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Activate Prior Knowledge. Humans have an estimated 20,000 to 25,000 genes. Why wouldn ’ t you expect all these genes to be transcribed and translated at the same time? The cell would run out of energy and raw materials (amino
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Activate Prior Knowledge Humans have an estimated 20,000 to 25,000 genes. Why wouldn’t you expect all these genes to be transcribed and translated at the same time? The cell would run out of energy and raw materials (amino acids and nucleotides). Proteins would accumulate in the cell if they were not needed. What might be an analogy to this in your own life? eating all the food in the house at one time, everyone in class speaking at once, spending savings all at once
FLT • Describe how prokaryotes turn genes on and off. • Explain how gene expression is regulated in eukaryotic cells.
KEY CONCEPT Gene expression is carefully regulated in both prokaryotic and eukaryotic cells.
Prokaryotic cells turn genes on and off by controlling transcription. • A promotor is a DNA segment that allows a gene to be transcribed. • An operator is a part of DNA that turns a gene “on” or ”off.” • An operon includes a promoter, an operator, and one or more structural genes that code for all the proteins needed to do a job. • Operons are most common in prokaryotes. • The lac operon was one of the first examples of gene regulation to be discovered. • The lac operon has three genes that code for enzymes that break down lactose.
Power Notes Promoter: aDNA segment that allows a gene to be transcribed; helps RNA polymerase find where a gene starts Operon: a region of DNA that has promoter, an operator, and one or more genes that code for all the proteins needed to do a specific task; typically found in prokaryotes lac operon: one of the earliest operons discovered; includes three genes involved in the breakdown of the sugar lactose that are all under the control of a single promoter and operator
The lac operon acts like a switch. • The lac operon is “off” when lactose is not present. • The lac operon is “on” when lactose is present.
Power Notes Without lactose: a repressor protein is bound to the operator and blocks RNA polymerase from transcribing the genes (off) With lactose: the repressor protein is bound to lactose, which keeps it off the operator, so RNA polymerase transcribes the genes that, in turn, break down lactose (on)
Eukaryotes regulate gene expression at many points. • Different sets of genes are expressed in different types of cells. • Transcription is controlled by regulatory DNA sequences and protein transcription factors.
Power Notes Controlling transcription in eukaryotic cells: transcription is regulated at many points in eukaryotic cells; control of the start of transcription is still an important point of regulation; have unique combinations of regulatory DNA sequences that are recognized by transcription factors; some genes control the expression of other genes and play an important role in development
Transcription is controlled by regulatory DNA sequences and protein transcription factors. • Most eukaryotes have a TATA box promoter. • Enhancers and silencers speed up or slow down the rate of transcription. • Each gene has a unique combination of regulatory sequences.
RNA processing is also an important part of gene regulation in eukaryotes. • mRNA processing includes three major steps.
mRNA processing includes three major steps. • Introns are removed and exons are spliced together. • A cap is added. • A tail is added.
Power Notes mRNA processing: hi mendez occurs after transcription but before mRNA leaves the nucleus. Introns are removed and the exons are spliced together. Introns are intervening sequences of DNA. - Exons are sequences of DNA that are expressed in the protein. - A cap is added that helps mRNA bind to a ribosome and prevents the strand from being broken down too quickly. - A tail is added that helps mRNA exit the nucleus.
