Genetics (Lac Operon)

T or: F

transcription and translation can be done simultaneously

true

What would be the most effective way for bacteria to control gene expression?

A: at level of transcription

B at level of translation

at level of transcription(cells are lazy)

Operon:

Set of structural genes in a bacterial cell, along with their common promoter and operator that control their ______.

transcription

Operator

-what binds to it

DNA sequence to which a regulator protein binds, affecting the rate of transcription of the structural genes

In lac operon, the RNA polymerase binds to what region?

regulatory region (promoter/operator)

Two lenses through which to examine bacterial gene expression.

Default state:

Inducible vs. repressible

Control Mechanism:

Negative vs. positive control

what does it mean if the operon is inducible?

it can be turned on; its default state is off

what does it mean if the operon is repressive?

can be deactivated; its default state is on

Describe what a positive control does in an operon

Describe what a negative control does in an operon

pos: increases rates of transcription

neg: decreases rates of transcription

one way to control bacterial transcription is for a _________ to interact with the operator

regulator protein

3 levels of transcription of functional protein

none

low

high

If the conditions inside the cell are:

Lactose present, glucose present, we would expect? why?

Low level of functional gene product

why? ecoli prefers to have glucose(less energy expensive)

what is the situation where we would have the highest level of gene expression in a lac operon?

lactose present, glucose absent

lactose high, glucose low

If the conditions inside the cell are:

Lactose present, glucose absent, we would expect?

High level of functional gene product

Repressor protein has a ______ effect/control. meaning?

negative

it has down regulation of transcription

Activator proteins(cap) has a ____ effect on the operon. meaning?

upregulation of transcription /positive control

what is cap?

- allows what levels of transcription?

an activator protein (catabolite activator protein)

1. binding of what to the operator BLOCKS transcription

2. binding of what ALLOWS transcription?

1. repressor protein

2. substrate molecule (allolactose)

LacI:

produces what protein?

Function:

Contains 2 binding sites. One for ______ and one for the ______, the inducer

produces repressor protein

operator;lactose

In the absence of lactose, would lacZ be expressed?

B) No/Low levels

What are the important mutants of LacI?

I- : unable to bind to operator

I^s: unable to bind to inducer(allolactase)

-transcription is repressed even when lactose is present

What happens to cap/camp when glucose is low?

high amounts of camp/cap--->high levels of transcription

what hapens to cap.camp when glucose is high

low amount of camp/cap----> low levels of transcription

what happens to lac operon when glucose is high?

little transcription

Which internal cellular environment results in the highest level of transcription of structural genes?

Low glucose, high lactose

what are the 3 protein producing genes in lac operon:

LacI

LacZ

LacY

LacZ

produces what protein?

produces B-galactosidase

What are the important mutants of LacZ?

LacZ- : no functional B-galactosidase

LacY

produces what protein?

produces permease

What are the important mutants of LacY?

LacY- no functional permease

WHat are the 2 regulatory sequence genes?

LacO (operator)

LacP(promoter)

What is the function of LacO

What is the mutant form and what does it do?

binds repressor protein to block transcription of operon genes

Mutant: Oc: fails to bind repressor protein

What is the function of LacP?

What is the mutant form and what does it do?

binds RNA polymerase

mutant: P- : fails to bind RNA polymerase (or does so weakly)

Mutant proteins

Is for low

Oc for high

which trumps which?

Is causes repression of operon even if lactose is present

Oc -causes transcription of operon even if lactose is absent

Oc trumps Is

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Which statement is correct regarding the lac operon system’s relationship with lactose? (Don’t worry about glucose)

a.transcription of lac operon genes occurs only in the presence of lactose

b.transcription of lacI repressor gene occurs only in the absence of lactose

c.binding of the repressor to the operator occurs only when lactose is present

d.binding of RNA polymerase to the promoter of lacI repressor gene occurs only when lactose is present

A

What happens in the lacOc mutant when lactose is absent? (Don’t worry about glucose)

a.the repressor cannot bind the operator and the lac operon is transcribed

-because Oc trumps Is

What happens in the lac I- mutant when lactose is absent?

the repressor cannot bind to the operon, leading to transcription of lac operon genes

Regardless of whether lactose is present or absent, which of the following statements describes transcription of lac operon genes in the lacIs mutant?

a. the repressor binds the operator and there is no transcription of the lac operon

Which statement describes what happens with an operon containing a lacP- mutant when lactose is present?

the transcriptional machinery cannot bind the promoter, and there is no transcription of the lac operon

What happens in the lacZ- mutant when lactose is present?

a.There is no transcription of the lac operon

b.There is no transcription of the lacZ gene

c.There is no translation of lacZ

d.Mutant b-galactosidase is produced

D

Lac Z- still transcribes, but the protein is non- functional

For a bacterium with the genotype: I+ P+ O+ Z- Y+ A+ name the FUNCTIONAL gene products produced at high levels in the presence of lactose (assume glucose absent)

A.Beta-gal only

B.Permease only

C.Both beta-gal and permease

D.Neither beta-gal nor permease

B

For a bacterium with the genotype: I+ P+ O+ Z- Y+ A+ name the FUNCTIONAL gene products produced at high levels in the absence of lact

A.Beta-gal only

B.Permease only

C.Both beta gal and permease

D.Neither beta-gal nor permeaseose (assume glucose absent)

D

Which of the following conditions will result in the greatest levels of transcription of the lac operon?

A.Lactose present, cAMP and glucose absent

B.Lactose and cAMP present, glucose absent

C.Lactose and glucose present, cAMP absent

D.Lactose, glucose, and cAMP present

B

Which statement best describes attenuation in the trp operon?

A.When tryptophan levels are low, transcription of the leader region stalls and transcription of the structural genes never begins

B.When tryptophan levels are high, translation of the leader region stalls, which creates a secondary structure that halts transcription of the structural genes

C.Tryptophan levels do not affect attenuation, which is the restriction of transcription of the structural genes via a repressor protein causing the ribosome to stall

B