Duplex DNA is isolated from bacteriophage lambda (cI857ind 1 Sam 7)

Therefore, those deoxyadenosine positions which are methylated in a wild type E. coli host are not methylated in the dam- host. This DNA gives complete digests with restriction endonucleases that are sensitive to dam methylation. Lambda DNA (N6-methyladenine-free) is 48,502 base pairs in length. The phage is isolated from a heat-inducible E. coli (dam-) lambda lysogen. The DNA is isolated from purified phage by phenol extraction and dialyzed against 10 mM Tris-HCl (pH 8.0) and 1 mM EDTA.

The bacteriophage is grown in an E. coli host which is deficient in adenine methylase (dam-).