Preparation of phage lambda DNA

The isolation of phage DNA from a purified phage preparation simply involves the removal of the proteins of the phage particle. This is done most easily by phenol extraction (Method A). The large amounts of high quality DNA needed for use as vectors or for markers for gel electrophoresis can be obtained in this way. However, sometimes (for example, when screening recombinant phages) only small amounts of phage DNA are required. The purity of these samples needs only to be sufficient for the restriction enzyme digests to provide distinct patterns on an agarose gel. Method B provides one way in which this can be done (1). A plate lysate is prepared and the released phage is subsequently lysed by the addition of SDS. The phage proteins and SDS are precipitated by the addition of potassium acetate to leave the phage DNA in solution. Finally, the DNA is recovered by ethanol precipitation.