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4.2_Gene_Expression_II(1) - Continuing with the theme of...

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Continuing with the theme of gene expression, we will examine the way that transcription and translation are regulated in cells. The basic question that will be addressed in this presentation is: How do cells only express the genes that are needed by that cell? 1
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We’ll look at a simple model of gene expression in prokaryotes such as bacteria and then examine a number of mechanisms that are currently being studied in eukaryotes. The concept of cell signaling which often leads to triggering gene expression will be introduced as well. By the way, this area of gene expression is a very hot area of research right now and is due, in part, to the newly developed, sophisticated biotechnology tools. 2
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3 The first example of gene regulation was discovered by Jacques Monod in the 50’s and 60’s. He was looking at how enzyme production was regulated in a common bacteria used in experiments called Escherichia coli or E. coli for short. He and other researchers developed the operon model of gene expression that is exemplified with the lac operon. The term operon refers to a segment of DNA that contains genes coding for enzymes and other DNA sections that serve to control the transcription of those genes. The term promoter is used to refer to a DNA sequence that RNA polymerase can attach to and begin the transcription. The term operator refers to a DNA sequence that a regulatory protein can bind to and by doing so influence whether the RNA polymerase can bind to the promoter or not. In the case of the lac operon, we are looking at how cells regulate the production of enzymes responsible for metabolizing lactose. Lactose, as we have studied previously, is a disaccharide made of glucose and galactose. Bacteria such as E. coli will produce enzymes to metabolize lactose only when the sugar is present; thus saving energy that would otherwise be wasted. In the absence of lactose, an active repressor protein binds to the operator, thus inhibiting the RNA polymerase binding to the promoter sequence. The repressor protein is constantly being produced by the expression of the regulatory gene. However, when lactose is present, it binds to the repressor and changes its shape and function. No longer does it bind to the DNA operator position and thus allows the transcription of the lactose enzymes. A messenger RNA with coding regions
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