One mammalian messenger RNA gets read into roughly 2,800 proteins
A gene does nothing on its own. Its instructions must be copied into RNA and, for most genes, translated into protein. The copying is surprisingly efficient: in mammals, each messenger RNA molecule is translated on average into about 2,800 protein molecules, a small production run from a single template.
Gene expression is the process that turns the information in a gene into a working product, either a protein or a functional RNA. The first step, transcription, is carried out by enzymes called RNA polymerases, which build an RNA strand one unit at a time by matching the DNA template, swapping the base thymine for uracil. Bacteria use a single kind of polymerase, while cells with a nucleus use three, each specialised for different classes of genes.
In eukaryotes the first RNA copy is a rough draft that needs editing. A protective chemical cap goes on one end, and a tail of about 200 adenine units goes on the other, both shielding the molecule from being chewed up. Most importantly, splicing removes stretches called introns and joins the remaining exons. Sometimes pieces are kept or left out selectively, so a single gene can yield several different transcripts and potentially several proteins. The nuclear membrane may make this elaborate editing possible, since it separates copying from translation; in bacteria the two happen at the same time.
Translation reads the finished messenger RNA three letters at a time. Each triplet, or codon, pairs with a matching anticodon on a transfer RNA carrying a specific amino acid, and the ribosome links those amino acids into a chain. Proteins meant for export or for cell membranes are flagged partway through, when a signal recognition particle steers the ribosome to a network of membranes called the endoplasmic reticulum.
Not every gene ends in a protein. Ribosomal and transfer RNAs are finished products themselves, trimmed and chemically modified by helper molecules, including about 150 kinds of small RNAs in the nucleolus. Controlling when and how much of each product appears lets cells adapt to changing conditions and underlies how they specialise into different tissues.
Source: Gene expression
