Written by Lauren Suarez
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The operon is a system within prokaryotes and bacteriophages that regulates groups of genes using a promoter and operator-repressor interactions (Russell 441-458). There are three basic components that all operons will have at least one of: a promoter, operator and a number of structural genes that code for specific proteins. The general structure of an operon is given by the figure below where P, is the promoter, O is the operator and A, B,C, and D represent structural genes.
The operon system can be compared to that of an assembly line system. Each part plays a specific role, and if one portion doesn't do it's job or stops, the final product is not made. The operon's purpose is to transcribe the genes that are present and then the products (proteins) that those genes code for can be made. The promoter is like the start button that starts the conveyor belt moving. For any gene to be transcribed, RNA polymerase is needed, without it; the rest of the operon is trivial because there is no transcription without RNA polymerase. The operator is similar to the person in charge of the assembly line. The operator decides based on the needs of the cell (or population) whether or not the genes (or goods) need to be produced. If they are not needed, then the operator stops the assembly line, just as in a cell if the products are not needed, transcription does not continue. The genes can be compared to the parts that are added to the system that help produce the final product. The genes are transcribed so that they can produce the required proteins to perform a task or supply the cell. (Russell 441-458)
The promoter, P, is at the beginning of the operon. A promoter is a sequence of DNA to which RNA polymerase can recognize and bind. Once RNA polymerase has bound to the DNA, transcription has been initiated and the operon DNA can begin translation so that the proteins the genes code for can be made. An operon can contain more than one promoter, and in such cases the promoters are not usually active at the same time. One promoter may be active if there is a particular substance present in the media, while the other promoter may only be active when this substance is absent.(Russell 441-458)
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The operator, O, is also called a controlling site. Its basic job is to be a site to which a repressor or in some cases an activator protein to bind to the operon. The state of the operator determines whether or not transcription will continue and if proteins will be synthesized. For example, an operator may contain a sequence of DNA that a protein in the media called a repressor will recognize. This repressor protein will then bind to the operon at the operator site and in doing so creates a blockage on the operon. This blocks the path of RNA polymerase and so transcription stops and the genes are not transcribed or transcription is very low. There are also cases where the binding of a protein to the operator does not result in genes not being transcribed but the presence of the protein actually enhances transcription and therefore expression of the genes (Russell 441-458). The operator can also regulate transcription through a process called attenuation. In this case, the operator is a place for transcription to begin and is also a place for a leader piece of DNA to be synthesized. This leader DNA is like a starter piece and will determine if transcription will continue further and if the genes will be expressed. (operon)
The operon can consist of any number of genes. However, in most cases, the genes on an operon are usually closely related to each other by their function or the product they produce. There are some genes within an operon that need to be expressed all of the time because there products are continuously needed. Since the products hey produce are necessary, these genes are referred to as constitutive gene (on a constitutive or always on operon). Genes whose products are not needed all the time are referred to as regulated genes, and their operons are controlled by the needs of the cell. There are also inducible genes. In this case, the presence of a particular substance requires that the operon be turned on and these inducible genes be expressed. The repressor proteins that were binding to the operator before now bind to the inducer (or substance present), and transcription is allowed to commence. (Russell 441-458)
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