Double-stranded cDNA is synthesized from mRNA

The formation of cDNA for the insulin gene. The insulin gene (with its two introns) is transcribed in the pancreas into pre-mRNA. The introns are removed by splicing, and A residues are added to the 3′ end to form polyadenylated mRNA. In the laboratory, mRNAs are isolated from pancreatic cells and a short oligo-dT primer is hybridized to the poly(A) tail of all mRNAs to prime synthesis of complementary DNA from the RNA template by reverse transcriptase. Reverse transcriptase synthesizes a stem-loop structure that acts as a primer for synthesis of the second cDNA strand, after the mRNA strand has been degraded (by treatment with NaOH or with RNAseH).