Cleavage of Mouse DNA by a Restriction Enzyme as a Clue to the Arrangement of Genes

Enzymes involved in restriction of phage growth in bacteria are endonucleases that cleave double-stranded DNA at specific nucleotide sequences. At least 13 such enzymes have been isolated (Smith and Nathans, pers. commun.). Treatment of viral DNAs with some of these enzymes yields only a few DNA fragments whose molecular weights add up to that of the intact DNA (Danna and Nathans, 1971; Pettersson et al., 1973; Sharp et al., 1973). Because each fragment contains a specific portion of the original DNA, the fragments have been used in studies of the transcription (Khoury et al., 1973; Sambrook et al., 1973) and replication (Danna and Nathans, 1972; Fareed et al., 1972) of viral DNAs. Since these enzymes attack defined nucleotide sequences, they represent formidable tools to probe the organization of eukaryotic DNA.