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.