Operon (genetics)

An operon is a group of genes with a single promoter. A promoter is an area of genes that starts transcription, or genetic copying, within an organism's deoxyribonucleic acid, or DNA. The DNA uses information from genes to direct the replication of cells, determining which cells will have which functions. The genes in the operon are all related to instructing the cells for function or task; they therefore share a promoter so that they are copied together. Operons are part of the cellular regulatory system in bacteria and viruses. They allow the cell to produce proteins when needed and restrict their production when not needed, helping the cell to conserve its resources.

Background

Every living thing has cells as its basic units of structure. Each cell has a specific function. That function is encoded in genes, which provide instructions to the cells for how to develop. There are two ways this information is passed on: from one generation of an organism to another during reproduction or replication, and within the organism during processes known as transcription and translation.

For many years, scientists knew that cells shared information, but they did not understand how this happened. The early years of the 1960s were an important time for scientists working to understand genetics. Nearly one hundred years after German monk Gregor Mendel first documented how specific physical traits were inherited by pea plants, scientists were just beginning to understand how cells used DNA to transcribe the information needed to replicate cells and generate the materials needed for life. Two French scientists working with bacteria DNA went public with a key discovery regarding how information is passed within cells of an organism in the early years of the decade.

The scientists, Francois Jacob and Jacques Monod, were working with the bacterium Escherichia coli, or E. coli, when they discovered it had three genes that were specifically used to form the proteins needed to break down lactose. Lactose is a milk sugar made up of glucose and galactose. The three genes could turn the production of the proteins off and on, and they were all controlled from one central point, or promoter. This group of E. coli genes controlled by one promoter was the first operon to be identified.

Jacob and Monod published their findings in 1960 and 1961 in the journal Proceedings of the French Academy of Sciences. They named their discovery an operon, based on the French verb operer, which means "to effect" or "work on." The French scientists had identified a part of the system that regulates gene function for the first time; for this, they earned a share of the 1965 Nobel Prize in Physiology or Medicine.

Overview

Operons are part of the transcription process that tells cells when to make proteins. DNA contains the blueprint and instructions needed to build proteins. This information is carried in ribonucleic acid, or RNA, and is carried to the cells by messenger RNA, or mRNA. The mRNA carries the instructions to the cells, which then create the protein per the instructions.

This transcription process has three stages known as initiation, elongation, and termination. Operons are part of the initiation of the transcription process. The RNA interacts with the promoter to begin the transcription of the gene and transfer the instructions it contains to a new cell. In some cases, multiple genes are attached to a promoter. This is because the genes contain instructions that must be used together to form the correct protein, similar to the way a recipe has multiple ingredients and steps that must be used together for it to come out properly. Multiple genes attached to one promoter are what Jacob and Monod identified as an operon.

During the transcription process, the RNA opens up the instructions contained in the genes by unwinding their DNA. The RNA begins a process that codes the instructions in the genes into mRNA; this is the elongation step of the transcription. The mRNA continues until it reaches a signal in the instructions that indicates their end. At this point, the transcription process is terminated and the DNA rewinds into its customary double helix configuration.

In the case of operons, the coding that is provided often triggers the formation of enzymes that initiate the formation of an amino acid. Amino acids are essential to the biosynthesis of many substances used by living organisms, including proteins, hormones, and enzymes. However, the promoter that is associated with an operon can be regulated by other influences that tell the promoter when to activate the genes associated with it and when to prevent them from activating. For example, in the E. coli bacteria initially studied by Jacob and Monod, they observed that the operon that controlled the release of the proteins needed to digest lactose was only triggered when lactose was present. When lactose was not present, these proteins were not needed and the lactose operon, or lac operon, was not triggered. This means that the operon performs an important duty in regulating the function of genes.

While operons are important, they have not been found in all living organisms. They are present in bacteria, viruses, and some other single-cell organisms (those known as prokaryotic organisms). Few have been identified in multicell organisms, and none have been found in humans.

Scientists are unsure why operons are not found in more advanced organisms. One theory is that they help the simpler organisms eliminate some of the extraneous signals or "noise" received from their environment and conserve resources for primary functions. Researchers continue to investigate operons and their functions because they might provide insight into how to provoke cells to perform actions they normally do not do, something that could be useful in fighting diseases.

Bibliography

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"Genetics of Bacteria and Viruses." Kean University, www.kean.edu/~jfasick/docs/Cell%20Biology/chapt18‗lecture%20%5BCompatibility%20Mode%5D.pdf. Accessed 24 June 2017.

Holmes, Randall K., and Michael G. Jobling. "Genetics." Medical Microbiology, edited by Samuel Baron, University of Texas Medical Branch at Galveston, 1996.

"I Think I've Just Thought Up Something Important – Francois Jacob (1920–2013)." National Geographic, 21 Apr. 2013, phenomena.nationalgeographic.com/2013/04/21/i-think-ive-just-thought-up-something-important-francois-jacob-1920-2013/. Accessed 24 June 2017.

"Mystery of Operon Evolution Probed." Science Daily, 30 Aug. 2012, www.sciencedaily.com/releases/2012/08/120830173502.htm. Accessed 24 June 2017.

"The Nobel Prize in Physiology or Medicine 1965: François Jacob, André Lwoff, Jacques Monod: Award Ceremony Speech." Nobelprize.org,www.nobelprize.org/nobel‗prizes/medicine/laureates/1965/press.html. Accessed 24 June 2017.

Osbourn, Anne E., and Ben Field. "Operons." Cellular and Molecular Life Sciences, vol. 66, no. 23, Dec. 2009, pp. 3755–3775, www.ncbi.nlm.nih.gov/pmc/articles/PMC2776167/. Accessed 24 June 2017.

Price, Morgan N., et al. "The Life-Cycle of Operons." PLOS, 28 June 2006, journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.0020096. Accessed 24 June 2017.

Semeniuk, Ivan. "Jacques Monod: A Scientist Whose Revolution Is Still Unfolding." Globe and Mail, 27 Sept. 2013, www.theglobeandmail.com/technology/science/jacques-monod-a-scientist-whose-revolution-is-still-unfolding/article14572219/. Accessed 24 June 2017.