Complementary DNA - Overview

Overview

According to the central dogma of molecular biology, when synthesising a protein, a gene's DNA is transcribed into mRNA which is then translated into protein. One difference between eukaryotic and prokaryotic genes is that eukaryotic genes can contain introns (intervening DNA sequences) which are not coding sequences, in contrast with exons, which are DNA coding sequences. During transcription, all intron RNA is cut from the RNA primary transcript and the remaining pieces of the RNA primary transcript are spliced back together to become mRNA. The mRNA code is then translated into an amino acid chain (sequence) that comprises the newly made protein. Prokaryotic genes have no introns, thus their RNA is not subject to cutting and splicing.

Often it is desirable to make prokaryotic cells express eukaryotic genes. An approach one might consider is to add eukaryotic DNA directly into a prokaryotic cell, and let it make the protein. However, because eukaryotic DNA has introns, and prokaryotes lack the machinery for removing introns from transcribed RNA, to make this approach work, all intron sequences must be removed from eukaryotic DNA prior to transferring it into the host. This 'intron-free' DNA is constructed using 'intron-free' mRNA as a template. Thus it is a 'complementary' copy of the mRNA, and is thus called complementary DNA (cDNA). To obtain expression of the protein encoded by the cDNA, prokaryotic regulatory sequences would also be required (e.g. a promoter).