24 terms

Ch. 18 - Regulation of Gene Expression

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Prokaryotes
- 1 promoter --> many genes
- 1 mRNA can result in multiple polypeptides, meaning multiple RNA sites
Operon
cluster of genes all controlled by same promoter
Eukaryotes
- 1 promoter --> 1 gene
- each gene is controlled by its own promoter
Two levels of control
1. regulate activity of metabolic enzymes
2. regulate expression of genes that encode metabolic enzymes
Operator
"on-off" switch, negatively regulated by a repressor protein
Repressor
inhibits expression of one or more genes by binding to the operator or associated silencers
Promoter
RNA polymerase and transcription factors bind here to begin transcription, result is no mRNA, no polypeptide product- serve mutation
Structural genes
encode metabolic enzymes (operator has)
Inducible operon
usually off, molecule called an inducer inactivates the repressor and turns on transcription (lac)
Lac operon
- contains genes that encode enzymes used to synthesize transcription
- inducible, catabolic
- If cell lacks lactose, cell turns lac operon OFF
~ lacI repressor binds lac operon's operator, thereby preventing binding of RNAP to the promoter
~ lac Z, Y, A genes are not transcribed
~ lactose absent: repressor active, operon off, no transcription, no lac mRNA synthesized
- If cell contains lots of lactose, the cell turns lac operon ON
- Inducer allolactose binds the lacI repressor causing it to change shape. The altered conformation of the repressor prevents it from binding the operator. Therefore, RNAP binds the promoter and transcribes lac Z, Y, A genes
~ Lactose present: repressor inactive, operon on, low level transcription, little lac mRNA synthesized, if lactose is present then allocates is present
Repressible operon
usually on, binding of repressor to the operator shuts off transcription (trp)
Trp operon
- contains genes that encode enzymes used to synthesize tryptophan
- If cell lacks tryptophan, the cell turns the trp operon ON
~ In the absence of the corepressor tryptophan, the trp r repressor cannot bind the operator of the trp operon. Therefore RNAP binds the promoter and initiates transcription
- If cell contains plenty of tryptophan, the cell turns the trp operon OFF
~ Tryptophan acts as a corepressor and binds the trp r repressor. Association of the repressor and corepressor allows the repressor to bind the operator of the trp operon, thereby preventing RNAP from binding the promoter and initiating transcription
Allosteric
can be induced to change shape
Adenylylcyclase
converts ATP to cAMP
- negatively regulated by glucose
- if there's lots of glucose then there's not very much cAMP
- glucose is preferred (if has both lactose and glucose, would prefer to use glucose)
CAP-cAMP
positively regulates the lac operon
- Lactose present, glucose scarce (cAMP level high): repressor inactive, operon on, positive regulation, high level transcription, abundant lac mRNA synthesis
~ In absence of glucose, CAP-cAMP binds the regulatory region of the lac operon- increases efficiency of RNAP to transcribe genes lac Z, Y, A
- Lactose present, glucose present (cAMP level low): repressor inactive, operon on, low level transcription, little lac mRNA synthesized
~ In presence of glucose, cAMP levels are low. As result, low levels of CAO-cAMP complex which is needed for positive regulation
- cAMP binds to CAP, so RNA polymerase is less likely to bind and you get a low level of transcription (10 units)
What would result in 0 units of B-galactosidase?
nonsense mutation in promoter region, mutation in area that encodes RNA polymerase
The lac operon:
a. is an anabolic (building up) operon [should be catabolic]
b. includes a promoter, operator, and four structural genes (lac I , Z, Y, A) [lacI is not physical component of lac operon]
c. is turned ON in the absence of lactose [OFF]
d. is regulated, in part, by the product of the lacI gene
e. all of the above
d. is regulated, in part, by product of the lacI gene
If the gene encoding the lac repressor is mutated such that the inducer allolactose is unable to bind to the lacl protein, will transcription of the lac operon occur?
a. Yes, but only when lactose is present
b. No, because RNA polymerase can't bind the promoter
c. Yes, because the operator will not be bound by repressor and RNA polymerase can transcribe the lac operon
d. No, because cAMP levels are low when the repressor is nonfunctional
b. No, because RNA polymerase can't bind the promoter
Which of the following mutations in the lac operon is most likely to result in an inability of a bacterial cell to metabolize lactose?
a. mutation in lacZ (gene encoding β-galactosidase)
b. mutation in lacI (cannot bind to operator)
c. mutation in operator (cannot bind repressor)
d. all of the above
a. mutation in lacZ (gene encoding B-galactosidase)
Regulation of the trp operon
- Tryptophan absent, repressor inactive, operon on, trp mRNA synthesized
~ In the absence of tryptophan, the repressor cannot bind the operator. Therefore, RNAP binds that promoter and transcribes the trp E, D, C, B, A genes
- Tryptophan present, repressor active, operon off, no trp mRNA synthesized
~ If tryptophan is present, it acts as a corepressor. This allows the repressor to bind the operator and prevent RNAp from transcribing the trp E, D, C, B, A genes
Which of the following statements regarding the trp operon is accurate?
a. the trp operon is an anabolic operon
b. tryptophan acts as an inducer (corepressor not inducer)
c. the trp r repressor binds the operon's promoter (binds operator)
d. one of the proteins encoded by this operon is β-galactosidase (β-galactosidase is part of lac operon)
e. all of the above statements are accurate
a. the trp operon is an anabolic operon
What is the most commonly regulated stage of gene expression?
Transcription
Heterochromatin
Barr body
- centromeres and telomeres are heterochromatin material= less transcription
- length of a poly tail can influence stability
- translation can be regulated
Since almost all cells in an organism are genetically identical, how do different cell types have different properties?
MicroRNAs are transcribed from noncoding regions found in genes, can bind to mRNA and cause it to be degraded or cause blockage of translation, negatively regulates production of polypeptide