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 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

GENE EXPRESSION 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 & enhancer regions 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 lac operon turned on? lactose in 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

trp operon 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 cells use 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 PACKAGING 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

DNA PACKAGING beaded strings are wrapped into tight helical fibers which in turn are coiled into supercoils looping & folding further compacts DNA

DNA PACKAGING 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