Central dogma states that biological information goes from DNA to RNA to protein (figure 1). However, there are times when information goes from RNA to DNA. Viruses such as HIV have RNA genomes that can be converted into DNA by an enzyme called reverse transcriptase. Molecular biologists realized that they could use reverse transcriptase to convert mRNA into complementary DNA and thus was born the term cDNA.
Figure 1. Central dogma: DNA to RNA to mRNA to protein. Coding sequence (purple) exons are spliced together and the 5' cap and 3' polyA tail is added to produce a mature mRNA molecule from the primary transcript. The mRNA is translated into protein.
