Regulation of gene expression

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Lac operon concept

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Regulation of gene expression

  1. 1. Regulation of geneexpressionDr. N.R.Hazari
  2. 2.  Gene: A DNA segment that contains theall genetic information required toencodes RNA and protein molecules. Genome: A complete set of genes of agiven species. Gene expression: A process of genetranscription and translation.
  3. 3.  Expression of many genes is relativelycontinuous. Some genes are expressed only under certain conditions, nutritional during differentiation and development after physiological stimulations(nervous,Hormonal etc) stressful situations. Thus there is mechanism by which thecells express or turn off certain genes.
  4. 4.  High cost of protein synthesis, regulationof gene expression is essential to makingoptimal use of energy. Regulation of gene expression isabsolutely essential for the growth,development, differentiation and veryexistence of an organism.
  5. 5.  There are two types of gene regulation 1. Positive Regulation 2. Negative regulation.
  6. 6. Positive regulation The expression of gene isincreased by the presence ofspecific regulatory element ,is saidpositive regulation. The molecule which exert positiveregulation is said to be positiveregulator or activator / inducer The process is called Derepressionor Induction
  7. 7. Negative regulation The expression of gene isdecreased by the presence ofspecific regulatory element ,issaid negative regulation. Molecule which exert the negativeregulation is said to be a negativeregulator or repressor. The process is called Repression.
  8. 8. Sites Gene regulation occurs at thetranscriptional, post-transcriptional,translational and post-translational level. Controlling gene expression is oftenaccomplished by controlling transcriptioninitiation in prokaryotes.
  9. 9.  The environmental and metabolic state ofthe cell has a direct and significant effect onthe control of gene expression. Usually small extracellular or intracellularmetabolites trigger the complex mechanisms. (stimulate or inhibit). All genes are not expressed at all time or inall tissues. Ex: Insulin gene expressed only inpancreatic cells but not in other tissues. Thatmeans insulin gene is in repression state inother tissue.
  10. 10. Type of gene expression Genes or gene expression areconsidered under two categories:1. Constitutive genes or expression: Some genes are essential andnecessary for life, and therefore arecontinuously expressed, such as thoseenzymes involved in TCA cycle. Thesegenes are called housekeeping genes.
  11. 11.  2. Inducible gene or expressionInduction and repressionThe expression levels of somegenes fluctuate in response to theexternal signals.Ex: Tryptophan pyrrolase inducedby tryptophan.
  12. 12. Prokaryotic gene regulation. Prokaryotes provide models for the study ofgene expression in mammalian cells. Some features of prokaryotes geneexpression are unique. In prokaryotes, genes involved in metabolicpathway are often present in linear arraycalled as OPERON. ( mRNA are polycistronic) Multiple genes are present on singletranscript and a single promoter initiates thetranscription all genes.
  13. 13. OPERONFrancois Jacob & Monod in 1961 firstdescribe operon model in E-coli.An operon is a groupof genes that aretranscribed at thesame time.They usually controlan importantbiochemical process.They are found only inprokaryotes.
  14. 14.  Their theory was based on observations oflactose metabolism in E-Coli. When E-Coli cells grown in glucosemedium cells don’t contain β-galactosidaseenzyme. But when cells are transferred to mediumcontain only lactose, then β-galactosidaselevel increases. β-galactosidase hydrolyses the lactose intogalactose & glucose.
  15. 15. Operon The gene for this enzyme isclustered with other two geneswhich produce two enzymes whichare involve in lactose metabolismwith β-galactosidase, i.e.galactoside permease andthiogalactoside transacetylase.
  16. 16.  Genes product involve in the samepathway and initiate the transcription ofthese genes by single promoter andregulate by a single gene called operons. Some operons induced when metabolicpathway needed.For prokaryotic systems:Operon is composed of structural genes,promoter, operator, and other regulatorysequences.
  17. 17. AYZOPIstructural genepermeaseregulatory siteoperatorpromoterCAP-binding siteregulatory genetransacetylaseβ galactosidaseLac Operon
  18. 18. Metabolism of lactose
  19. 19. Adapting to the environment E. coli can use either glucose, which is amonosaccharide, or lactose, which is adisaccharide. However, lactose needs to be hydrolysed(digested) first. So the bacterium prefers to use glucose whenit can present and its genes are constitutive.
  20. 20. Regulation of Lac -operon Four situations are possible1. When glucose is present and lactose is absentthe E. coli does not produce β-galactosidase.2. When glucose is present and lactose is presentthe E. coli does not produce β-galactosidase.3. When glucose is absent and lactose is absentthe E. coli does not produce β-galactosidase.4. When glucose is absent and lactose is presentthe E. coli does produce β-galactosidase
  21. 21. Regulation of lac-operon : glucose ispresent and lactose is absent IN the absences of lactose, the cell hasno need to produce lactose metabolizingenzymes. Hence lac repressor protein , preventsexpression of genes of lactosemetabolism.
  22. 22. AYZOPImRNARNA polSituation I
  23. 23. Situation IIWhen lactose is present, lacZ, lacY, and lacAgenes are expressed.AYZOPIrepressormRNAlactoseallolactoseRNA polgalactosidase
  24. 24. Situation IIIAYZOPI RNA polWhen glucose is present, the [cAMP] is low, no CAP-cAMP isformed and the expression of the lac operon is still low.
  25. 25. Situation 4AYZOPICAPcAMPRNA polWhen glucose is absent and lactose is present, the CAP-cAMP complex binds to the CAP site to activate the lac gene.
  26. 26. CarbohydratesActivatorproteinRepressorproteinRNApolymeraselac Operon+ GLUCOSE+ LACTOSENot boundto DNALifted offoperator siteKeeps fallingoff promotersiteNotranscription+ GLUCOSE- LACTOSENot boundto DNABound tooperator siteBlocked bythe repressorNotranscription- GLUCOSE- LACTOSEBound toDNABound tooperator siteBlocked bythe repressorNotranscription- GLUCOSE+ LACTOSEBound toDNALifted offoperator siteSits on thepromotersiteTranscriptionSummary
  27. 27. THANK YOU!RNA pol

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