In genetics, complementary DNA (cDNA) is double-stranded 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).
Although 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, T, G and C) to their DNA complements (T, A, C and G, respectively).
Here I stand a broken man
Broken dreams slipped trough my hands
What once was is now gone
I can't go on, I am done
Last call
Last change to make things right
Pick up the pieces and mend my life
But how can I heal a broken trust
It feels so hard, it rips my guts