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Transcriptional control of the Lactose Operon.

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Describe the transcriptional control of the Lactose Operon.

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1) First, we need to understand WHEN lactose operon is activated.

The Lactose Operon or Lac operon is an inducible gene. This means that the lactose operon is ONLY activated (or induced) under certain conditions. The presence of lactose and absence of glucose ACTIVATES the lac operon. Activation of the lactose operon gene leads to production of an enzyme called beta galactosidase, which helps the cell metabolize lactose for energy. However, glucose is the preferred source of energy for cell metabolism and cell function. So, if both glucose and lactose are present, the lactose operon WILL NOT be activated (or transcribed. Transcription leads to specific RNA and translation of proteins regulated by the lactose gene, which is beta galactosidase).

Hypothetically, let's say your favorite food is pizza and you like peanut butter sandwiches. If given the choice between pizza and peanut butter sandwiches, you will choose pizza. This is like a cell saying, "glucose is my favorite food, but I like lactose." So if a cell is given an option between glucose and lacotse, the cell will choose glucose, like you would choose pizza.

Now, let's say you missed lunch ...

Solution Summary

The expert describes the transcriptional control of the Lactose Operon.

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Lac Operon - I gene

In the table below, predict (yes or no) whether or not the E. coli lac operon will be transcriptionally active in the presence or absence of glucose or lactose as indicated and respond
Lactose Glucose Lac expression?
No Yes
Yes Yes
Yes No
to questions "a" and "b."

a. Explain each of your answers in terms of the molecular mechanisms that are known to underlie the regulation of the lac operon.

b. Which mechanism is considered to be negative control and which is considered to be positive control? Explain.

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