Operon Term paper

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Lin Malaitham

AP Bio

January 18, 2001

Operon

The operon is a set of coding regions of DNA clustered together that includes structural genes and it is under the control of a single regulatory region. The operator regulates transcription, which is a repressor protein. When the operator binds to a segment of the regulatory region, transcription is shut down.

E. Coli will be used as an example of how an inducible operon works. E. Coli’s main source of nutrition is glucose. If glucose is not available, it can utilize lactose. But the necessary enzymes used to digest lactose aren’t normally made by E. Coli. When lactose is available in the environment, certain activities will take place allowing E. Coli to digest lactose. Lactose induces transcription of enzymes to utilize lactose.

It takes three enzymes for E. Coli to digest lactose: beta-glycosidase, permease, and transacetylase. The coding regions for these three enzymes are linked together under the control of a single regulatory region called the lac operon.

The I gene makes the lac repressor protein. The I gene constitutively transcribes mRNA for the repressor protein. The repressor is a regulator protein. It can bind to either lactose or the operator DNA. The binding of a repressor protein to either DNA or lactose is reversible, so the repressor is bound most often to whichever of the two it finds in the highest concentration. The lac repressor protein is highly attracted to the operator sequence of the lac promoter. When it is bound, transcription does not occur. If lactose levels rise to high concentrations, the lactose molecules will quickly bind to a lactose molecule rather than the one operator. The operator would no longer be repressed and transcription will proceed.

The coding region of the lac operon is transcribed as a single polycistronic mRNA. When the three lactose-digesting enzyme are available, they will digest the lactose. Lactose levels will go down and the repressor molecule will bind to the operator when there is no more lactose to metabolize. Also as the availability of lactose gets low, the lactose molecules that were bound to repressor molecules can be degraded. This frees up repressor molecules. When the enzymes are no longer needed, transcription is shut down.

Lactose alone cannot induce the lac operon. E. Coli prefers to utilize glucose rather than lactose because glucose is...

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