RNA synthesis, also known as transcription, is the process by which living organisms produce RNA from DNA. This process is necessary, as RNA has a central role in protein synthesis, a vital life function. RNA synthesis occurs in a series of steps that temporarily unzip a strand of DNA so that transcription can take place. When the process is complete, the new strand of RNA detaches, ready to begin protein synthesis. Due to the different cell structures of prokaryotic and eukaryotic organisms, the process of RNA synthesis differs slightly between them.
DNA contains the genetic instructions for protein synthesis. Yet if DNA was directly involved in this process, genetic damage might occur, causing harm to the organism. Though genetically unique from DNA, RNA acts as a disposable copy of these genetic instructions that cells use for protein synthesis. As proteins are necessary to build new cells, RNA synthesis is a continuous process.
The process begins with RNA polymerase, an enzyme that unzips DNA's double helix and starts RNA synthesis. The unzipping process breaks apart DNA's paired nucleotides. RNA polymerase attracts the base pairs for the exposed nucleotides and begins constructing a single strand of RNA. Though in many ways identical to DNA, the RNA under construction has defining differences.
Unlike DNA, RNA is a single strand of genetic material without any matching base pairs. This feature is necessary so RNA can be "read" during protein synthesis. The other main difference is that instead of including the base pair thymine, RNA polymerase attaches uracil in thymine's place. This substitution allows the cell to tell RNA and DNA apart from one another.
When RNA polymerase reaches the area of DNA known as a terminator region, RNA synthesis stops. The new strand of RNA, known as messenger RNA (mRNA), detaches from RNA polymerase and leaves the cell nucleus. The mRNA migrates to the mitochondria and ribosomes. Together with ribosomal RNA (rRNA) and transfer RNA (tRNA), mRNA work together to produce proteins, a process known as translation.
Though RNA synthesis occurs in all living organisms, the process is somewhat different in prokaryotes, single-celled organisms that lack a cell nucleus. Bacteria and archaea are prokaryotes, whereas all other living organisms are eukaryotes. As there is no nucleus in prokaryotes, RNA synthesis occurs in the cell's cytoplasm alongside the process of protein synthesis. As these two processes happen in the same space, prokaryotes are the only organisms on earth where transcription and translation happen simultaneously. As a cell nucleus separates the locations of transcription and translation in eukaryotes, there is always a brief delay between transcription and translation.