Control MechanismsThere are 42 000 genes that code for proteins in

humansHowever, not all proteins are required at all times.

E.g. Insulin is only required in a cell when glucose levels are high. It would be inefficient and wasteful for a cell to transcribe,

translate the insulin gene when glucose levels are low

Regulation is therefore vital to an organism’s survivalRegulation – the turning on or off of specific genes

depending on the requirements of an organism

Why Turn Genes On and Off?Cell Specialization

each cell of a multicellular eukaryote expresses only a small fraction of its genes

Developmentdifferent genes needed at different points in life cycle

of an organism afterwards need to be turned off permanently

Responding to organism’s needscells of multicellular organisms must continually turn

certain genes on & off in response to signals from their external & internal environment

The control of gene expression can occur at any step in the pathway from gene to functional protein

The focus of today’s lesson will be on the regulation of gene expression at the transcriptional level.

Figure 19.3

Signal

NUCLEUS

Chromatin

Chromatin modification:DNA unpacking involvinghistone acetylation and

DNA demethlation

Gene

DNAGene availablefor transcription

RNA Exon

Transcription

Primary transcript

RNA processing

Transport to cytoplasm

Intron

Cap mRNA in nucleus

Tail

CYTOPLASM

mRNA in cytoplasm

Degradationof mRNA

Translation

Polypetide

CleavageChemical modificationTransport to cellular

destination

Active protein

Degradation of protein

Degraded protein

Gene ExpressionGene expression for all genes falls into one of two

categories. (prokaryotes and eukaryotes)

1. constitutive expression – genes which are always turned on

known as housekeeping genes

2. induced expression – genes which are only turned on as needed

lac Operonlac - lactose

operon – several genes in a sequence all controlled by a single promoter (mainly in prokaryotes; some eukaryotes). It also includes an operator which is the “on” and “off” switch.

promoter gene 1 gene 2 gene 3

operator

lac OperonCells mainly use glucose as a source of energy.

The lac operon is only turned on when glucose is absent, but lactose is present.

lac Operon Animation

lac Operon DetailsOperon codes for 3 enzymes – found in E. coli

1. beta-galactosidase (lacZ gene) enzyme which breaks down lactose

2. permease (lacY gene) protein transporter which brings lactose into cell

3. transacetylase (lacA gene) adds acetyl group to galactose

Repressor Proteinrepressor protein (lacI gene)

transcribed by a different gene from the lac operonbinds to the operator portion in the presence of glucoseprevents RNA polymerase from transcribing genes

when bound to operator Lactose is not needed as an energy source because glucose

is present

When do you want the repressor to bind / not bind to the operon?

When lactose is absent:enzymes are not needed to metabolise lactoserepressor binds to the operator to inhibit transcription

Figure 18.22a

DNA

mRNA

ProteinActiverepressor

RNApolymerase

NoRNAmade

lacZlacl

Regulatorygene

Operator

Promoter

Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator.

(a)

5’

3’

When lactose is present:Lactose (or allactose) binds to the repressor proteinlactose-repressor complex cannot bind to the operatortranscription can proceed

mRNA 5'

DNA

mRNA

Protein

Allolactose(inducer)

Inactiverepressor

lacl lacz lacY lacA

RNApolymerase

Permease Transacetylaseβ-Galactosidase

5’

3’

(b)Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced.

mRNA 5’

lac operon

Figure 18.22b

Effector MoleculesSince lactose is the molecule that determines when

the operon is turned on or off, it is known as an effector molecule.

effector molecule – any molecule that can regulate the activity of a protein

inducer – effector molecule that binds repressor protein to cause it to fall off operator

DNA

mRNA

ProteinActiverepressor

RNApolymerase

NoRNAmade

lacZlacl

Regulatorygene

Operator

Promoter

Lactose absent, repressor active, operon off. (a)

5’

3’

mRNA 5'

DNA

mRNA

Protein

Allolactose(inducer)

Inactiverepressor

lacl lacz lacY lacARNApolymerase

5’3’

(b) Lactose present, repressor inactive, operon on.

mRNA 5’

lac operon

lac Operon Animation

lac Operon Animation

trp Operontrp – tryptophan

The genes of the trp operon are used to make the amino acid tryptophan.

It is turned off when enough tryptophan is in the cell.

Tryptophan is the effector molecule.

trp Operon

Operon codes for 5 genes – found in E. coli

Five polypeptides combine to make three enzymes.each enzyme participates in a step to make tryptophan

Repressor Proteinrepressor protein (trpR)

transcribed as a different gene from trp operonbinds to operator when tryptophan is presentprevents RNA polymerase from transcribing genes

when bound to operator

When do you want the repressor to bind / not bind to the trp operon?

When tryptophan needs to be made:enzymes are required to make tryptophanrepressor is NOT bound to operatortranscription can proceed

(a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes.

Genes of operon

Inactiverepressor

Protein

Operator

Polypeptides that make upenzymes for tryptophan synthesis

Promoter

Regulatorygene

RNA polymerase

Promoter

trp operon

5’

3’mRNA 5’

trpDtrpE trpC trpB trpAtrpRDNA

mRNA

E D C B A

When cell has enough tryptophan:tryptophan binds to repressorrepressor can now bind operator to prevent

transcriptionDNA

mRNA

Protein

Tryptophan(corepressor)

Active repressor

No RNA made

Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein.

(b)

RNA Polymerase

Effector MoleculeSince tryptophan is the molecule that determines when

the operon is turned on or off, it is known as an effector molecule.

corepressor – effector molecule that binds repressor protein to cause it to bind to the operator

(a) Tryptophan absent, repressor inactive, operon on.

Genes of operon

Inactiverepressor

Protein

Operator

Polypeptides that make upenzymes for tryptophan synthesis

Promoter Promoter

trp operon

5’

3’5’

trpDtrpE trpC trpB trpAtrpRDNA

mRNAE D C B A

DNA

mRNA

Protein

Tryptophan(corepressor)

Active repressor

No RNA made

Tryptophan present, repressor active, operon off.

(b)

http://www.youtube.com/watch?v=8aAYtMa3GFU

Classwork/HomeworkSection 5.5 Questions pg. 258 #1-6