Biology Homework

Study Guide chapter 18-answers

18.3_Bacterial Genome_____________________________________________________

1. Describe the replication of a bacterial chromosome.

Binary fission (see chapter 12 study guide for a more detailed description of this process)

18.4 Bacterial Gene Expression

2. Define the following terms:

Promoter- DNA sequence that the RNA polymerase binds to transcribe the operon genes.

Operator- DNA sequence within the promoter that the repressor protein binds to when the inducer is absent.

Operon- 1 promoter (with operator within the promoter) and several genes.

Repressor- regulatory protein that can switch the operator “off”

Regulatory gene- gene that codes for the repressor protein

Inducer- small molecule that binds to the repressor protein, changing its conformation so it can no longer bind to the operator (ex: lactose is the inducer for the lac operon)

lac operon

3. Is the lac operon a repressible operon or an inducible operon? Why?

Inducible, because it is usually turned “off” and can be turned “on” in the presence of an the inducer molecule lactose.

4. Describe the mechanism of lac operon function when...

A. the lac operon is said to be “Turned OFF”?

Lac Operon “OFF”

The lac operon is usually turned “off” (not actively transcribing the gene of the operon).

A regulatory gene (Lac I) is transcribed and translated producing the repressor protein that binds to the operator within the promoter. This repressor protein blocks RNA polymerase from binding to the promoter and transcribing the genes of the lac operon.

B. lac operon is said to be “Turned ON”?

Lac Operon “ON”

The lac operon is only turned “on” when lactose is present in the environment of the bacterial cell. Lactose (the inducer molecule) binds to the repressor protein and changes its conformation. In this changed conformation the repressor protein cannot bind to the operator. The RNA polymerase can now bind to the promoter and transcribe the genes of the lac operon. The transcript is subsequently translated into the 3 enzymes that metabolize lactose.

5. A certain mutation in an E. coli bacterial cell’s DNA changes the lac operator so that the repressor cannot bind. How would this affect the cell’s production of B-galactosidase (the protein product of one of the genes in the lac operon)?

The lac operon would no longer be turned “off” by the repressor molecule. RNA polymerase would bind to the promoter and transcribe the genes of the lac operon. One of these genes is the gene that codes for the enzyme B-galactosidase. Therefore B-galactosidase would be produced continuously