Lesson 2.3 - lac operon Flashcards
What is the flow of information?
DNA to mRNA to protein (polypeptide)
What are genes?
DNA sequence that codes for a product (include protein, rRNA, tRNA)
What does RNA polymerase look for on mRNA?
An available 3’ -OH
mRNA is ___ and ___ to the template strand
complementary; antiparallel
______ result in polycistronic mRNA
Operons
Why use operons?
- Single switch can turn on or off a cluster of functionally related genes
- Saves time and energy, makes lots of protein
- Bacteria & Archaea, rarely Eukarya
mRNA has ribosome binding sites in _____
5’ UTR
Define polysome.
Multiple ribosomes on a single mRNA
Role of homeostasis
- Maintain internal stability by expressing and repressing different genes
What does the lac operon create?
- Enzymes needed for catabolism of lactose
- Disaccharide aka milk sugar
What is the default state of the lac operon?
Off; only expressed when needed (inducible)
Function of lacZ
- Codes for ß-galactosidase
- Cuts lactose
Function of lacY
- Codes for permease
- Membrane transport protein for lactose
Function of lacA
- Codes for transacetylase
- Enzyme that transfers acetyl group (C-C) from acetyl-COA to ß-galactosides
- Function in operon unclear
(2) Functions of ß-galactosidase
- Cuts lactose to galactose & glucose
- Enter glycolysis
- Converts lactose to allolactose
- Inducer of lac operon
What is the lacI gene?
- Located on side of chromosome; not next to lac protein
- Codes for repressor protein LacI
What is the LacI protein?
- Binds to operator sequence & prevents binding of RNA poly. & transcription of lacZ,Y, and A
- Fits major groove of operator
- Head pieces bind succssive turns
- Inducer changes orientation of head piece (allosteric site)
What occurs when lactose is absent?
The lac operon is OFF. Repressor protein stops 96% of transcription.
What occurs when lactose is present?
- LacI (repressor) detaches and allows RNA poly. to initiate transcription.
- Lactose converted to allolactose (inducer) and binds to repressor protein.
Lac operator (Olac)
- Overlaps promoter; binding site for repressor
- Inverted repeat (snug fit)
- Match b/t operator & repressor
Catabolite repression
Turning off of genes due to plenty of glucose (E. coli prefers glucose before other C sources)
What is cAMP?
- Binds to cAMP receptor protein (CAP) & hyper-stimulates lac operon transcription
- CAP - cAMP complex binds to promoter
- Increased when [glucose] decreases
When glucose is low, positive control of lac operon results in…
Increase in transcription when cAMP - CAP complex binds to prmoter (unless repressor is bound to operator)
When glucose is high, positive control of lac operon results in…
catabolite repression; glucose inhibits cAMP synthesis
Conditions affecting homeostasis (table)
