section 2 chapter 10. protein synthesis in prokaryotes both prokaryotic and eukaryotic cells are...
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Section 2
CHAPTER 10
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PROTEIN SYNTHESIS IN PROKARYOTES
• Both prokaryotic and eukaryotic cells are able to regulate which genes are expressed and which are not, depending on the cell’s needs.
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PROTEIN SYNTHESIS IN PROKARYOTES• The piece of DNA that overlaps the
promoter site and serves as the on-off switch is called an operator.
• In bacteria, a group of genes that code for enzymes involved in the same function, their promoter site, and the operator that controls them all function together as an operon.
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OPERON• The operon that controls the
metabolism of lactose is called the lac operon.
• When there is no lactose in the bacterial cell, a repressor turns the operon off.
• A repressor is a protein that binds to an operator and physically blocks RNA polymerase from binding to a promoter site.
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PROTEIN SYNTHESIS IN EUKARYOTES
• Eukaryotic cells contain much more DNA than prokaryotic cells do.
• Eukaryotic cells must continually turn certain genes on and off in response to signals from their environment; however, eukaryotes lack operons.
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CONTROLLING THE ONSET OF TRANSCRIPTION• Most gene regulation in eukaryotes controls the onset of
transcription—when RNA polymerase binds to a gene.
• Transcription factors (regulatory proteins in eukaryotes) help arrange RNA polymerases in the correct position on the promoter.
• A gene can be influenced by many different transcription factors.
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CONTROLLING THE ONSET OF TRANSCRIPTION• An enhancer is a sequence of DNA
that can be bound by a transcription factor.
• Enhancers typically are located thousands of nucleotide bases away from the promoter.
• A loop in the DNA may bring the enhancer and its attached transcription factor (called an activator) into contact with promoter site – which then activates transcription.
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ENHANCERS FOR CONTROL OF GENE EXPRESSION• In eukaryotes, many genes are
interrupted by introns—long segments of nucleotides that have no coding information.
• Exons are the portions of a gene that are translated (expressed) into proteins.
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ENHANCERS FOR CONTROL OF GENE EXPRESSION
• After a eukaryotic gene is transcribed, the introns in the resulting mRNA are cut out by complex assemblies of RNA and protein called spliceosomes.