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!)

http://www.youtube.com/watch?v=oBwtxdI1zvk

Seatwork/Homework

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