Building a cDNA library from mRNA. A cell’s total mRNA includes transcripts from thousands of genes, and the cDNAs generated from this mRNA are correspondingly heterogeneous. Reverse transcriptase can synthesize DNA on an RNA or DNA template. Eukaryotic mRNAs end with a long sequence of A residues (poly(A); see Chapter 15), and thus a poly(dT) oligonucleotide is used to prime synthesis of the first DNA strand. To prime the synthesis of a second DNA strand, oligonucleotides of known sequence are ligated to the 3′ end of the first strand, and the double-