Chapter Summary
16.1 Prokaryotic Gene Expression Is Regulated in Operons
Some proteins are synthesized only when they are needed. Proteins that are made only in the presence of a particular compound—
an inducer—are called inducible proteins. Proteins that are made at a constant rate regardless of conditions are called constitutive proteins. Review Focus: Key Figure 16.1 Figure 16.2 An operon consists of a promoter, an operator, and two or more structural genes. Promoters and operators do not code for proteins, but serve as binding sites for regulatory proteins. Review Figure 16.4
Regulatory genes code for regulatory proteins, such as repressors. When a repressor binds to an operator, transcription of the structural gene is inhibited. Review Figure 16.5, Animations 16.1, 16.2
The lac operon is an example of an inducible system, in which the presence of an inducer (lactose) keeps the repressor from binding the operator, allowing the transcription of structural genes for lactose metabolism.
Transcription can be enhanced by the binding of an activator protein to the promoter. Review Figure 16.6
Catabolite repression is the inhibition of a catabolic pathway for one energy source by a different, preferred energy source.
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16.2 Eukaryotic Gene Expression Is Regulated by Transcription Factors
Eukaryotic gene expression can be regulated before, during, and after transcription. Review Figure 16.7, Activity 16.1
Transcription factors and other proteins bind to DNA and affect the rate of initiation of transcription at the promoter. Review Figures 16.8, 16.9, Animation 16.3
The interactions of these proteins with DNA are highly specific and depend on protein domains and DNA sequences.
Genes at distant locations from one another can be coordinately regulated by transcription factors and promoter elements. Review Figure 16.10
16.3 Viruses Regulate Their Gene Expression during the Reproductive Cycle
Viruses are not cells, and rely on host cells to reproduce.
The basic unit of a virus is a virion, which consists of a nucleic acid genome (DNA or RNA); a protein coat, called a capsid; and in some cases, an envelope of lipids that surrounds the capsid.
Bacteriophages are viruses that infect bacteria.
Viruses undergo a lytic cycle, which causes the host cell to burst, releasing new virions.
Some viruses have promoters that bind host RNA polymerase, which they use to transcribe their own genes and proteins. Review Figure 16.11
Some viruses can also undergo lysogeny, in which a molecule of their DNA, called a prophage, is inserted into the host chromosome, where it replicates for generations. Review Figure 16.12
A retrovirus uses reverse transcriptase to generate a cDNA provirus from its RNA genome. The provirus is incorporated into the host’s DNA and can be activated to produce new virions. Review Figure 16.13
16.4 Epigenetic Changes Regulate Gene Expression
Epigenetics refers to changes in gene expression that do not involve changes in DNA sequences.
Methylation of cytosine residues generally inhibits transcription. Review Figure 16.14
Modifications of histone proteins in nucleosomes make transcription either easier or more difficult. Review Figure 16.15
Changes in the environment, such as drought stress, can prompt epigenetic changes to occur. Review Investigating Life: Gene Expression and Behavior
16.5 Eukaryotic Gene Expression Can Be Regulated after Transcription
Alternative splicing of pre-
mRNA can produce different proteins. Review Figure 16.17 Small RNAs (microRNAs and small interfering RNAs) do not code for proteins but regulate the translation and longevity of mRNA. Review Figure 16.18
The translation of mRNA to proteins can be regulated by translational repressors.
A proteasome can break down proteins, thus affecting protein longevity. Review Figure 16.20
See Activity 16.2 for a concept review of this chapter.
Go to LearningCurve (in LaunchPad) for dynamic quizzing that helps you solidify your understanding of this chapter. LearningCurve adapts to your responses, giving you the practice you need to master each key concept.