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16.1 Gene expression is controlled by regulating transcription.
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• Regulatory sequences are short stretches of DNA that function in transcriptional control but are not transcribed themselves.
• Regulatory proteins recognize and bind to specific regulatory sequences on the DNA.
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16.2 Regulatory proteins read DNA without unwinding it.
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• Regulatory proteins possess structural motifs that allow them to fit snugly into the major groove of DNA, where the sides of the base-pairs are exposed.
• Common structural motifs include the helix-turn-helix,
homeodomain, zinc finger, and leucine zipper.
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1. How do regulatory proteins identify specific nucleotide sequences without unwinding the DNA?
2. What is a helix-turn-helix motif? What sort of developmental events are homeodomain motifs involved in?
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16.3 Bacteria limit transcription by blocking RNA polymerase.
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• Many genes are transcriptionally regulated through repressors, proteins that bind to the DNA at or near the promoter and thereby inhibit transcription of the gene.
• Genes may also be transcriptionally regulated through activators, proteins that bind to the DNA and thereby stimulate the binding of RNA polymerase to the promoter.
• Transcription is often controlled by a combination
of repressors and activators.
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3. Describe the mechanism by which the transcription of trp genes is regulated in Escherichia coli when tryptophan is present in the environment.
4. Describe the mechanism by which the transcription of lac genes is regulated in E. coli when glucose is absent but lactose is present in the environment.
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16.4 Transcriptional control in eukaryotes operates at a distance.
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• In eukaryotes, RNA polymerase cannot bind to the promoter unless aided by a family of transcription factors.
• Anything that interferes with the activity of the transcription factors can block or alter gene expression.
• Eukaryotic DNA is packaged tightly in nucleosomes within chromosomes. This packaging appears to provide some inhibition of transcription, although regulatory proteins and RNA polymerase can still activate specific genes even when they are so packaged.
• Gene expression can also be regulated at the posttranscriptional
level, through RNA splicing, translation repressor proteins, and the selective
degradation of mRNA transcripts.
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5. How do transcription factors promote transcription in eukaryotic cells? How do the enhancers of eukaryotic cells differ from most regulatory sites on bacterial DNA?
6. What role does the methylation of DNA likely play in transcriptional control?
7. How does the primary RNA transcript of a eukaryotic gene differ from the mRNA transcript of that gene as it is translated in the cytoplasm?
8. How can a eukaryotic cell control the translation of mRNA transcripts after they have been transported from the nucleus to the cytoplasm?
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