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dreamv Rising GMAT Star Default Avatar
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operon Post Sat Feb 04, 2012 5:19 pm
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  • Lap #[LAPCOUNT] ([LAPTIME])
    The operon theory of gene regulation explains how the production of proteins is regulated in bacteria. A complex of genes, the operon consists of two regions: a coding region, for enzymes involved in a metabolic pathway, and a regulatory region that determines whether the gene is switched “on” or “off.” There are two types of operons. In an inducible operon, protein production is “off” by default, and turns “on” when a sufficient quantity of inducer is present. In a repressible operon, protein production is “on” by default, and turns “off” when an active repressor complex is present.

    The lac operon is an example of an inducible operon. Normally, the lac operon's protein production is inactive; since the lac repressor is bound to the regulatory region, the gene does not produce enzymes. However, when lactose, or milk sugar, is present in high concentrations, the lac repressor binds instead to the lactose. Since the regulatory region is now free of a repressor, the gene activates and begins to produce proteins.

    The process is neatly self-regulating. The proteins produced by the now-activated genes lactose-digesting enzymes, which in turn down the lactose. Once all lactose is broken down, there is no remaining lactose that can bind to the lac repressor. The repressor returns to the regulatory region, turning the operon “off” and arresting the production of enzymes and the digestion of lactose.

    Which of the following can be inferred from the passage?

    A. All genes are either inducible or repressible.
    B. Lactose is the inducer of the lac operon.
    C. The lac repressor prefers to bind to lactose rather than the regulatory region.
    D. The repressible operon does not normally have anything bound to its regulatory region.
    E. Lactose is the main source of energy for bacteria.

    Source is Kaplan CAT. Please choose your answer choice and explain why it is correct by quoting some sentences from the passage.

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    sam2304 GMAT Titan
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    Post Sun Feb 05, 2012 1:49 am
    Quote:
    Normally, the lac operon's protein production is inactive; since the lac repressor is bound to the regulatory region, the gene does not produce enzymes. However, when lactose, or milk sugar, is present in high concentrations, the lac repressor binds instead to the lactose.
    IMO C.

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    dreamv Rising GMAT Star Default Avatar
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    Post Mon Feb 06, 2012 10:55 pm
    Thank you, but OA is B. My choice was D. The following is Kaplan's explanation about Choice B.

    "Choice B is correct. While it is not explicitly stated that lactose is the inducer, this can be inferred from the fact that the concentration of lactose is what activate the gene."

    Even after I went through the passage sentence by sentence, it was hard for me to choose B.

    Please refer to Kaplan's explanation for the other choices.

    A. While only inducible and repressible genes are mentioned in this passage, this does not preclude other types.

    C. goes beyond what is stated in the passage. Although lactose in high concentrations binds to the lac repressor and takes away from the regulatory region, it cannot be inferred that the lac repressor prefers lactose to the regulatory region.

    D. Incorrect. The repressible operon is merely mentioned at the end of the first paragraph, but it is not stated whether anything is normally bound to the regulatory region.

    E. Out of scope. The passage does not mention the significance of the role that lactose plays in bacterial metalbolism.

    Hope it helps.

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    sam2304 GMAT Titan
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    Post Tue Feb 07, 2012 9:38 am
    Thanks for posting the OA and OE. The question seems too broad. Generally inference questions will be focusing on a specific point in the passage and we can answer reading the localized area but this question requires reading each choice and validating with the whole passage. Took more time to solve and yet got it wrong Smile

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