AndyK

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Question 5 from practice test 1:

The lac operon:
A. is found in eukaryotic cells
B. codes for the sequence of amino acids in lactase
C. regulates translation of mRNA
D. regulates transcription by turning on or off the production of a repressor protein
E. regulates DNA replication by turning on or off the production of an inducer protein

A, C and E are obviously false. The answer is D but I don't understand how it's not B. If I understand correctly, the repressor is constantly being made by the repressor gene and the only thing that dictates whether or not the gene for lactase (i guess that's short for beta-galactosidase) is being transcribed is the presence of the inducer (lactose) -

Here is Cliff's solution:
The lac operon in bacteria regulates gene expression by turning on or off the production of a repressor protein. The repressor protein, in turn, occupies the operator region of the operon, blocking transcription of the structural gene for lactase. When the sugar lactose is available, it inactivates the repressor protein, thereby allowing transcription of the structural gene for lactase. Lactase is then produced, and lactose digestion proceeds.

Am I missing something??
 
May 15, 2009
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Question 5 from practice test 1:

The lac operon:
A. is found in eukaryotic cells
B. codes for the sequence of amino acids in lactase>>The operon is cluster of three different genes (LacZ, Y, and A), each of which codes for a particular enzyme that assumes a particular role in digestion of lactose. So, lactase is not the only enzyme coded for by the operon.
C. regulates translation of mRNA
D. regulates transcription by turning on or off the production of a repressor protein
E. regulates DNA replication by turning on or off the production of an inducer protein

A, C and E are obviously false. The answer is D but I don't understand how it's not B. If I understand correctly, the repressor is constantly being made by the repressor gene and the only thing that dictates whether or not the gene for lactase (i guess that's short for beta-galactosidase) is being transcribed is the presence of the inducer (lactose) -

Here is Cliff's solution:
The lac operon in bacteria regulates gene expression by turning on or off the production of a repressor protein. The repressor protein, in turn, occupies the operator region of the operon, blocking transcription of the structural gene for lactase. When the sugar lactose is available, it inactivates the repressor protein, thereby allowing transcription of the structural gene for lactase. Lactase is then produced, and lactose digestion proceeds.

Am I missing something??
I agree that this is somewhat tricky! But if I was to eliminate choice B the above would be my reasoning.
 
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AndyK

AndyK

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I agree that this is somewhat tricky! But if I was to eliminate choice B the above would be my reasoning.
"The lac operon in bacteria regulates gene expression by turning on or off the production of a repressor protein. "

Isn't this patently false though? How does the lac operon regulate the PRODUCTION of the repressor?
 
May 15, 2009
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"The lac operon in bacteria regulates gene expression by turning on or off the production of a repressor protein. "

Isn't this patently false though? How does the lac operon regulate the PRODUCTION of the repressor?
I read this material a while ago, so I don't remember all the details. But if I remember correctly one of the gene products of the operon induces the regulatory gene to produce the repressor. So, the operon does in fact have some effect on transcription of the regulatory gene.
 
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AndyK

AndyK

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7+ Year Member
Sep 24, 2007
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Houston, TX
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I read this material a while ago, so I don't remember all the details. But if I remember correctly one of the gene products of the operon induces the regulatory gene to produce the repressor. So, the operon does in fact have some effect on transcription of the regulatory gene.
Well here's what cliffs says:

"A regulatory gene, lying OUTSIDE THE OPERON REGION, produces repressor proteins, substances that occupy the operator region and block the action of RNAPol...."

So they are just contradicting their own answer with the material in the book.
 
May 15, 2009
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Well here's what cliffs says:

"A regulatory gene, lying OUTSIDE THE OPERON REGION, produces repressor proteins, substances that occupy the operator region and block the action of RNAPol...."

So they are just contradicting their own answer with the material in the book.
Well, Cliffs is not the bible and most certainly it doesn't cover all the details.
What I was saying was that some of the gene products of the operon can induce the transcription of the regulatory gene, which results in production of the repressor protein.
Perhaps you can google this and read some of the details.