Cutting and Joining DNA Molecules
CUTTING DNA MOLECULES :
It is worth recalling that prior to 1970 there was simply no method available for cutting a duplex DNA \molecule into discrete fragments. DNA biochemistry was circumscribed by this impasse. It became apparent that the related phenomena of host-controlled restriction and modification might lead towards a solution to the problem when it was discovered that restriction involves specific endonucleases. The favourite organism of molecular biologists, E. coil K12, was the first to be studied in this regard, but turned out to be an unfortunate choice. Its endonuclease is perverse in the complexity of its behaviour. The breakthrough in 1970 came with the discovery in Hae,nophilus influenzae of an enzyme that behaves more simply. Present-day DNA technology is totally dependent upon our ability to cut DNA molecules at specific sites with restriction endonucleases. An account of host-controlled restriction and modification therefore forms the first part of this chapter.
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