phage DNA library is analogous to a thousand little ferry boats, each transporting one automobile, where each automobile holds a single man. The collection of ferries carries thousands of automobiles, and thus thousands of different men. The goal is to detect and acquire a single ferry (containing the man of interest), and to discard the rmwanted ferries. Once the correct ferry boat has been found, it is child s play to open the door, to let the man out, and to study the man. 

After removal of the plasmid from the phage, the plasmid can be used in reaction mixtures used for DNA sequencing. The dideoxy technique for sequencing DNA is shown below. 

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