Regulating Gene Expression

Turning Genes On and Off

Procaryotes

Procaryotes are bacteria and blue green algae

They have no nucleus They have circular chromosomes made only

of DNA

Procaryotes

On procaryotic chromosomes there are many genes

We will concentrate on how the genes that code for lactase is controlled

Lactase is the enzyme that digests the milk sugar lactose

Lac Operon

The genes that code for lactase and its associated control mechanisms is called the Lac Operon.

The Lac Operon was first described by Jacob and Monod

Lac Operon

RNA Polymerase Binding Site (Promoter)

Repressor Binding Site (Operator)

Genes coding forLactase

Lac Operon

RNA Polymerase Binding Site (Promoter)

Genes coding forLactase

RNA Polymerase

Transcription of lactase can start when theRNA polymerase attaches to the DNA

RNA Polymerase can only attach at the binding site

Transcription of Lactase Genes

Genes coding forLactase

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

Usually the sugar lactose is not present The genes for Lactase should be turned off

most of the time

RNA Polymerase Binding Site

Repressor Binding Site

Genes coding forLactase

Lac Operon

Genes are turned off by blocking transcription

RNA Polymerase Binding Site

Repressor Binding Site

Genes coding forLactase

Lac Operon

The presence of a repressor protein blocks the RNA polymerase binding site

RNA Polymerase Binding Site

Repressor Binding Site

Genes coding forLactase

RNA Polymerase

Repressor Protein blocks RNA polymerase

Lac Operon

Lactose can act as an Inducer by deactivating the repressor protein

RNA Polymerase can now bind

Genes coding forLactase can now beTranscribed

RNA Polymerase

Deactivated Repressor ProteinInducer (lactose)

Transcription of Lactase Genes

Genes coding forLactase

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

Go to this site http://midpac.edu/~biology/Intro%20Biology/PH%20Biology%20Lab%20Simulations/lacoperon/intro.html

and do the simulation Then take the self quiz –PRINT OUT to

turn in tomorrow.

Operon – a group of genes and its control mechanismPromoter- RNA polymerase binding siteOperator- Repressor protein binding siteRepressor- when active can bind to operator to block RNA polymerase (turns OFF transcription)Lac genes- genes to digest lactoseLactose- milk sugar also the inducerInducer- deactivates repressor and turns ON transcription.

Regulatory sites

Promoter(RNA polymerase binding site)

Start transcription

DNA strand

Stop transcription

Typical Gene StructureSection 12-5

Eukaryotic Gene Regulation

Promoter- RNA polymerase binding siteEnhancers- DNA binding site for transcription factorsTranscription factors – can help or hinder RNA polymerase from binding to the promoter, can help to unwind chromatin to allow access to DNATATA box – helps position RNA polymerase by marking beginning of transcriptionIntrons- non-coding sections of DNAExons- codes for proteins

Transcription factors (TF)

TF can bind to the TATA box and help the RNA Polymerase (POL) to bind to the promoter to start transcription.

TF can also hinder transcription – hence they can control genes.

Enhancers

In eukaryotic cells, enhancer regions of the DNA help transcription

Enhancers

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

RNA Polymerase

Enhancers

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

RNA Interference

RNA interference produces molecules that will breakdown mRNA, thus silencing the expression of a gene

miRNA

Silencing complex

video

Homeotic Genes

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

Homeobox genes

Within the Homeotic gene there is a 180-base sequence which is called the homeobox. The homeobox gene controls for transcription

factors that activate other genes that affect development and differentiation. Ex: control of wing or leg development.

These same genes are found in many different organisms – controlling the same areas of the body.

HOX genesHOX genes determine basic body plan.come from a common ancestorHOX 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.Differentiation – cells become specialized during developmentIn eukaryotes only specific genes are turned on to control differentiation