Many procedures in molecular biology require an initial pure sample of DNA. These procedures include the polymerase chain reaction, sequencing, gene cloning, blotting, and DNA profiling. Purification of DNA involves removing it and other constituents from the cell, separating it from the various other cell constituents, and protecting it from degradation by cellular enzymes. Isolation procedures must also be gentle enough that the long DNA strands are not sheared by mechanical stress.

DNA can be isolated from almost any cellular source. White blood cells and cheek cells taken directly from humans are most commonly used for diagnostic purposes, but skin, hair follicles, semen, and other tissues can be used for forensic analysis. Cells grown in petri dishes or in suspension can also be used. The cells are isolated from any surrounding fluid (such as blood serum) by centrifuging them—spinning them at high speeds...