Chapter 12 DNA & RNA

FOOTHILL HIGH SCHOOL SCIENCE DEPARTMENT

Chapter 12DNA & RNA

Section 12 – 5Section 12 – 5Gene RegulationGene Regulation


Gene Regulation

How Does A Cell Know?How Does A Cell Know?

Which Gene To Which Gene To ExpressExpress

&&

Which Gene Should Stay Which Gene Should Stay SilentSilent??


Gene Regulation

• When a Gene is When a Gene is ExpressedExpressed::– It Is Transcribed Into mRNAIt Is Transcribed Into mRNA

• When a Gene is When a Gene is SilentSilent::– It Is Not TranscribedIt Is Not Transcribed


Gene Regulation

• Expression Regulated ByExpression Regulated By– PromotersPromoters

• RNA Polymerase Binding SitesRNA Polymerase Binding Sites• Certain DNA Base Pair SequencesCertain DNA Base Pair Sequences

– StartStart & & StopStop Base Pair Sequences Base Pair Sequences

– Regulatory SitesRegulatory Sites• DNA Binding ProteinsDNA Binding Proteins• Regulate TranscriptionRegulate Transcription


Gene Regulation


Prokaryote Gene Regulation:lac Operon

• What is an What is an OperonOperon??• Group of Genes That Operate TogetherGroup of Genes That Operate Together

• For Example:For Example:– E. coliE. coli ferments lactose ferments lactose

• To Do That It Needs Three Enzymes To Do That It Needs Three Enzymes (Proteins), It Makes Them All At Once!(Proteins), It Makes Them All At Once!

– 3 Genes Turned On & Off Together. This is 3 Genes Turned On & Off Together. This is

known as the known as the lac Operonlac Operon (lac(lactosetose Operon) Operon)


Gene Regulation: lac Operon

The The laclac Operon Operon– Regulates Lactose MetabolismRegulates Lactose Metabolism– It Turns On Only When It Turns On Only When Lactose Is PresentLactose Is Present & &

Glucose is AbsentGlucose is Absent..

Lactose is a Lactose is a DisaccharideDisaccharide– A Combination of Galactose & Glucose A Combination of Galactose & Glucose

To Ferment Lactose To Ferment Lactose E. coliE. coli Must: Must:1.1. Transport Lactose Across Cell MembraneTransport Lactose Across Cell Membrane

2.2. Separate The Two SugarsSeparate The Two Sugars



Each Task Requires A Specific ProteinEach Task Requires A Specific Proteinbutbut

Proteins Not Needed If Glucose Present Proteins Not Needed If Glucose Present (why waste energy if you already have food?)(why waste energy if you already have food?)

sosoGenes Coding For Proteins Expressed Genes Coding For Proteins Expressed

Only When There Is Only When There Is No GlucoseNo Glucose Present But Present But Lactose Is PresentLactose Is Present



= Lactose

ADD LACTOSE



Key ConceptKey Concept::

The The laclac Genes Genes Are: Are:Turned OffTurned Off By Repressors By Repressors

AndAnd

Turned OnTurned On By The Presence By The Presence Of LactoseOf Lactose


lac Gene Expression

• Operon Has 2 Operon Has 2 Regulatory Regulatory RegionsRegions– PromoterPromoter (RNA (RNA

Polymerase Polymerase Binding)Binding)

– OperatorOperator (O (O region) Bound To region) Bound To A A lac Repressorlac Repressor


lac Gene Expression

• laclac Repressor Repressor– When Bound To When Bound To O O

RegionRegion : Prevents : Prevents Binding of RNA Binding of RNA Polymerase To Polymerase To PromoterPromoter

– Turns The Turns The OperonOperon ““OFFOFF””


lac Gene Expression

• lac Repressorlac Repressor Also Also Binds To LactoseBinds To Lactose– Higher Affinity For Higher Affinity For

LactoseLactose• When Lactose PresentWhen Lactose Present

lac Repressorlac Repressor Is Is Released From Released From O RegionO Region– Allows Transcription Allows Transcription

of All Three Genesof All Three Genes


Regulation Can Be:

1.1. Based On RepressorsBased On Repressors

2.2. Based On EnhancersBased On Enhancers

3.3. Regulated At Protein SynthesisRegulated At Protein Synthesis


Eukaryotic Gene Regulation

OperonsOperons Usually UsuallyNOTNOT Found In Eukaryotes Found In Eukaryotes

Key ConceptKey Concept::Most Eukaryotic Genes Are Most Eukaryotic Genes Are Controlled Controlled

IndividuallyIndividually And Have Regulatory And Have Regulatory Sequences That Are Sequences That Are Much More Much More ComplexComplex Than Prokaryotic Gene Than Prokaryotic Gene RegulationRegulation



• TATA BoxTATA Box– About 30 Base Pairs LongAbout 30 Base Pairs Long

– Found Before Most GenesFound Before Most Genes

– Positions RNA PolymerasePositions RNA Polymerase

– Usually Usually TATATATATATA or or TATAAATATAAA– PromotersPromoters Usually Occur Just Before Usually Occur Just Before

The The TATA BoxTATA Box


Eukaryotic Promoters

Enhancer SequencesEnhancer Sequences– Series of Short DNA SequencesSeries of Short DNA Sequences– Many TypesMany Types

Enormous Number Of Proteins Can Bind Enormous Number Of Proteins Can Bind To Enhancer SequencesTo Enhancer Sequences– Makes Eukaryote Enhancement Very Makes Eukaryote Enhancement Very

ComplexComplex


Eukaryotic Promotors

• Some Enhance Transcription By Some Enhance Transcription By Opening Up Packed ChromatinOpening Up Packed Chromatin

• Others Attract RNA PolymeraseOthers Attract RNA Polymerase

• Some Block Access To GenesSome Block Access To Genes

• Key To Cell SpecializationKey To Cell Specialization– All Cells Have Same ChromosomesAll Cells Have Same Chromosomes– Some Liver, Skin, Muscle, etc.Some Liver, Skin, Muscle, etc.


Regulation & Development

• hox Geneshox Genes– Control Organ & Tissue Development Control Organ & Tissue Development

In The EmbryoIn The Embryo

– Mutations Lead To Major ChangesMutations Lead To Major Changes• Drosophila With Legs In Place of Drosophila With Legs In Place of

AntennaeAntennae



hox Geneshox Genes Present In All Eukaryotes Present In All Eukaryotes– Shows Common AncestryShows Common Ancestry

–Pax 6 hox genePax 6 hox gene• Controls eye growth in Drosophila, Mice Controls eye growth in Drosophila, Mice

& Man& Man

• Pax 6Pax 6 from Mouse Placed In Knee from Mouse Placed In Knee Development Sequence Of Drosophila Development Sequence Of Drosophila Developed Into Eye Tissue.Developed Into Eye Tissue.

Common Ancestor >600M Years AgoCommon Ancestor >600M Years Ago

Chapter 12 DNA & RNA

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