18.1 Gene expression is controlled by regulating transcription.
An Overview of Transcriptional Control
• One way to control gene expression is to regulate the initiation of transcription by regulating promoter access. (p. 362)
• Gene expression is controlled at the transcriptional and posttranscriptional levels, with transcriptional control being the most common form of control (p. 362)
18.2 Regulatory proteins read DNA without unwinding it.
How to Read a Helix Without Unwinding It
• Regulatory proteins identify DNA sequences without unwinding the helix by inserting DNA-binding motifs into the major groove where the edges of the base-pairs are exposed. (p. 363)
Four Important DNA-Binding Motifs
• The four most important DNA-binding motifs are the helix-turn-helix motif, the homeodomain motif, the zinc finger motif, and the leucine zipper motif. (pp. 364-365)
18.3 Prokaryotes regulate genes by controlling transcription initiation.
Prokaryotic Gene Regulation
• How prokaryotes respond to changes depends on the nature of the proteins encoded by relevant genes. (p. 366)
• Prokaryotic genes are often organized into operons, multiple genes that are part of a single gene expression unit. (p. 366)
• Repressors are proteins that bind to regulatory sites on DNA and prevent or decrease initiation of transcription. (p. 367)
• Activators can bind to DNA to stimulate the initiation of transcription. (p. 368)
18.4 Transcriptional control in eukaryotes operates at a distance.
Transcriptional Control in Eukaryotes
• Eukaryotic transcription factors fall into two categories: basal transcriptional factors and specific transcriptional factors. Along with enhancers, coactivators, and mediators, these factors provide flexibility in controlling eukaryotic gene expression. (pp. 370-372)
The Effect of Chromatin Structure on Gene Expression
• Despite the additional complexity of DNA packaging in eukaryotes, transcriptional control of gene expression can still occur in eukaryotes. (p. 373)
Posttranscriptional Control in Eukaryotes
• Posttranscriptional control of gene expression is exercised by proteins and small RNAs. (p. 374)
• Small RNAs such as miRNAs and siRNAs are thought to regulate gene expression through RNA interference and, in some cases, through the alteration of DNA packaging. (pp. 374-375)
• Proteins interact with small nuclear RNA and carry out alternative splicing of RNA transcripts. (p. 376)
• RNA transcript editing can be caused by chemical alterations of specific bases. (pp. 376-378)
• Following modification of the primary transcript, further gene expression control occurs by translation repression and selective degradation of mRNA transcripts. (p. 378)