Chapter 13. Prokaryotic Regulation & Mutagens

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1 Chapter 13 Prokaryotic Regulation & Mutagens

2 How often do you buy new swimsuits or bags of ice in the winter? How about space heaters and sweaters in the summer? We buy what we need when we need it. We also might not buy a product without certain perks Efficiency (gas mileage) Opportunities (Groupon) Good deals (2-for-1. dollar menus) Gifts (Costco samples, raffles) Organisms are opportunistic. They only put energy into necessary or advantageous situations Regulation

3 In 1961, Francois Jacob and Jacques Monod discovered the first operon in a sample of E. coli An operon is a group of genes that work together as a single unit. Their function is to regulate when genes are active or not The operon includes the following elements: Structural gene: the sequence or sequences of DNA that code for enzymes (sequence to be transcribed) Promoter: A short sequence of DNA in front of the structural gene. RNA polymerase attaches here to begin transcription Operator: A short section of DNA in between the promoter and structural gene. When a repressor protein attaches to this site, RNA polymerase cannot attach to the promoter Prokaryotic Regulation

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5 Outside the operon is an additional gene called a regulator. The regulator sequence is the gene for transcribing the repressor enzyme that attaches to the operator. Jacob and Monod discovered that most of the time operons are in the on position. Why? In case the operon is broken, the failsafe is that the gene will be transcribed Is it better for your car to be permanently off, or on? Is it better to be paralyzed, or constantly in motion? Prokaryotic Regulation

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7 The trp operon codes for a sequence of enzymes that build the amino acid tryptophan If tryptophan is already present in the environment, the cell doesn t need to waste energy making more The repressor enzyme has a co-factor site that fits perfectly with tryptophan. When the co-factor site is activated, the enzyme is turned on and can bind with the operator, thus shutting down the promoter site. In short, the presence of tryptophan signals the cell to stop making tryptophan This is called a repressible operon because the presence of tryptophan represses (turns off) the operon The trp Operon

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9 Lactose is a dissacharide made up of a glucose and a galactose. When E. coli does not get glucose but does get lactose, the genes that metabolize lactose are turned on The three genes are controlled by the same operon and are adjacent on the DNA strand Thus, they are under control by the same promoter and operator Normally, this operon is turned off thanks to the presence of a repressor protein The lac Operon

10 Lactose is an allosteric noncompetitive inhibitor to the repressor It binds with the repressor s allosteric site, changes the shape of the enzyme, and prevents the enzyme from binding to the operator Now that the repressor is off the operator, RNA polymerase can bind to the promoter and begin transcription of the enzymes that metabolize lactose. When all the lactose is metabolized, there s nothing left to bind with the repressor. It returns to the operator and stops transcription We re out of lactose. The genes aren t needed anymore anyway This is called an inducible operon because the presence of lactose induces (turns on) the operon The lac Operon

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12 Mutations are permanent changes in DNA nucleotide sequences. Somatic mutations occur in body cells. Lots more opportunities for a mutation, but it rarely affects more than a handful of cells in a single tissue layer Germ-line mutations occur in gamete and sex cells. Don t usually affect the host, but can be passed on to the next generation. Spontaneous mutations are mistakes caused by biological enzymes or processes Induced mutations are mistakes caused by exposure to mutagens Mutations

13 Mutagens are any environmental agent that increases the chances of a mutation. Most mutagen exposure results in a cancerous cell Mutagens that cause cancer are called carcinogens The most common forms of mutagens are radiation (X-ray, UV) or chemicals (cigarette smoke, bugspray) The most common cells affected by mutagens are in skin, thyroid, liver, lungs and kidneys Why: the function of these cells have something to do with filtration Mutagens

14 UV Radiation UV radiation has a strong effect on pyramidines such as thymine and cytosine. Any DNA sequence that has multiple thymines in a row and is exposed to UV can cause thymines to bond to each other This results in a kink in the strand. Normally, repair enzymes notice the kink and break it, repairing the nucleotides. Unfortunately the gene for the repair enzymes also contain lots of thymines in a row, making them susceptible to the kinks as well Either way, the result is a high percentage of cancerous cells Mutagens

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16 Tobacco Smoke Tobacco smoke is another carcinogen that causes cancer by mispairing nucleotides in DNA If the mispairing occurs in a strand of RNA, the cell still has a chance to survive. But changing DNA means changing the master blueprint. One example of mispairing is a chemical called 5-bromouracil. 5-bromouracil bonds with thymine, but resembles a cytosine on the opposite end. During replication or transcription, the polymerase believes the thymine is a cytosine and incorrectly adds a guanine instead of an adenine. Mutagens

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