The complementary deoxyribonucleic acid (cDNA) is a DNA form of mRNA molecule synthesized by the enzyme reverse transcriptase. The cDNA is frequently used to clone the eukaryotic genes in prokaryotes. In eukaryotes, a poly (A) tail is used as primer site for reverse transcription, which differentiates the mRNA from tRNA and rRNA. Before creating cDNA, the purified mRNA is added to the enzyme reverse transcriptase, the four dNTPs, and a short primer of polymerized dTTP residue called an oiligo-dT primer.
The primer molecule will anneal to the poly (A) tail of the mRNA that is being copied. By utilizing the mRNA molecule as a template, reverse transcriptase enzyme catalyses the production of a single-stranded DNA molecule beginning from the oligo-dT primer. Then, the cDNA can be copied into a double-stranded cDNA by DNA polymerase.
The cDNA molecule cannot be synthesized by tRNA or rRNA molecules because the tRNA contains anticodons that functions to translate the amino acids in the correct order according to the DNA coding. And the ribosomal RNA (rRNA) is large macromolecular complexes that are the major components of ribosomes that gather the amino acids to form a protein. The rRNAs fold up by intra-molecular base pairing into stable secondary structures, which are only responsible for operating the important steps in protein synthesis.