The next step is to convert the mRNA back into a DNA molecule in the test tube. This can be thought of simply as reversing what went on in the cell when the gene DNA was switched on and mRNA was made by base pairing.

This is a two-stage process. First, each mRNA is copied into a new DNA strand using base pairing to form a mRNA-DNA duplex. Next, the mRNA is chopped up and removed, and the DNA strand is used to make a second DNA strand. This double-stranded DNA is called complementary DNA or cDNA.

Thus, each cDNA in the test tube originally came from a specific mRNA in the cell.