Chapter 18

Question 1

What are the two main ways of controlling metabolism in bacterial cells?

Answer 1

Cells can adjust the activity of enzymes already present

Cells can adjust the production level of certain enzymes; they can regulate the expression of the genes encoding enzymes

Question 2

Operator

Answer 2

segment of DNA

Controls the access of RNA polymerase to the genes

Question 3

Operon

Answer 3

The operator, the promoter, and the genes they control, the entire stretch of DNA required for enzyme production for the tryptophan pathway

Question 4

Repressor

Answer 4

Operon can be switched off by this protein (trp)
Binds to the operator and blocks attachment of RNA polymerase to the promoter, preventing transcription of the genes
Specific to the operon

Question 5

The top repressor is the protein product of a

Answer 5

Regulatory gene called trpR

Located some distance from the top operon and has its own promoter

Question 6

Corepressor

Answer 6

A small molecule that cooperates with a repressor protein to switch an operon off

Question 7

Inducer

Answer 7

Inactivates the repressor

Question 8

Activator

Answer 8

A protein that binds to DNA and stimulates transcription of a gene

Question 9

The differences between cell types are due to

Answer 9

Differential gene expression, the expression of different genes by cells with the same genome

Question 10

Alternative RNA splicing

Answer 10

Different mRNA molecules are produced from the same primary transcript, depending on which RNA segments are treated as exons and which as introns

Question 11

What is the difference between inducible and repressible operons?

Answer 11

A repressible operon is usually on, but can be inhibited (repressed) when a specific small molecule binds allosterically to a regulatory protein. One example of a repressible operon is the trp operon (trp for tryptophan).
An inducible operon is usually off but can be stimulated (induced) when a specific small molecule interacts with a regulatory protein. One example of an inducible operon is the lac operon (for lactose).

Question 12

Compare and contrast the lac operon and the trp operons

Answer 12

In both lac and trp operons, the entire transcription unit is under the command of one main operator and promoter. The lac operon is an inducible operon, while the trp operon is repressible. In both operons, regulation involves negative control of genes, because operons are switched off by the active form of the repressor protein.

Question 13

What happens when a repressor is bound to the operator?

Answer 13

It blocks attachments of RNA polymerase to the promoter, preventing transcription of the genes

Question 14

What is CAP? How does CAP work?

Answer 14

CAP refers to catabolite activator protein, and is a regulatory protein that binds to DNA and
stimulates transcription of a gene.

Question 15

Explain why CAP binding and stimulation of gene expression is positive regulation

Answer 15

By facilitating the binding of RNA polymerase to the promoter and thereby increasing the rate of transcription, the attachment of CAP to the promoter directly stimulates gene expression, qualifying CAP as positive regulation

Question 16

Describe the relationship between glucose supply, cAMP, and CAP

Answer 16

If the amount of glucose in the cell increases, the cAMP concentration falls, and without camp, CAP detaches from the operon. Because CAP is inactive, RNA polymerase binds less efficiently to the promoter, and transcription proceeds only at a low level.

Question 17

How can both repressible and inducible operons be negative regulators?

Answer 17

Both repressible and inducible operons can be negative regulators as long as the operons are switched off by the active form of the repressor protein. In the case of the lac operon, allolactose induces enzyme synthesis not by acting directly on the genome, but by freeing the lac operon from the negative effect of the repressor. Remember that gene regulation is said to be positive only when a regulatory protein interacts directly with the genome to switch transcription on