CONTROL MECHANISMS 5.5

Controlling Transcription and Translation of Genes  Housekeeping Genes: needed at all times: needed for life functions vital to an organism.  Constantly transcribed and translated.  not all proteins are needed at all times  Ex// insulin only need when glucose levels are high  Transcription factors: proteins that switch on genes by binding to DNA and helping the RNA polymerase to bind.  Gene Regulation: turning on or off specific genes depending on requirements of organism.  Vital to survival.

Control of Gene Expression in Eukaryotic Cells

Operons Operon: cluster of genes under control of a promoter and operator (prokaryotic). Acts as a simple regulatory loop.

Repressor Protein: regulatory molecule that binds to an operator site and prevents the transcription of an operon.

Two Major Operons (in E. Coli)  lac operon  Required for transport and metabolism of lactose  trp operon  Codes for the production of tryptophan  an amino acid.

lac operon (1)  Lactose: ____________ + ______________  E. Coli bacteria found in mammalian intestines can use energy from lactose for growth.  β-galactosidase: enzyme that catalyzes lactose.  When lactose not available, B-g is not made. When would B-g cease on being produced by E. Coli in a mammalian intestine? ______________________________________________

lac operon (2)  Consists of three genes:  lacZ: gene for the enzyme β-g.  LacY : gene for β-g Permease: allows lactose to be permeable to cell membrane.  lacA: unknown function.

lac operon: If Lactose not Present  Lacl protein: repressor protein of lactose metabolism  Binds to the lac operon operator, preventing RNA polymerase from transcribing the lac operon genes.  Operon and operator actually overlap, so RNA poly cannot bind to DNA.

lac operon: If lactose IS present  If lactose present, lacl must be removed from operator so lac operon genes can be transcribed.  Lactose: known as an ‘inducer.’ Binds to LacI protein  change conformation of LacI  cannot stay bound to operator region  no more block!

trp operon (1)  Tryptophan: an _______________ used by E.Coli to make ________________.  E. Coli in inside of mammalian intestine can absorb trp from diet. If no host, needs to produce trp.  5 genes responsible in the metabolic pathway of the trp operon.

Trp operon: when trp is not present  Inactive repressor is not blocking operator.  RNA polymerase can transcribe operon.

Trp operon: when trp is present  Tryptophan acts as a corepresser: binds to the repressor protein  conformation change  can bind to the operator  block transcription. (NO NEED FOR ENZYMES THAT MAKE TRP WHEN TRP IS ALREADY PRESENT!)

Seatwork/Homework  Page258, #1-6.