Cloning of DNA

The cloning of DNA confers the ability to replicate and amplify individual pieces of genes. Cloning can be performed with genomic DNA or complementary DNA (cDNA). cDNA is synthesized artificially from mRNA in vitro with the aid of reverse transcriptase. Cloned genomic DNA may contain any stretch of DNA, either introns or exons, whereas cloned cDNA consists only of exons. For cloning (see Figure 2-5 for outline of the process), the desired pieces of DNA (often called "inserts") are connected with the DNA of genetically engineered vectors or plasmids, and the vectors are introduced into hosts, such as bacteria or mammalian cells. A DNA library is a collection of cloned restriction fragments of the DNA of an organism that consists of random pieces of genomic DNA (i.e., genomic library) or cDNA (i.e., cDNA library). Complete cDNA libraries contain all of the mRNA molecules expressed in a certain tissue. Sometimes cDNA libraries can be made from a specific tissue in a distinct circumstance. For example, a cDNA library could be made from cerebral cortex in rats undergoing transient forebrain ischemia (Abe et al. 1993).

FIGURE 2-5. Outline of gene cloning.

Transformation and selection ol clone containing desired gene

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