Section 1

Control of Gene Expression

• Gene expression is the activation of a gene that results in transcription and the production of mRNA. Only a fraction of any cell's genes are expressed at any one time.

• A promoter and an operator regulate the transcription of structural genes. In prokaryotes, the structural genes, the promoter, and the operator collectively form an operon.

• A promoter is the segment of DNA that is recognized by the enzyme RNA polymerase, which then initiates transcription. An operator is the segment of DNA that acts as a "switch" by controlling the access of RNA polymerase to the promoter.

• A repressor protein can inhibit genes from being expressed. Repressor proteins are coded for by regulator genes. A repressor protein attaches to the operator, physically blocking the advancement of RNA polymerase.

Vocabulary gene expression (p. 217) genome (p. 217) structural gene (p. 218) operator (p. 218)

operon (p. 218) lac operon (p. 218) repressor protein (p. 219) regulator gene (p. 219)

• An inducer is a molecule that initiates gene expression. In E. coll, lactose serves as an inducer. An inducer binds to the repressor protein. As a result, the shape of the repressor protein changes, and the repressor protein detaches from the operator. RNA polymerase can then advance to the structural genes.

• Eukaryotes do not have operons. The genomes of eukaryotes are larger and more complex than those of prokaryotes.

• Eukaryotic genes are organized into noncoding sections, called introns, and coding sections, called exons.

• In eukaryotes, gene expression can be controlled after transcription—through the removal of introns from pre-mRNA—or at the onset of transcription—through the action of transcription factors.

inducer (p. 219) euchromatin (p. 220) intron (p. 220) exon (p. 220)

pre-mRNA (p. 221) transcription factor (p. 222) enhancer (p. 222)

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