module 9 Flashcards
What is an operon?
- in prokaryotes, a group of bacterial structural genes
- under the control of a single protein
- produces a single mRNA strand that produces different proteins
- all transcribed together
- regulates the expression of genes by controlling transcription
Describe the parts of the typical operon
- promoter and regulator = codes for a regulatory gene (either positive or negative which will bind to the operon)
- promoter and operator and gene - translates into proteins = regulator molecules (which are inducible/repressible)
what is the short definition of an operon?
- a group of structural genes plus sequences that control transcription
What is the role of the regulator?
- regulator works to control the amount of expression of a gene by increasing or decreasing its transcription
- despite playing a part in the operon it is not considered a part of the operon
what are the two types of transcriptional control?
- negative control: regulator protein binds to the operator and inhibits transcription
- positive control: regulator protein binds to the operator and initiates transcription
What is the regulator molecule called?
- a metabolite: a product or input of the metabolic pathway
What are the two forms of operons?
- inducible: transcription is generally off: regulator molecule binds to regulator protein and initiates transcription
- repressible: transcription is generally on
- regulator molecule binds to regulatory protein and turn transcription off
what is the process called when the regulator molecule binds to the regulator protein?
- allosteric inhibition: regulator molecule binds to allosteric site of the repressor, DNA binding site changes shape and DNA can no longer bind!
What are the two sites on the repressor called?
- DNA binding site and the allosteric site
What is the lac operon of E.coli? Who discovered it?
- the lac operon is an example of a negative inducible operon
- Jacob / monod first to describe operon model for the genetic control of lactose metabolism in E.Coli
Describe lactose metabolism
- Permease brings lactose molecules into the cell
- B-Galactosidase converts lactose into glucose and galactose
- B-galactosidase also converts lactose in allolactose which is then converted in glucose and galactose
Which enzymes are required for lactose metabolism?
- permease and B-galactosidase
Which enzymes have a common promoter?
- permease, B-galactosidase, transacetylase are transcribed in adjacent genes in the lac operon: promoter is lac-P
- Permease: Lac Y
- B-galactosidase: lac Z
- transacetylase: lac A
how did Jacob and monod determine the structure and function of the lac operon?
- by analyzing mutations that affected the metabolism of the E.Coli
- used haploid strains of E.Coli
- mutations on the bacterial DNA and the plasmid showed that some parts of the lac-operon were cis acting and some parts were transacting (control gene expression on OTHER DNA molecules)
what are the types of lac operon mutations ?
- operator mutations
- promoter mutations
- regulator-gene mutations
- structural mutations
Describe structural gene mutations
- mutations on Lac Z and Lac Y structural genes altered the amino acids and affected structure of proteins
- independany, usually only affected the product of the mutated genes
- b-galactosidase and permase produced normally in the presence of lactose
Describe regulator gene mutations
lacI+ is trans dominant: which means that it can work on other DNA molecule too
- when lactose is absent, it can bind to both operators and repress transcription
- when lactose present, genes are transcribed as repressor inactivated
LacI s is a trans dominant Super repressor that does not bind lactose: blocks transcription at both operators ALWAYS (when lactose present and absent)
Describe the operator mutations
- lacOc is cosnittuive operon: cis acting, consiutive: means it’s always on!
Describe promoter mutations
- designated lacP- interferes with the binding or RNA polymerase to the promoter (cis-acting)
- E.coli strains with lace-mutations don’t produce lac proteins either in presence or absence of lactose
What is catabolite repression?
- the idea that efficient transcription of the lac operon only occurs when glucose is absent or low and lactose is present
- this is because glucose is preferred thanks to the low energy required to metabolize it, so when it is available other genes that metabolize other sugars are shut down in catabolic repression
What is a structural gene? -
- any gene that doesn’t code for a regulator: essentially any protein coding gene
Are the lac operons found in prokaryotes or eukaryotes?
- prokaryotes (bacteria: E.Coli)
