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| Modern Genetic Analysis Chapter 10. Recombinant DNA Technology Cloning a Specific Gene Figure 10-9. The synthesis of double-stranded cDNA from mRNA. A short oligo(dT) chain is hybridized to the poly(A) tail of an mRNA strand. The oligo(dT) segment serves as a primer for the action of reverse transcriptase, which uses the mRNA as a template for the synthesis of a complementary DNA strand. The resulting cDNA ends in a hairpin loop. When the mRNA strand has been degraded by treatment with NaOH, the hairpin loop becomes a primer for DNA polymerase I, which completes the paired DNA strand. The loop is then cleaved by S1 nuclease (which acts only on the single-stranded loop) to produce a double-stranded cDNA molecule. (From J. D. Watson, J. Tooze, and D. T. Kurtz, Recombinant DNA: A Short Course. Copyright © 1983 by W. H. Freeman and Company.)

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