1Control of Gene Expression

2Variation in cell morphology with the same genome

A neuron and alymphocyte sharethe samegenome

3An Overview of Gene Expression

The different cell types of a multicellular organismcontain the same DNA (experiments to prove) Different cell types produce different sets of proteins A cell can change the expression of its genes in

response to external signals Gene expression can be regulated at many of the

steps in the pathway from DNA to RNA to protein

4All cells contain all the genetic instructionsnecessary for a complete organism formation

5All cells contain all the genetic instructionsnecessary for a complete organism formation

6Different cell types produce differentsets of proteins

All proteinexpressed in aspecific cell typecan becharacterized byproteomic analysis

7Different cell types produce differentsets of proteins

All protein expressed in a specific cell type can becharacterized by proteomic analysis

Slide 14 of 23

8Gene expressed in cells

Housekeeping proteins To maintain basic living of a cell Protein for cytoskeleton, cell replication, protein synthesis

machinery, energy production and other basic metabolism

Specialized proteins Proteins for cells distinctive properties Ex., hemoglobin for reticulocytes, myosin for muscle cell,

pigment for skin and eyes,

The expression of a different collection of genes ineach cell type causes the large variations in size,shape, behavior and function of differentiated cells.

9Cells change the expression of theirgenes in response to external signals

Most of the specialized cells are capable ofaltering their patterns of gene expression inresponse to extracellular cuesHormones, temperature, chemicals, smallmolecules, pH,

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Gene expression can be controlledat several different steps

Main site of regulation

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How Transcriptional Switches Work?

Transcription iscontrolled by proteinsbinding to regulatory DNAsequences

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Transcriptional regulation

Usually at initiation step at promoter region andregulatory DNA sequences The size of regulatory sequence can range from

5 to 10000 base pairs which will be recognizedand bound by one or more proteins. Protein (factor) binds to DNA by interacting with

base pairs in major groove through hydrogenbond, ionic bonds and hydrophobic interaction.

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Protein bind to the major grooveof a DNA

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DNA binding motifs on DNA binding proteinactivator vs repressor

Gene regulatoryproteins contain avariety of DNA-binding motifs

(A) (B);Homeodomain(C) Zinc finger(D) Leucine zipper

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Transcription regulation inprokaryotes

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Concept of Operon in prokaryotes

Operona set of genes that is transcribed into a single mRNAworked and regulated coordinately

Only observed in prokaryotes Famous example: Lac operonGuanine operon Trp operon

Figure 8-24 Essential Cell Biology ( Garland Science 2010)

Riboswitch

A riboswitch controls purine biosynthesis genes in bacteria

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Tryptophan operon

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The constitutively expressed Trp operon can beswitched off with repressor proteins in the presenceof free Trp molecules (feedback regulation)

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Trp operon also regulated by attenuationat mRNA level

Trp operon regulated by attenuation

lac operon is controlled by two signalspresence of lactose vs. absence of glucose

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Activator CAP binds cyclic AMP before it can bind to DNA andinitiate transcription in response to low glucose environment

lac operon is controlled by two signalspresence of lactose vs. absence of glucose

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Transcription regulation ineukaryotes

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Differences of transcription ineukaryotes from prokaryotes

Three types of RNA polymerasesRequire general transcription factors to

initiate transcriptionRegulatory sequences could be far away

from promoter sequence Affected by chromosome structural

complexity

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I. RNA polymerases in eukaryotes

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II.

Transcriptioninitiation ineukaryotesrequires manyfactorsFactors assembling andRNA polymerasephosphorylation by TFIIH(protein kinase) to begin itstask of making RNA)

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Binding of TBP and distortion of DNA

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Phosphorylation of RNA polymerase

Phosphorylation of RNApolymerase II also allowsRNA-processing proteins toassemble on its tail.

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III. Gene activation can occur at a distancein eukaryotes through enhancer (DNA) andmediator (protein)

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IV. Activator or repressor direct localalterations in chromatin structure

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The molecular mechanisms thatcreate specialized cell types

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The molecular mechanisms for cellsdifferentiation

Eukaryotic genes are regulated by combinations ofproteins Expression of different genes can be coordinated by

a single protein Combinatorial control can create different cell types Stable patterns of gene expression can be

transmitted to daughter cells Formation of an entire organ can be triggered by a

single gene regulatory protein

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I. Factors work as acommitteetocontrol gene expression

Regulatoryproteins worktogether as acommittee to control theexpression ofan eukaryoticgene

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II. A single protein can coordinate expressionof many genes bycompletingthecombination of committee

Several genescan be turnon/offsimultaneouslyby completingthe complexeswith the samesingle protein

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A single gene regulatory protein iscapable of cell type conversion

Fibroblasts from theskin have beenconverted to musclecells byexperimentallyinduced expression ofthe myoD gene.

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Combinations of afew gene regulatoryproteins cangenerate manydifferent cell typesduring development.

III. Cell differentiation during development

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IV. Cell memory results from a positive feedbackloop to maintain the same phenotype for all itsprogenitor cells

A key generegulatory proteinactivates its owntranscription inaddition to othercell-type specificgenes.

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IV. Cell memory results from maintainingstates of chromatin structure

Figure 8-22 Essential Cell Biology ( Garland Science 2010)

DNA methylation patterns can be faithfully inherited

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Expression of the Drosophila ey gene in theprecursor cells of the leg triggers thedevelopment of an eye on the leg

Post-transcriptional control

Riboswitches Provide An Economical Solutionto Gene RegulationThe Untranslated Regions (UTR) of mRNAs

Can Control Their TranslationSmall Regulatory RNAs (miRNA) Control the

Expression of Thousands of Animal and PlantGenesRNA Interference (RNAi) Destroys Double-

Stranded Foreign RNAsScientists Can Use RNA Interference to Turn

Off Genes

Figure 8-25 Essential Cell Biology ( Garland Science 2010)

Controlled by regulating translation initiation

Figure 8-26 Essential Cell Biology ( Garland Science 2010)

miRNA

(microRNA)

Figure 8-27 Essential Cell Biology ( Garland Science 2010)

RNAi vs. siRNAs

(small interference RNAs)

(RNA interference)

vs

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How we know aboutgene regulation

The story of eve

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Molecules localized at the ends of the Drosophilaegg control its anterior-posterior polarity

The first demonstration of asymmetric distribution in cell (embryo)

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The early Drosophila embryo shows a nonuniformdistribution of four gene regulatory proteins

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A reporter gene reveals the modular constructionof the eve gene regulatory region

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The regulatory module for eve stripe 2 contains bindingsites for four different gene regulatory proteins

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The end