REGULATION OF GENE EXPRESSION

Prokaryotes vs. Eukaryotes

Gene Expression

is the process by which information from a gene is used in the synthesis of a functional protein or other gene product such as tRNA or rRNA.

As you can imagine, not all genes are expressed at the same time in an organism.

Regulation of Gene Expression Both prokaryotic and eukaryotic cells,

have the ability to turn gene expression on or off depending on what the cell or organism needs at any given time.

Prokaryotic cells have evolved to be able to regulate gene expression in response to different environmental conditions.

Eukaryotes regulate gene expression to maintain different cell types, in embryonic development, etc.

Why Regulate Gene Expression? Bacterial cells that can conserve energy

have the selective advantage over cells that are unable to do so.

Natural selection has favored bacteria who express genes only when their products are needed by the cell.

Why regulate gene expression? E. coli can adjust their metabolism to

changing environments i.e. They can adjust the activity of enzymes

already present in the cell by Feedback inhibition where the product inhibits the first enzyme in a metabolic pathway.

Cells can adjust the production level of the enzyme and only produce it as needed.

The operon model was dicovered in 1961 The mechanism by which bacterial cells

can switch genes on or off Synthesis or metabolism of molecules

(metabolism) occurs in steps (pathway) Each reaction in a pathway is catalyzed

by a specific enzyme. The genes for the enzymes that work

together are usually clustered on the same chromosome.

The Lac Operon

Promoter – a DNA segment that signals the beginning of a gene (RNA polymerase attaches)

Operator – a DNA segment that controls the access of RNA polymerase to the genes

Operon – a DNA segment that includes a cluster of genes, the promoter and the operator

Repressor – a protein that binds to a specific operator, blocking attachment of RNA polymerase & thus turning the operon off

The Lac Operon

Regulatory genes – code for repressor proteins

The Lac Operon

Genes LacI – regulatory gene, codes for the lac

repressor LacZ – codes for βgalactosidase (hydrolysis

lactose) LacY – codes for permease (membrane

protein that transport lactose into the cell) LacA – codes for transacetylase ( transfers

an acetyl group from acetyl CoA to βgalactosidase but function unclear)

How it works

The dissacharide lacose is available to E.coli in the human colon if the host drinks milk.

Lactose metabolism begins with hydrolysis into ___ and _____.

This reaction is catalyzed by β-galactosidase.

In the absence of glucose, only a few molecules are present in the E.coli cell.

If lactose is added to the environment, the number of molecules of enzyme increase a thousandfold

E.coli uses 3 enzymes to metabolize lactose. The genes for these enzymes are all clustered together on a molecule of DNA in the lac operon .

How it Works

How it Works

Matching

1. β-galactosidase2. Allolactose3. Operator4. Promoter5. Regulator gene6. Repressor7. Gene in operon

A. Is inactivated when attached to allolactose

B. Codes for repressor protein

C. Hydrolyzes lactose

D. Repressor attaches here

E. RNA polymerase attaches here

F. Acts as an inducer that inactivates repressor

G. Usually codes for an enzyme

Prokaryotes vs. Eukaryotes

Control expression by transcription.

No Nucleus to separate transcription and translation

Nucleus provides opportunity for post transcription control.

Transcription RNA processing mRNA transport mRNA translation mRNA degradation Protein degradation

FIGURE 18.6Signal

NUCLEUSChromatin

Chromatin modification:DNA unpacking involvinghistone acetylation and

DNA demethylationDNA

Gene

Gene availablefor transcription

RNA ExonPrimary transcript

Transcription

Intron

RNA processing

Cap

Tail

mRNA in nucleus

Transport to cytoplasm

CYTOPLASM

mRNA in cytoplasm

TranslationDegradationof mRNA

Polypeptide

Protein processing, suchas cleavage and

chemical modification

Active proteinDegradation

of proteinTransport to cellular

destination

Cellular function (suchas enzymatic activity,structural support)

Control Points of Gene Expression Chromatin Modification Transcription RNA Processing (Post transcriptional) mRNA degradation (Post transcriptional) Translation Protein Degradation (Post translational)

Chromatin modification

Chromatin is the DNA – protein complex in eukaryotic cells

Each chromosome consists of 1 long DNA strand bound and wound around positively charges proteins called histones.

DNA is wound around 4 histones to form a nucleosome

Some areas of chromatin are coiled more tightly than others

FIGURE 18.7

Amino acidsavailablefor chemicalmodification

Histone tails

DNA double helix

Nucleosome(end view)

(a) Histone tails protrude outward from a nucleosome

Unacetylated histones Acetylated histones

Acetylation of histone tails promotes loose chromatinstructure that permits transcription

Genes within regions where chromatin is tightly coiled (heterochromatin) are usually not transcribed or expressed.

Histones can be modified to regulate gene expression

DNA can be modified to regulate gene expression

Histone tails protrude from the nucleosome and are accessible to enzymes that catalyse the addition or removal of certain chemical groups.

Acetyl, methyl, phosphate groups Acetylation – promotes transcription

(loosens chromatin) Methylation – prevents transcription

(tightens chromatin)

Epigenetic inheritance

Once methylated, genes usually stay that way through successive generations

Enzymes methylate daughter strands according to the template strand in each round of replication.

Chromatin modification such as methylation can be passed from generation to generation.

Epigenetics is the inheritance of traits transmitted by mechanisms not involving the nucleotide sequence.

Changes in nucleotide sequences (mutations)are permanent.

Changes in chromatin (methylation, acetylation) is reversible.

Alterations in normal patterns of DNA methylation are seen in some cancers (inappropriate gene expression)

Transcriptional Control

Includes transcription factors Enhancers Promoters Hormones

A Typical Eukaryotic Gene

Consists of a promoter sequence – a sequence where RNA polymerase binds and starts transcription

Introns Exons

Upstream from the promoter sequence are: Enhancer (distal control elements) Proximal Control Elements

RNA polymerase attaches to the promoter sequence

FIGURE 18.10-3

ActivatorsDNA

EnhancerDistal controlelement

PromoterGene

TATA box

Generaltranscriptionfactors

DNA-bendingprotein

Group of mediator proteins

RNApolymerase II

RNApolymerase II

RNA synthesisTranscriptioninitiation complex