Regulating Gene Expression Turning Genes On and Off

Procaryotes ä Procaryotes are bacteria and blue green algae ä They have no nucleus ä They have circular chromosomes made only of DNA

Procaryotes ä On procaryotic chromosomes there are many genes ä We will concentrate on how the genes that code for lactase is controlled ä Lactase is the enzyme that digests the milk sugar lactose

Lac Operon ä The genes that code for lactase and its associated control mechanisms is called the Lac Operon. ä The Lac Operon was first described by Jacob and Monod

Lac Operon RNA Polymerase Binding Site (Promoter) Repressor Binding Site (Operator) Genes coding for Lactase

Lac Operon RNA Polymerase Binding Site (Promoter) Genes coding for Lactase RNA Polymerase Transcription of lactase can start when the RNA polymerase attaches to the DNA RNA Polymerase can only attach at the binding site

Transcription of Lactase Genes Genes coding for Lactase RNA Polymerase mRNA that codes for Lactase If Lactose is present, then the bacteria will transcribe the lactase genes that will make the enzyme to digest lactose

Lac Operon ä Usually the sugar lactose is not present ä The genes for Lactase should be turned off most of the time RNA Polymerase Binding Site Repressor Binding Site Genes coding for Lactase

Lac Operon ä Genes are turned off by blocking transcription RNA Polymerase Binding Site Repressor Binding Site Genes coding for Lactase

Lac Operon ä The presence of a repressor protein blocks the RNA polymerase binding site RNA Polymerase Binding Site Repressor Binding Site Genes coding for Lactase RNA Polymerase Repressor Protein blocks RNA polymerase

Lac Operon ä Lactose can act as an Inducer by deactivating the repressor protein RNA Polymerase can now bind Genes coding for Lactase can now be Transcribed RNA Polymerase Deactivated Repressor Protein Inducer (lactose)

Transcription of Lactase Genes Genes coding for Lactase RNA Polymerase mRNA that codes for Lactase If Lactose is present, then the bacteria will transcribe the lactase genes that will make the enzyme to digest lactose

Online Quiz ä Go to this site y/PH%20Biology%20Lab%20Simulations/l acoperon/intro.html y/PH%20Biology%20Lab%20Simulations/l acoperon/intro.html y/PH%20Biology%20Lab%20Simulations/l acoperon/intro.html and do the simulation ä Then take the self quiz –PRINT OUT to turn in tomorrow.

ä Operon – a group of genes and its control mechanism ä Promoter- RNA polymerase binding site ä Operator- Repressor protein binding site ä Repressor- when active can bind to operator to block RNA polymerase (turns OFF transcription) ä Lac genes- genes to digest lactose ä Lactose- milk sugar also the inducer ä Inducer- deactivates repressor and turns ON transcription.

Regulatory sites Promoter (RNA polymerase binding site) Start transcription DNA strand Stop transcription Typical Gene Structure Section 12-5

Eukaryotic Gene Regulation ä Promoter- RNA polymerase binding site ä Enhancers- DNA binding site for transcription factors ä Transcription factors – can help or hinder RNA polymerase from binding to the promoter, can help to unwind chromatin to allow access to DNA ä TATA box – helps position RNA polymerase by marking beginning of transcription ä Introns- non-coding sections of DNA ä Exons- codes for proteins

Transcription factors (TF) ä TF can bind to the TATA box and help the RNA Polymerase (POL) to bind to the promoter to start transcription. ä TF can also hinder transcription – hence they can control genes.

Activators ä Activators unwind the DNA molecule ä The DNA must be unwound to allow the RNA polymerase to bind to the promoter (RNA polymerase binding site). ä Even if the Repressor protein is deactivated, if the DNA is not unwound by the activator no transcription will take place - this is a second level of control over the gene.

Enhancers ä In eukaryotic cells, enhancer regions of the DNA help transcription

Enhancers ä Factors bound to an enhancer region of DNA help to bind initiation factors that activate the RNA polymerase. RNA Polymerase

Enhancers ä If an enhancer allows the RNA polymerase to bind more easily - more transcription will take place

RNA Interference ä RNA interference produces molecules that will breakdown mRNA, thus silencing the expression of a gene miRNA Silencing complex video

Homeotic Genes ä Master control genes that regulates the development of groups of organs in specific parts of the body

Homeobox genes ä Within the Homeotic gene there ä is a 180-base sequence which is ä called the homeobox. ä The homeobox gene controls for transcription factors that activate other genes that affect development and differentiation. Ex: control of wing or leg development. ä These same genes are found in many different organisms – controlling the same areas of the body.

HOX genes ä HOX genes determine basic body plan. ä come from a common ancestor ä HOX genes-a cluster of homeobox genes that control the differentiation of cells and tissue in each segment of the embryo – determines where body parts go. ä Differentiation – cells become specialized during development ä In eukaryotes only specific genes are turned on to control differentiation