Control of Gene Expression Chapter 16

Contolling Gene Expression What does that mean? Regulating which genes are being expressed  transcribed/translated  i.e. made into proteins Not all genes are expressed all of the time

Controlling Gene Expression Why do cells control gene expression? Each cell in an organism contains the exact same set of DNA (i.e. 6 billion bp, ~30,000 genes) What is the difference, then, between a skin cell and a nerve cell?  The proteins found within the cell, i.e. the genes that are expressed  Allows for the process of differentiation

Controlling Gene Expression All organisms regulate when and for how long a gene is on This regulation allows for the conservation of energy In eukaryotes differential gene expression is what creates the different cell types Too much or too little expression can lead to disease, aging, etc.

Control of Genes Regulatory proteins interact with DNA, RNA or other proteins to control the expression of genes Transcription factors are regulatory proteins which interact with DNA at specific sequences to regulate gene activity Two types of control: Negative control slows down or stops gene activity Positive control promotes gene activities

Gene Control in Prokaryotes No nucleus separates DNA from ribosomes in cytoplasm Translation occurs even before mRNA transcripts are finished Control functionally related genes together by grouping them into units called Operons  E.g. enzymes in a biosynthesis pathway

Operons in Prokaryotes Consist of a Regulatory Gene, Operator, Promoter and the Structural genes they control Transcription of these genes is initiated by one promoter, and controlled by a single operator Transcribes as 1 unit, and a single mRNA is made, which is later translated into one polypeptide, which is later cleaves into individual proteins - polycistronic

Polycistronic Expression

Inducible Operon - Lac Operon Encodes genes necessary to process lactose Not needed unless lactose is present If there is no lactose: Lac Operon RNA Polymerase Repressor Regulatory gene Structural genes not transcribed Operator

Inducible Operon - Lac Operon What happens when lactose is present? Need to make lactose-digesting enzymes Lactose binds allosterically to regulatory protein:

Inducible vs. Repressible Operons Inducible operon (e.g. Lac operon) usually functions in catabolic pathways, digesting nutrients to simpler molecules produce enzymes only when nutrient is available cell avoids making proteins that have nothing to do Repressible operon (e.g. Tryp operon) usually functions in anabolic pathways synthesizing end products when end product is present cell allocates resources to other uses

Eukaryotic Gene Expression MUCH more complicated that prokaryotic control Most genes in eukaryotic cells are turned off at any given point Only 5-10% of genes are being expressed at any point Controlling gene expression occurs at several different points in the process:

Mechanisms of Gene Control in Eukaryotes

Chromatin Structure  Eukaryotic DNA wraps around histones, is further structured into nucleosomes Promoters inaccessible  Chromatin remodeling makes gene promoters more accessible Activators recruit remodeling complexes that displace nucleosomes Activators recruit enzyme that acetylates and loosens histone assocation with DNA

Chromatin Remodeling

Eukaryotic gene organization DNA Regulatory sequences Enhancer TATA box Promoter 5' UTR Transcription unit of gene Exon Intron 3' UTR

Transcription in Eukaryotes Transcription factors bind to the TATA sequence within the promoter of gene to be transcribed RNA polymerase binds to the transcription factors (initiates low levels of transcription) Activators bind to enhancer sequences (may be located far from the gene) Activators bind to RNA polymerase and trigger it to begin transcription (high level of transcription)

Transcription in Eukaryotes Transcription begins Initial general transcription factor Additional general transcription factors Transcription complex TATA box Promoter RNA polymerase Site where transcription starts DNA The first general transcription factor recognizes and binds to the TATA box of a protein-coding gene’s promoter. 1 2 Additional general transcription factors and then RNA polymerase add to the complex, and then transcription begins.

Controlling transcription in Eukaryotes

Transcription regulation in Eukaryotes DNA sequences Promoters Enhancers Other regulatory sequences Proteins Transcription factors Activators Inhibitors

Eukaryotic Gene Expression Regulates cell cycle Controls development (Homeo box genes) Controls differentiation ….and lots, lots more!

Apoptosis Programmed cell death Signals unleash molecular weapons of self-destruction Cancer cells do not commit suicide on cue