Alkaline Lysis
Alkaline lysis is the method of choice for isolating circular plasmid DNA, or even RNA, from bacterial cells. It is probably one of the most generally useful techniques as is a fast, reliable and relatively clean way to obtain DNA from cells. If necessary, DNA from an alkaline lysis prep can be further purified.
Alkaline lysis depends on a unique property of plasmid DNA. It is able to rapidly anneal following denaturation. This is what allows the plasmid DNA to be separated from the bacterial chromosome.
Typically, you will grow up E coli cells that contain the plasmid you want to isolate, then you will lyse the cells with alkali and extract the plasmid DNA. The cell debris is precipitated using SDS and potassium acetate. This is spun down, and the pellet is removed. Isopropanol is then used to precipitate the DNA from the supernatant, the supernatant is removed, and the DNA is resuspended in buffer (often TE). A mini prep usually yields 5-10 ug. This can be scaled up to a midi prep or a maxi prep, which will yield much larger amounts of DNA (or RNA).
Specific protocols for alkaline lysis differ widely from lab to lab, and even from scientist to scientist. The basic principles behind the procedure, however, are fairly uniform. Here they are: