A much smaller DNA library—one that includes only the genes transcribed in a particular tissue at a particular point in time—can be made from complementary DNA, or cDNA (Figure 18.5B). Making cDNA involves isolating mRNA from cells, then making cDNA copies of that mRNA by complementary base pairing in a process called reverse transcription. Reverse transcription is catalyzed by the enzyme reverse transcriptase.

A collection of cDNAs from a particular tissue at a particular time in the life cycle of an organism is called a cDNA library. Because mRNAs do not last long in the cytoplasm, the types and amounts of mRNAs present in a cell are a good representation of the transcription rates of all the genes in that cell. So a cDNA library is a “snapshot” that captures the transcription pattern of a set of cells at a given point in time. cDNA libraries have been invaluable for comparing gene expression in different tissues at different stages of development. For example, researchers have found that up to one-third of all the genes of an animal are expressed only during development. cDNA is also a good starting point for cloning eukaryotic genes, because the clones contain only the coding sequences of the genes (the introns having been spliced out; see Figure 14.7). Also, if a eukaryotic gene is highly expressed in a particular tissue, a cDNA library made from that tissue will be enriched for the gene, making it easier to identify and clone the gene.

387

Reverse transcriptase can be used in *PCR procedures to create and amplify a specific cDNA sequence so that it is not necessary to construct a library. In this case, RNA is isolated from an organism or tissue, and reverse transcriptase is used to make cDNA from the RNA. Then PCR is used to amplify a specific sequence directly from the sample of cDNA. This procedure, called RT-PCR, has become an invaluable tool for studying the expression of particular genes in cells and organisms.