The extraction of DNA strands including a given sequence of bases is a crucial step in the Adleman-Lipton extract model of DNA computing. In this paper, a special type of PCR is presented with a related algorithm which performs a specified extraction from a given pool of DNA double stranded (shortly dsDNA) molecules. This kind of PCR, called Cross Pairing PCR (shortly XPCR) was tested in several situations, and used in a biotechnological procedure which implements the extraction algorithm.