Regulation of gene expression
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REGULATION of GENE EXPRESSION. GENE EXPRESSION. all cells in one organism contain same DNA every cell has same genotype phenotypes differ skin cells have different structure & function from muscle cells. GENE EXPRESSION. differences -due to differences in gene expression

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  • all cells in one organism contain same DNA

  • every cell has same genotype

  • phenotypes differ

  • skin cells have different structure & function from muscle cells


  • differences -due to differences in gene expression

  • some genes are turned on

  • others are turned off in different cells

  • functionally eliminates particular cell from doing certain functions

  • cell cannot make proteins needed to do certain functions


  • expression of most genes is controlled at transcription

  • some genes are actively transcribed

  • others remain quiescent

  • some function at all times

  • 30,000 are expressed in nearly all cell types

  • housekeeping genes

    • carry out basic metabolic processes

  • called constitutive

  • other genes are regulated

    • turned on or off as needed

Transcription Factors

  • proteins which bind to promoter & enhancerregions of DNA to turn on (or off) genes

  • ability to be turned on is inducible

  • ability to be turned off is repressible

  • genes are most often regulated as a group

  • located next to one another on a chromosome

  • these genes along with their regulatory sequences of DNA are called an operon

The Lac Operon

  • E. coli cells

    • use different sugars for energy

    • glucose & lactose

    • ability to use lactose requires special enzymes

    • transacetylase

    • lactose permease

    • beta-galactosidase

  • genes for these enzymes are found on a single unit-operon

The Lac Operon

  • tells cell machinery to make or not to make enzymes

  • Consists of genes that make enzymes , promoter & operator-control sequences

  • promoter region

    • transcription enzyme-RNA polymerase attaches

    • begins transcription

  • operator

    • functions as switch

    • determines if RNA polymerase can attach to promoter region

Lac Operon

  • transcription of 3 enzymes is repressed-turned off by repressor protein

    • binds to operator

    • blocks attachment of RNA polymerase

    • regulatory gene located outside operon codes for repressor

  • regulatory gene is expressed all the time

  • if regulatory gene is always being transcribed

  • there is always repressor protein to stop transcription of enzymes needed to use lactose

  • How is lacoperon turned on?

  • lactosein environment

Lac Operon

  • lactose binds to repressor protein changes its shape.

  • new shape means it cannot bind to active site of operatorsite is turned on

  • RNA polymerase attaches

  • transcription of enzymes needed to metabolize lactose begins

  • genes that code for enzymes that lets cell use lactose are made only when lactose is present

  • induction

    • presence of a small molecule causes enzymes to be made


  • bacteria

  • repressor-inactive alone

  • to be active combines with specific small molecule

  • that small molecule is amino acid- tryptophan

  • E. coli can make tryptophan using enzymes in trp operon but if tryptophan do not make their own

  • tryptophan binds to repressor

  • activates repressor

  • turns off operon

  • when tryptophan is not present repressor is not active operon is turned ontryptophan is made

Repressor Operon

  • arginine is an essential amino acid

  • when plentifule. coli cellsuse it

  • arginine not presente. coli must make it

  • requires enzymes

  • mechanism allows e. coli cells to save cellular resources by shutting genes off for particular substance when substance is available

Gene Regulation in Eukaryotes

  • cells differ in appearance & function

  • inherit same, complete set of genetic information

  • differences in appearance & function is not due to different genes

  • differences due to genes being turned on or off

  • cells performing particular functions are termed specialized

  • during development cells differentiate & stay differentiated

  • terminally differentiated

Gene Expression-Eukaryotes

  • begins at chromosome level

  • DNA in one chromosome is about 4 cm long

  • entire amount can fit into nucleus because of way it is packaged


  • DNA helix is wound around small proteins- histones

  • DNA-histone complex looks like beads on a string

  • each bead-nucleosome

  • segment of DNA wound around 8 histones

  • short DNA segments-linkers make up string part between nucleosomes


  • beaded strings are wrapped into tight helical fibers

  • which in turn are coiled into supercoils

  • looping & folding further compacts DNA


  • extreme packaging is important in gene regulation

  • prevents gene expression by preventing transcription proteins from contacting DNA

  • some regions-heterochromatin

  • so condensed-never transcribed

    • 10% of genome

  • remainder of complex- euchromatin

  • less condensed

  • can be transcribed

  • 10% is active at any given time

Fine Control of Transcription in Eukaryotic Cells

  • fine tuning is done with control of RNA synthesis-transcription

  • most important way of regulating gene expression

Control of Transcription in Eukaryotic Cells

  • regulatory proteins bind to DNA to turn transcription of genes on & off

  • each eukaryotic gene has its own promoter & other control sequences

  • Activator proteins are more important in eukaryotic cells than in prokaryotic cells

  • in most eukaryotic organisms genes are turned off

  • small percentage of genes must be turned on for any one particular cell to make proteins required to carry out its particular job

Control of Transcription in Eukaryotic Cells

  • regulatory proteins in eukaryotic cells are transcription factors

  • required for RNA polymerase to transcribe DNA

Control of Transcription in Eukaryotic Cells

  • first step in gene transcription is binding of transcription factors to DNA sequences-enhancers

    • usually far away from genes they regulate

  • binding of activators to enhancers causes DNA to change shape

  • it bends

  • with bending bound activators can interact with transcription factor proteins which act as a complex at promoter area of gene

  • this complex promotes attachment of RNA polymerase to promoter transcription begins

  • there are also repressor proteins- silencers

  • inhibit transcription

Splicing & Regulation

  • transcription of DNA  mRNA

  • used to make a specific protein by translation

  • mRNA can be regulated by splicing

Splicing & Regulation

  • during splicing certain segments of RNA are eliminated

  • the way a piece of mRNA is spliced giving rise to different types of mRNA

  • gives rise to different proteins

Regulation of Translation

  • after mRNA has been fully processed and is in the cytoplasm other regulatory processes may occur

  • mRNA breakdown

  • initiation of translation

  • protein activation

  • protein breakdown

mRNA Breakdown

  • mRNA molecules do not stay intact forever

  • broken down by enzymes

  • time of breakdown is important

  • regulates amount of protein that is made

  • longer living mRNAs can make more protein

Initiation of Translation

  • many proteins control initiation of translation of RNA

  • in red blood cells, translation does not occur unless heme is present

Protein Activation

  • after translation is complete proteins often need altering to become functional

  • many made as proenzyme

  • Inactive

  • cleaving part of protein makes it functional

Protein Breakdown

  • proteins can be broken down after a short or after a long time

  • broken down after short timehave limited time to carry out functions

  • may be important in short term regulatory activity in cells

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