Text Questions Chapter 12 biol 222

10. *What is an enhancer? How does it affect the transcription of distant genes?

An enhancer is a DNA sequence that, when bound to transcriptional activator proteins, can affect the transcription of a distant gene. Transcription at a distant gene is affected when the DNA sequence between the gene's promoter and the enhancer loops out, bringing the promotor and the enhancer close together and allowing the transcriptional activator proteins to directly interact with the basal transcription apparatus at the promoter, which stimulates transcription.

7. *What changes take place in chromatin structure and what role do these changes play in eukaryotic gene regulation?

Changes in chromatin structure can result in the repression or stimulation of gene expression. The acetylation of histone proteins increases transcription. The reverse reaction, deacetylation, restores repression. Chromatin-remodeling complexes bind directly to the DNA, altering chromatin structure without acetylating histone proteins and allowing transcription to be initiated by making the promoters accessible to transcription factors. The methylation of DNA sequences represses transcription. The demethylation of DNA sequences often increases transcription.

2. *Name six different levels at which gene expression might be controlled.

Gene structure, transcription, mRNA processing, mRNA stability, translation, and protein modification.

13. *How does bacterial gene regulation differ from eukaryotic gene regulation? How are they similar?

Much of the gene regulation in bacteria takes place at the level of transcription, whereas gene regulation in eukaryotes often takes place at multiple levels. The modification of chromatin structure plays an important role in regulating eukaryotic transcription; chromatin structure is absent in bacteria. Many bacterial genes are organized in operons and coordinately expressed. In contrast, most eukaryotic genes have their own promoters. Eukaryotic transcription is controlled by more-complex machinery than that of bacteria; this complex machinery includes numerous transcription factors and transcriptional activators. RNA processing plays a larger role in eukaryotic gene regulation. Small RNA molecules (siRNAs and miRNAs) play an important role in eukaryotic gene regulation but are absent from most bacteria.

20. *A mutation prevents the catabolite activator protein (CAP) from binding to the promoter in the lac operon. What will the effect of this mutation be on the transcription of the operon?

RNA polymerase will bind the lac promoter poorly, significantly decreasing the transcription of the lac structural genes.

5. *Briefly describe the lac operon and how it controls the metabolism of lactose.

The lac operon consists of three structural genes—lacZ, lacY, and lacA, which encode β-galactosidase, permease, and thiogalactoside transacetylase, respectively. All three genes share a promoter and operator region. Upstream from the lactose operon, the lacI gene encodes the lac operon repressor, which binds at the operator region and inhibits the transcription of the lac operon by preventing RNA polymerase from successfully initiating transcription. When lactose is present in the cell, the enzyme β-galactosidase converts some of it into allolactose, which binds to the lac repressor, altering its shape and reducing the repressor's affinity for the operator. Because the allolactose-bound repressor does not bind to the operator, RNA polymerase can initiate transcription of the lac structural genes from the lac promoter.