gene_reg - Gene Regulation Intro What causes gene products...

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Gene Regulation
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Intro What causes gene products to be synthesized in some cells under some conditions, but not in others? A large part of research in molecular biology in aimed at trying to determine this. We are going to talk about a few basic systems, but a whole course could easily be devoted to the subject. For most genes, the essential regulation point is transcription: whether the gene is transcribed or not. Regulation also occurs at other points: availability of the DNA to be transcribed at all, whether the mRNA is translated, stability of the mRNA, how quickly the protein is degraded, etc.
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Gene Regulation in Prokaryotes The first system of gene regulation that was understood was the lac operon in E. coli, worked out by Francois Jacob and Jacques Monod in 1962. Many other prokaryotic genes are regulated in a similar fashion, and the basic principles carry over into eukaryotes. The lac operon codes for enzymes involved in the degradation of lactose. Lactose is a disaccharide that can be used as food in the absence of glucose. A lac- mutant is a chemoauxotroph that can’t use lactose.
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Structure of the lac Operon The lac operon consists of 3 protein-coding genes plus associated control regions. The 3 genes are called z, y, and a. lacZ codes for the enzyme beta-galactosidase, which splits lactose into glucose plus galactose. lacY codes for a “permease” protein that allows lactose to enter the cell, and lacA codes for an enzyme that acetylates lactose. Together these three genes are called the “structural genes”. We will mainly focus on lacZ. All 3 genes of the lac operon are transcribed on the same messenger RNA. Ribosomes translate the 3 proteins independently. This is a feature of prokaryotes that is only very rarely seen in eukaryotes, where 1 gene per mRNA is the rule.
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Control Regions Near the lac operon is another gene, called lacI, or just “i”. It codes for the lac repressor protein, which plays an essential role in lac operon control. The lac repressor gene is expressed “constitutively”, meaning that it is always on (but at a low level). It is a completely separate gene, producing a different mRNA than the lac operon. Just upstream from the transcription start point in the lac
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gene_reg - Gene Regulation Intro What causes gene products...

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