Regulation of Gene Expression

Regulation of GeneExpression

Sec. 6.4 (p.267)

Control Mechanisms42,000 genes code for proteins in Humans

Are the proteins for all of these genes beingmade all the time??

No!

Why?? Gene Regulation• Proteins are needed at certain times (E.g. Hormones and proteins)• Do the islets of langerhans continually

produce insulin?

What would happen if genes were expressed all the time?

If genes were always expressed, theywould always make protein.

Example: Insulin• Insulin is only required in cells with high glucose

levels . • Insulin causes cells in the liver, muscle, and

fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle.

• If we produce too much insulin then too much glucose is taken out of the blood and you get low blood sugar.

Transcription Factors

Genes can be turned “on” or “off” by proteins thatswitch on genes by binding to DNA.• In turn, they help RNA polymerase to bind

RNA Polymerase can't find the promoter.

RNA Polym-erase

RNA Polym-erase

promotor

promotor

transcription factors act like flags for RNA Polymerase, helping it to find the promoter

Certain Transcription Factors bind to specific regions on the DNA.

What do you think are the functions of the following transcription Factor Types and DNA sequences?

Which ones do you think pair up and bind together?

Transcription Factor DNA SequenceActivator SilencerRepressor Enhancer Basal Level Transcription Factors TATA Box

Types of Transcription Factors

House Keeping Genes(AKA: Constitutive genes)

• Genes are turned ‘ON’ all the time –not regulated.

• Vital to an organism’s life• Examples??

Four Levels of Control

• The extent to which they are transcribed is regulated (transcriptional and

posttranscriptional)• The extent to which they are translated is

regulated (translational and postranslational)

Gene Regulation: OperonsGene regulation mechanisms in prokaryotes

Operon: A cluster of genes grouped togetherunder the control of one promotor.

• Each operon contains a promoter, operator and genes

General Structure Definitions

The operon consists of:Promoter• a section of DNA where RNA polymerase bindsOperator• a section of DNA that interacts with the repressor

protein to alter transcriptionRegulatory genes• code for the repressor proteinStructural genes• code for the target proteins

Lactose

• Type of Macromolecule?• Lactose makes up around 2-8% of the solids in

milk

• Consists of Glucose and Galactose

Lactose Intolerance

Undigested lactose remains in the gut and bacteria feed on it, causing the typical symptoms:

- stomach cramps- nausea- bloating- acid reflux- flatulence

Can't digest lactose because don't have enough lactase

lac Operon

Produces B-galactosidase- enzyme that breaks down lactose- remember: lactase breaks down lactose andB- galactosidase degrades it- produced after lactose has been ingested(therefore when lactose levels are high)

Inducible operon: lactose present induces transcription

trp Operon

Produces tryptophan- A non-essential amino acid- Produced when none has been ingested(when tryptophan levels are low)

Trp operon is normally active, unless a repressor turns it off.

• How is regulation in Eukaryotes different?• See pg 269-270

• Read section 6.4 & try question25,27,28,29 pg 269,

• Pg 272 questions;

Gene Regulation in Eukaryotes5 levels of control

pre-transcriptional

transcriptionalpost-transcriptionaltranslationalpost-translational

Histones and nucleosomeskeep DNA tightly wrapped

transcription factors mRNA modification RNA interference Protein modification of

shape or ubiquitin tag

Eukaryotic gene control

• Genes are not organized into operons• They require a much more elaborate control• Ie. many activators, and enhancer regions on

DNA to regulate gene control under certain conditions. Pg 270

• Homework

• More review if needed• Page 267-272 #2-8