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The study of Escherichia coli lactose operon laid the foundation of modern molecular biology. It contributed to the concept of genetic regulation, proposed by Jacob and Monod nearly 50 years ago. The operon structure, consisting of structural and regulatory genes has been elaborated and their regulatory response to small molecules, such as inducer, glucose and cyclic AMP (cAMP), have been elucidated. Gene regulation of the lactose operon led to the discovery of messenger ribonucleic acid (mRNA), to the identification of the Lac repressor and to the development of the theory of allostery (Ullmann, 2009). Bacteria are interesting when it comes to genetic to phenotypic flow. Since they are asexual, they must transfer genetic variation in other in other ways. They use the horizontal gene transfer which allows for DNA to pass through organisms without sexual reproduction and they have been doing this for millions of years. Lac operon, required for the transport and metabolism of lactose, is usually inactive and must be turned on. Being an inducible gene, it needs an inducer to turn it on. Two regulators turn the operon "on" and "off" in response to lactose and glucose levels: the lac repressor and catabolite activator protein (CAP) based on feedback regulation. The lac repressor acts as a lactose sensor. It normally blocks transcription of the operon but stops acting as a repressor when lactose is present. In this case, what turns it on are small amounts of lactose converted to allolactose. At night when we go to bed, there is no lactose to metabolize (Lac-). Lac repressor protein is active and binds to the lac operator site, thus blocking transcription, and in turn, conserving energy, and maintaining homeostasis (Bauman, 2018). In the morning, let's say I get up and drink a glass of milk; in doing so, I essentially turn on the process. Lactose being present (Lac+), small amounts of it convert to allolactose, which binds to the repressor causing a change in the repressor. The change alters the operator binding site; the repressor protein falls off the operator. Allolactose binds the repressor protein; transcription begins (Bauman, 2018). cAMP is present and able to bind to CAP, which can bind to the binding site, thus allowing RNA polymerase to bind to the promoter and transcribe a polycistronic mRNA containing the Lacz, Lacy, and Laca genes, which are all necessary for metabolism. An example of a repressible gene is the tryptophan operon. Unlike the lac operon, the tryptophan operon is always turned on unless levels are high, it is then repressed (Khan Academy, 2020). While you are sleeping (Lac+), to maintain homeostasis, the tryptophan operon is active, the RNA polymerase attaches to the promoter and begins transcription into mRNA. The mRNA then translates this into enzymes and tryptophan biosynthesis occurs. The next day when you consume that turkey bacon sandwich (Lac-), the Aporepressor and Tryptophan(corepressor) bind the repressor binding site ceasing transcription.

References Bauman, R. (2018). Microbiology with diseases by body system (5th ed., pp. 212-217). Prentice Hall. Gene Regulation, Khan Academy. (2020). Retrieved 10 November 2020, from https://www.khanacademy.org/science/biology/gene-regulation/gene-regulation-inbacteria/a/the-trp-operon?modal=1.

Ullmann, A. (2009, March 15). Wiley Online Library | Scientific research articles, journals, books, and reference works. Escherichia coli Lactose Operon - Ullmann - - Major Reference Works - Wiley Online Library. Retrieved November 11, 2020, from http://onlinelibrary.wiley.com/doi/full/10.1002/9780470015902.a0000849.pub2 Biologydictionary.net Editors. (2018, September 17). Gene Flow. Retrieved from https://biologydictionary.net/gene-flow/...