Summary

Concept 11.1: Many Prokaryotic Genes Are Regulated in Operons

• Gene expression can be regulated at the levels of transcription, RNA processing, translation, or posttranslation. Review Figure 11.1 and 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
• 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 11.3
• 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 11.4 and 11.5 and ANIMATED TUTORIALS 11.1 and 11.2
• A metabolic pathway can be regulated either by allosteric regulation of an enzyme or by regulation of enzyme synthesis. Review Figure 11.6
• Sigma factors direct RNA polymerase to specific promoters in prokaryotes.
• Viruses provide examples of gene regulation as they convert the host cell into a virus factory. Review Figure 11.7

Concept 11.2: Eukaryotic Genes Are Regulated by Transcription Factors

• 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 11.8 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 Figure 11.9

Concept 11.3: Gene Expression Can Be Regulated via Epigenetic Changes to Chromatin

• The term epigenetic refers to changes in gene expression that do not involve changes in DNA sequences.
• Chromatin remodeling via the modification of histone proteins in nucleosomes also affects transcription. Review Figure 11.14
• Methylation of cytosines in DNA generally inhibits 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 ACTIVITY 11.2 for a concept review of this chapter.