A Subtraction Library is a type of DNA library that contains cDNA clones from one tissue of an organism but not in another. For example, A subtraction library could have the genes that are expressed in the roots of a plant, but not in the stems, leaves, and flowers.
A subtraction library is prepared by combining cDNA generated from your tissue of interest with tagged cDNA from the tissues you want to have "subtracted" from the first. By heating up and cooling down this mixture, complimentary cDNA from your first tissue will hybridize with excess tagged cDNA. Then, the tags are used to selectively remove the hybridized cDNA molecules from the solution, and what are left are cDNAs made from genes expressed in your first tissue, but not the second. These cDNAs are transformed into cloning vectors as per the usual process to generate a DNA library.
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