Concept 11.1 Several Strategies Are Used to Regulate Gene Expression

• Gene expression can be regulated at the levels of transcription, RNA processing, translation, or posttranslation. Review Figure 11.1 and WEB ACTIVITY 11.1
   
• Some genes are always expressed (constitutive genes), whereas others are expressed only at certain times and in certain cells (inducible genes).
   
• Transcription factors are regulatory proteins that bind DNA and regulate gene expression. Activators positively regulate gene expression. Repressors negatively regulate gene expression. Review Figure 11.2
   
• Viruses provide examples of gene regulation as they convert the host cell into a virus factory. Review Figures 11.3 and 11.4
   

Concept 11.2 Many Prokaryotic Genes Are Regulated in Operons

• A metabolic pathway can be regulated either by allosteric regulation of an enzyme or by regulation of enzyme synthesis. Review Figure 11.6
   
• In prokaryotes, several genes can be part of a single transcriptional unit called an operon, which consists of a promoter, an operator, and two or more structural genes. Review Figure 11.7
   
• An inducible operon is turned off unless its expression is needed, whereas a repressible operon is turned on unless its expression is not needed. When an operon is turned off, it has a repressor protein bound to its operator, preventing transcription.
   
• The lac operon is an example of an inducible system, whereas the trp operon is an example of a repressible system. Review Figures 11.8 and 11.9 and ANIMATED TUTORIALS 11.1 and 11.2
   
• Sigma factors direct RNA polymerase to specific promoters in prokaryotes.
   

Concept 11.3 Eukaryotic Genes Are Regulated by Transcription Factors and DNA Changes

• Eukaryotic gene expression is regulated both during and after transcription.
   
• General transcription factors bind to the core promoter sequences of protein-coding genes and direct RNA polymerase II to the promoter. Review Figure 11.10 and ANIMATED TUTORIAL 11.3
   
• Specific transcription factors (activators and repressors) bind to specific DNA elements near the promoter and affect the rate of transcription initiation. Review Figures 11.11 and 11.12
   
• The term epigenetic refers to changes in gene expression that do not involve changes in DNA sequences.
   
• Methylation of cytosine residues generally inhibits transcription. Review Figure 11.13
   
• Chromatin remodeling via the modification of histone proteins in nucleosomes also affects transcription. Review Figure 11.15
   
• Epigenetic changes can be induced by the environment and can be inherited.
   

Concept 11.4 Eukaryotic Gene Expression Can Be Regulated after Transcription

• Alternative splicing of pre-mRNA can produce different proteins. Review Figure 11.16
   
• A microRNA (miRNA) is a small noncoding RNA that inhibits the translation of specific mRNAs by causing their premature degradation. Review Figure 11.17
   
• The translation of mRNA to proteins can be regulated by translational repressors. Review Figure 11.18
   
• A proteasome can break down proteins, thus affecting protein longevity. Review Figure 11.19
   

See WEB ACTIVITY 11.2 for a concept review of this chapter.