(Molecular biology) Of a gene; being converted into its functional product (either RNA or protein) and thus contributing to the phenotype (the physical structure and function) of an organism.

Genes in themselves are merely inactive stretches of DNA; they cannot actually do anything useful. The function of genes is to provide the information for the generation of useful, active molecules made of RNA or protein. The expression of a gene is the process by which the cell converts genetic information into chemical function.

Generally, the expression of a gene consists of four distinct stages. The first is transcription, in which an enzyme called RNA polymerase zips along one strand of an unwound DNA double helix, faithfully making an RNA copy of the gene on that strand. Some RNA molecules then proceeds to the second stage of gene expression, known as RNA splicing, in which different segments of the gene are chopped out or joined together to create a variety of alternative transcripts. Some types of RNA, such as transfer and ribosomal RNA, simply fold up on themselves to form useful molecules. The RNA encoded by most genes, however, is non-functional until it has been converted into protein.

This type of RNA, known as messenger RNA or mRNA, enters the third phase of gene expression called translation. In this phase the beginning of the mRNA molecule (known as the 5' end) is bound by a large molecular complex known as the ribosome. The ribosome reads along the mRNA molecule until it reaches the three-nucleotide sequence AUG, also known as the start codon. This codon is translated into the amino acid methionine, which becomes the first amino acid in the emerging protein. The ribosome continues reading along the coding region of the mRNA molecule, converting each codon into an amino acid according to the genetic code, until it reaches one of the three stop codons - UAA, UAG or UGA - at which point it dissociates from the mRNA and release the completed protein. The protein then folds into its native state, at which point (in many cases) it can perform its normal biological role.

Many proteins, however, undergo a fourth process known as post-translational modification. This is the covalent addition of various chemical groups, such as phosphate, methyl or sugars, to the folded protein. Some proteins such as glycogen phosphorylase (which breaks down glycogen into glucose molecules) will not function until they have been post-translationally modified (in this case, by the addition of a phosphate group, a process known as phosphorylation). Many proteins require post-translational modification in order to be transported to the correct part of the cell - in this case, the addition of sugar groups (a process known as glycosylation) plays a major role. Post-translational modification also helps to regulate protein degradation, with damaged or poorly-folded proteins being targeted for destruction by the covalent addition of a ubiquitin tag, which directs the proteins to the proteasome where they are chopped up into their constituent amino acids and recycled.

This simplistic account gives some idea of the complexity of the process by which a single gene generates its functional product. Imagine, then, the mind-boggling complexity of human development, during which more than 30,000 genes, encoding more than 100,000 alternative transcripts and unknown numbers of post-translationally modified proteins, interact with one another to create hundreds of distinct cell and tissue types organised together into a working, breathing human being from a single cell. The fact that any of us exist at all should be a cause for ceaseless wonder.