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 operatorsite 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 ontryptophan is made

Repressor Operon• arginine is an essential

amino acid• when plentifule. coli

cells use it• arginine not presente.

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 timehave limited time to carry out functions

• may be important in short term regulatory activity in cells