Chapter 18 Gene expression

Regulation of Gene Expression

Overview of Gene Expression

Gene expression is the process by which information from a gene is used to synthesize a functional gene product, typically a protein. Regulation of gene expression allows cells to control which genes are active, adapting to internal and external conditions. - Key Point 1: Cells use only about 5-20% of their genes at any given time. - Key Point 2: Gene usage depends on cell type, internal conditions (such as cell cycle stage and nutrient availability), and external signals (such as environmental stress and growth factors). - Example: Muscle cells express muscle-specific genes, while heat shock proteins are expressed in response to elevated temperatures.

Major Points of Regulation

Gene regulation can occur at multiple levels, but two major points are: - Controlling enzyme activity: Feedback inhibition, such as in tryptophan synthesis, where the end product inhibits the pathway. - Controlling transcription (TXN): Regulation of gene transcription, such as the trp operon.

Transcription and Translation

Transcription Initiation

Transcription is initiated when RNA polymerase binds to the promoter region of DNA, often assisted by transcription factors. - Promoter: DNA sequence where RNA polymerase attaches. - Transcription factors: Proteins that regulate the binding of RNA polymerase. - Transcription initiation complex: Assembly of RNA polymerase and transcription factors at the promoter.

RNA Processing in Eukaryotes

After transcription, eukaryotic pre-mRNA undergoes processing: - Addition of a 5' cap and a poly-A tail. - Removal of introns and joining of exons. - Processed mRNA is exported from the nucleus to the cytoplasm for translation.

Gene Regulation in Prokaryotes: Operons

Operon Structure and Function

An operon is a cluster of genes under the control of a single promoter, transcribed together as one mRNA. - Operator: DNA segment within the promoter that acts as a regulatory switch. - Regulatory gene: Encodes a repressor protein that can inhibit transcription. - Example: The trp operon controls tryptophan synthesis.

Negative Regulation: Repressible Operons (trp Operon)

Repressible operons are usually "on" but can be turned "off" when the end product is abundant. - trp Operon: When tryptophan levels are high, tryptophan binds to the repressor, activating it. The active repressor binds the operator, blocking transcription. - Feedback inhibition: Tryptophan also inhibits enzyme activity in its biosynthetic pathway.

Positive Regulation: Inducible Operons (lac Operon)

Inducible operons are usually "off" but can be turned "on" in response to a substrate. - lac Operon: Controls genes for lactose metabolism. - Negative regulation: The lac repressor binds the operator and blocks transcription when lactose is absent. - Induction: When lactose is present, allolactose binds the repressor, inactivating it and allowing transcription. - Positive regulation: When glucose is low, cAMP levels rise, activating CRP (cAMP receptor protein), which stimulates transcription.

Summary Table: Repressible vs. Inducible Operons

Type	Default State	Regulation	Example
Repressible	ON	Can be turned OFF	trp operon (anabolic pathway)
Inducible	OFF	Can be turned ON	lac operon (catabolic pathway)

Restriction Enzymes and DNA Tools

Restriction Enzymes

Restriction enzymes are proteins produced by bacteria to defend against viruses. They cut DNA at specific nucleotide sequences, often palindromic. - Restriction site: Specific sequence recognized and cut by the enzyme. - Sticky ends: Overhanging ends produced by staggered cuts, useful for DNA recombination. - Example: EcoRI cuts at the sequence GAATTC.

Applications of Restriction Enzymes

Restriction enzymes are used in molecular biology for: - DNA cloning - Genetic engineering - DNA fingerprinting

Additional info:

- The notes reference figures from textbook chapters 17 and 18, which cover gene expression and regulation. - The cell cycle and feedback inhibition are briefly mentioned, relevant to chapters 12 and 8. - The lab agenda references restriction enzyme digests and gel electrophoresis, which are part of DNA tools and biotechnology (chapter 20). ----------------------------------------