Definition of "Complementary" in the DNA synthesis context: DNA is a helical double strand of complementary nucleotide base-pairs. There are only four different nucleotide bases that comprise DNA. These are adenine (A), thymine (T), guanine (G) and cytosine (C). A always chemically pairs with T, and T always pairs with A. Similarly, G always pairs with C, and C always pairs with G. This phenomenon is called complementary base-pairing.
Synthesis of complementary DNA (cDNA) is only accomplished in the laboratory (i.e., outside of the body). (1) The first step is the isolation of natural mRNA from the body. As described above, mRNA, although derived from DNA, does not contain introns (i.e., non-coding base regions) of DNA because the introns are removed during the natural synthesis of mRNA. For the purposes of cDNA synthesis in the laboratory, mRNA has an entirely different function than it does in the body. In the body mRNA serves as an intermediate in the process of building a protein. On the other hand, in the laboratory natural mRNA can be used as a template to build a strand of complementary nucleotides to the mRNA template. (2) The natural mRNA is added to a solution containing a very small string of nucleotides (oligo-dT shown here) known as a primer, which will combine with the mRNA to initiate a double-stranded molecule. (3) A solution of single nucleotide bases, with all four base types (i.e., A, T, C and G) included, is added to the mixture along with an enzyme called Reverse Transcriptase (RT). This enzyme facilitates the step-wise addition of the individual bases to the chain in a complementary fashion to the bases in the mRNA template. The result is a double-stranded molecule consisting of the mRNA template combined with a newly synthesized complementary strand. (4) Another enzyme, RNAse is added to the mixture to degrade the mRNA, leaving only the single strand of DNA complementary to the original mRNA. (5) Another enzyme, DNA polymerase, is added to again facilitate complementary base pairing, this time with the newly synthesized, but now single-stranded DNA. The result is a double-stranded cDNA molecule made up of strands synthesized entirely in the laboratory. Since cDNA is built entirely from mRNA, which does not contain introns (non-coding base regions), the cDNA necessarily does not contain introns. Natural DNA, always contains introns, because it replicates in the body from a DNA template, never an mRNA template. The take-away is that natural DNA always contains introns, and thus is chemically distinct from cDNA, which does not contain introns. Since the cDNA is a man-made molecule that is chemically distinct from natural DNA, it has been deemed by the US Supreme Court in Myriad to be patent eligible.