alleles.
..... Click the link for more information. , site on a bacterial chromosome containing genes that control protein synthesis (structural genes) together with a gene that determines whether the structural genes are active or not (operator gene). See nucleic acid nucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. .
operon
Genetic regulatory system of single-celled organisms (prokaryotes) and their viruses, in which genes coding for functionally related proteins are clustered along the DNA, enabling their expression to be coordinated in response to the cell's needs. By providing a means to produce proteins only when and where they are required, the operon allows the cell to conserve energy. A typical operon consists of a group of structural genes that code for enzymes involved in a metabolic pathway, such as the biosynthesis of an amino acid. A single unit of messenger RNA is transcribed from the operon and is then translated into separate proteins. Operons are controlled by various regulatory elements that respond to environmental cues. The operon system was first proposed by Francois Jacob and Jacques Monod in the early 1960s.
operon [′äp·ə‚rän]
Operon
A group of distinct genes that are expressed and regulated as a unit. Each operon is a deoxyribonucleic acid (DNA) sequence that contains at least two regulatory sites, the promoter and the operator, and the structural genes that code for specific proteins (see illustration). The promoter (p) site is the location at which ribonucleic acid (RNA) polymerase binds to the operon. RNA polymerase moves down the operon catalyzing the synthesis of a messenger RNA (mRNA) molecule with a sequence that is complementary to DNA. This process is called transcription. The mRNA is used as a template by ribosomes to synthesize the proteins coded for by the structural genes (in the original DNA) in a process called translation. This mRNA is referred to as polycistronic because its sequence directs the synthesis of more than one protein. The operator (o) site is located between the p site and the beginning of the coding region for the first structural gene. It is at this site that molecules called repressors can bind to the DNA and block RNA polymerase from transcribing the DNA, thus shutting off the operon. Some systems can be derepressed by the addition of small molecules called effectors, which bind to the repressor protein and cause a conformational (shape) change that makes it no longer able to bind to the DNA at the operator site. See Deoxyribonucleic acid (DNA), Ribonucleic acid (RNA)
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Activation is believed to arise from the binding of a protein immediately adjacent to the promoter. The protein provides additional locations with which RNA polymerase can interact; the extra interactions result in an increased amount of polymerase binding to the promoter. Activators are more frequently involved in the regulation of genes in eukaryotes than in prokaryotes.
Once RNA polymerase begins transcribing a gene, it continues making RNA until a termination site is reached. Antiterminators are proteins that prevent termination at certain sites. In the presence of these antiterminators, RNA polymerase continues along the genome and transcribes the genes following the termination site until a different class of termination site is encountered.
Attenuation is the premature termination of the mRNA translation. Although the exact mechanism of attenuation has not been determined, it is thought that attenuation is due to the formation of a translation termination site in mRNA. See Gene, Gene action
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