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Exam 15: Control of Gene Expression in Prokaryotes

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Question 1

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Functionally related genes in bacterial cells are frequently clustered together as a single transcriptional unit termed an operon. A typical operon includes several structural genes, a promoter for the structural genes, and an operator site to which the product of a regulator gene binds. -What is the difference between a structural gene and a regulator gene?

A) Structural genes are transcribed into mRNA, but regulator genes aren't.
B) Structural genes have complex structures; regulator genes have simple structures.
C) Structural genes encode proteins that function in the structure of the cell; regulator genes carry out metabolic reactions.
D) Structural genes encode proteins; regulator genes control the transcription of structural genes.

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D

Question 2

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In attenuation, transcription is initiated but terminates prematurely. When tryptophan levels are low, the ribosome stalls at the tryptophan codons and transcription continues. When tryptophan levels are high, the ribosome does not stall at the tryptophan codons, and the 5' UTR adopts a secondary structure that terminates transcription before the structural genes can be copied into RNA (attenuation) . -Attenuation results when which regions of the 5' UTR pair?

A) 1 and 3
B) 2 and 3
C) 2 and 4
D) 3 and 4

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Question 3

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The lac operon of E. coli controls the transcription of three genes needed in lactose metabolism: the lacZ gene, which encodes β- galactosidase; the lacY gene, which encodes permease; and the lacA gene, which encodes thiogalactoside transacetylase. The lac operon is negative inducible: a regulator gene produces a repressor that binds to the operator site and prevents the transcription of the structural genes. The presence of allolactose inactivates the repressor and allows the transcription of the lac operon. -In the presence of allolactose, the lac repressor

Regulatory proteins that bind DNA have common motifs that interact with sequences in the DNA. -How do amino acids in DNA-binding proteins interact with DNA?

The trp operon is a negative repressible operon that controls the biosynthesis of tryptophan. In a repressible operon, transcription is normally turned on and must be repressed. Repression is accomplished through the binding of tryptophan to the repressor, which renders the repressor active. The active repressor binds to the operator and prevents RNA polymerase from transcribing the structural genes. -In the trp operon, what happens to the trp repressor in the absence of tryptophan?

Gene expression can be controlled at any of a number of points along the molecular pathway from DNA to protein, including DNA or chromatin structure, transcription, mRNA processing, RNA stability, translation, and posttranslational modification. -Why is transcription a particularly important level of gene regulation in both bacteria and eukaryotes?

There are two basic types of transcriptional control: negative and positive. In negative control, when a regulatory protein (repressor) binds to DNA, transcription is inhibited; in positive control, when a regulatory protein (activator) binds to DNA, transcription is stimulated. Some operons are inducible; transcription is normally off and must be turned on. Other operons are repressible; transcription is normally on and must be turned off. -In a negative repressible operon, the regulator protein is synthesized as

Genes are DNA sequences that are transcribed into RNA. Regulatory elements are DNA sequences that are not transcribed but affect the expression of genes. Positive control includes mechanisms that stimulate gene expression, whereas negative control inhibits gene expression. -What is a constitutive gene?

In spite of its name, catabolite repression is a type of positive control in the lac operon. The catabolite activator protein (CAP) , complexed with cAMP, binds to a site near the promoter and stimulates the binding of RNA polymerase. Cellular levels of cAMP are controlled by glucose; a low glucose level increases the abundance of cAMP and enhances the transcription of the lac structural genes. -What is the effect of high levels of glucose on the lac operon?