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In genetics, complementary DNA (cDNA) is DNA synthesized from a messenger RNA (mRNA) template in a reaction catalysed by the enzyme reverse transcriptase. cDNA is often used to clone eukaryotic genes in prokaryotes. When scientists want to express a specific protein in a cell that does not normally express that protein (i.e., heterologous expression), they will transfer the cDNA that codes for the protein to the recipient cell. cDNA is also produced naturally by retroviruses (such as HIV-1, HIV-2, Simian Immunodeficiency Virus, etc.) and then integrated into the host's genome where it creates a provirus. The term cDNA is also used, typically in a bioinformatics context, to refer to an mRNA transcript's sequence, expressed as DNA bases (GCAT) rather than RNA bases (GCAU).
According to the central dogma of molecular biology, when synthesizing a protein, a gene's DNA is transcribed into mRNA which is then translated into protein. One difference between eukaryotic and prokaryotic mRNA-coding genes is that eukaryotic genes can contain introns which are non coding sequences, in contrast with exons, sequences that code for mRNA. 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 constitutes 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. It is therefore 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).
Though there are several methods for doing so, cDNA is most often synthesized from mature (fully spliced) mRNA using the enzyme reverse transcriptase. This enzyme, which naturally occurs in retroviruses, operates on a single strand of mRNA, generating its complementary DNA based on the pairing of RNA base pairs (A, U, G and C) to their DNA complements (T, A, C and G respectively).
To obtain eukaryotic cDNA whose introns have been removed:
Complementary DNA is often used in gene cloning or as gene probes or in the creation of a cDNA library. When scientists transfer a gene from one cell into another cell in order to express the new genetic material as a protein in the recipient cell, the cDNA will be added to the recipient (rather than the entire gene), because the DNA for an entire gene may include DNA that does not code for the protein or that interrupts the coding sequence of the protein (e.g., introns). Partial sequences of cDNAs are often obtained as expressed sequence tags.
With amplification of DNA sequences via polymerase chain reaction (PCR) now commonplace, one will typically conduct reverse transcription as an initial step, followed by PCR to obtain an exact sequence of cDNA for intra-cellular expression. This is achieved by designing sequence-specific DNA primers that hybridize to the 5' and 3' ends of a cDNA region coding for a protein. Once amplified, the sequence can be cut at each end with nucleases and inserted into one of many small circular DNA sequences known as expression vectors. Such vectors allow for self-replication inside cells, and potentially integration in the host DNA. They typically also contain a strong promoter to drive transcription of the target cDNA into mRNA, which is then translated into protein.
On June 13, 2013, the United States Supreme Court ruled in Association for Molecular Pathology v. Myriad Genetics that while human genes cannot be patented, cDNA can be.
Some viruses also use cDNA to turn their viral RNA into mRNA (viral RNA → cDNA → mRNA). The mRNA is used to make viral proteins to take over the host cell.