A major goal of molecular cloning experiments is to learn the functions of the DNA sequences and the proteins they encode. The DNA fragments used in cloning procedures are obtained from several sources that we will describe in this section: random fragments of chromosomes that are maintained as gene libraries, complementary DNA obtained by reverse transcription from mRNA, products of the polymerase chain reaction (PCR), and artificially synthesized or mutated DNA.
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A cDNA library represents the RNAs made by a particular cell or tissue type at a particular point in time.
Reverse transcriptase can be used in PCR procedures to create and amplify a specific cDNA sequence such that construction of a library is not necessary.
Artificial DNA can be synthesized using organic chemistry.