Transcription is the process by which the enzyme RNA polymerase forms messenger RNA (mRNA) with a nitrogenous base sequence that is complementary to the sequence of the gene on a DNA strand.
With the exception of the gametes (sex cells), every cell in an organism that has a nucleus has a complete copy (two copies of each gene in diploid organisms) of the organism's genome, or genetic material. Despite having the same DNA, muscle cells, nerve cells, and skin cells each have a different form and a different function. Cells are different because of differences in their proteomes that result from differences in gene expression. A proteome is the proteins expressed by the genome of an organism or a cell throughout its life or at a specified time under certain environmental conditions.
The information encoded in deoxyribonucleic acid (DNA) is sufficient to instruct a cell, or an entire multicellular organism, to build each of its properly functioning parts. However, that information must be translated from genes, which are written in the four-letter language of DNA, into proteins that will facilitate the chemical reactions necessary to build an organism. The central dogma of biology is the concept that genetic information flows from DNA to RNA to proteins, through the processes of transcription and translation. The shortened form of the central dogma is simply "genes to RNA to proteins."
The first step is transcription—the formation of messenger RNA from the template DNA strand to be used to build proteins. Messenger RNA (mRNA) is an RNA molecule made from a DNA template that contains the complementary gene sequence—that is, the sequence opposite to the DNA strand. mRNA acts as a messenger that carries information about how to make a protein. In eukaryotes, transcription of mRNA takes place in the nucleus. RNA polymerase, an enzyme used in the formation of RNA, synthesizes the mRNA strand and binds to the DNA template at a precise sequence of nucleotides that gives RNA polymerase the instruction to begin transcription. The promoter is the sequence of DNA to which RNA polymerase binds to initiate the process of transcription. After binding, RNA polymerase unwinds the DNA and starts an mRNA transcript.
When unwound, the DNA strands have an asymmetric configuration, with one end, called the 5′—or five prime—end, containing a carbon attached to a phosphate group. The other end, called the 3′—or three prime—end, contains a carbon attached to a hydroxyl group. This asymmetry to the DNA strands is what gives DNA its directionality. The 5′ end of a nucleic acid strand is identified by matching the direction of the 5′ carbon sticking off of the sugar ring of each nucleotide. The carbons are numbered in clockwise order starting to the right of the oxygen on the sugar ring. Opposite to the 5′ end is the 3′ end. The RNA polymerase begins moving along one strand of DNA, building a strand of mRNA complementary to the DNA sequence, one nucleotide at a time. Each RNA nucleotide is added to the 3′ end of the growing RNA strand. For this reason, the DNA strand that is not transcribed, which is also complementary to the template, is called the coding strand. Synthesis of mRNA continues until RNA polymerase reaches a terminator sequence. A terminator is a stop signal on the DNA strand that halts synthesis of mRNA. Because the DNA strands are not changed by RNA synthesis, many transcripts can be synthesized serially, allowing for quick production of messages and therefore rapid synthesis of many copies of a protein.