Lecture 26

Question 1

Where is the operator located compared to the promoter

Answer 1

it is downstream to the promoter

Question 2

why are operons useful/economical?

Answer 2

it puts all the enzymes and proteins involved in the same process under her control of a single promoter

Question 3

polycistronic mRNA

Answer 3

multiple genes on a single mRNA stand, but they are translated independently

Question 4

housekeeping genes

Answer 4

genes that are constantly expressed throughout the lifetime of the organism. eg ones in metabolic process

Question 5

What does expressing the gene mean?

Answer 5

for the gene to show a phenotype. (DNA-mRNA-protien). Some do not to the protein stage.

Question 6

Activators

Answer 6

they are proteins that increase the expression level of genes by binding to DNA, increasing activity of the RNA polymerase

Question 7

Repressors

Answer 7

they are proteins that reduce the expression levels of genes by binding to DNA, decreasing the activity of the RNA polymerase

Question 8

Effectors

Answer 8

they bind to an activator or repressor to alter the affinity the activator or repressor has for the gene. It can induce or repress gene expression

Question 9

positive regulation

Answer 9

when an activator binds to DNA enhancing the activity of the RNA polymerase

Question 10

negative regulation

Answer 10

when a repressor bind to DNA blocking the activity of the RNA polymerase

Question 11

what is an Inducer

Answer 11

When an effector decreases the affinity of a repressor to DNA so gene expression occurs (negative regulation)

Question 12

When an effector decreases the affinity of an activator a DNA…

Answer 12

it stops gene expression

Question 13

What is a co-repressor?

Answer 13

When an effector increases the affinity of a repressor to DNA helping it stop gene expression (negative regulation)

Question 14

When an effector increases the affinity of an activator to DNA…

Answer 14

it helps transcription occur. It cannot activate gene expression without the effector.

Question 15

What genes encode repressors?

Answer 15

regulator genes

Question 16

What is the lac operon?

Answer 16

it is 3 genes that make the enzymes to catalyze lactose into glucose and galactose when glucose is not present.

Question 17

What is lacZ?

Answer 17

structural gene that encodes B-galactosidease

Question 18

What is lacY?

Answer 18

structural gene that encodes galactoside permease and its used for transporting lactose across the bacterial membrane

Question 19

What is lacA?

Answer 19

structural gene that encodes thiogalactoside transacetylase to export unwanted toxins that come into the cell from the permease activity

Question 20

what is the lacI?

Answer 20

its a regulatory gene adjacent to the lac operon that makes the lac repressor protein LacI (lac inhibitor). it binds to the lac operator when lactose is absent

Question 21

Negative regulation of the lac operon

Answer 21

When lactose is present, the B-gal turns it into glucose and galactose. It also changes some of it into its isomer allolactose which then acts like an inducer on the lac repressor. That inhibits the lac repressor from blocking the expression of the lac operon and gene expression occurs.

Question 22

how does the repressor bind to the operon

Answer 22

it binds at 35 base pairs in a near perfect symmetry called the helix turn helix motif. It binds as dimers

Question 23

Positive regulation of the lac operon

Answer 23

an effector called cAMP binds to the activator protein CRP (cAMP receptor protein) to increase affinity for the lac promoter. it increases expression because it distorts the DNA enough for RNA pol to bind more effectively

Question 24

where does cAMP come from?

Answer 24

it is produce by adenylate cyclase

Question 25

Why is cAMP produced when glucose is absent?

Answer 25

glucose inhibits adenylate cyclase

Question 26

What are the 3 scenarios for lac operons

Answer 26

1) no lactose
2) lactose present, glucose absent
3) both lactose and glucose present

Question 27

what happens when no lactose is present

Answer 27

the lac operon is off, no expression

Question 28

what happens when lactose is present but glucose is not

Answer 28

high levels of expression because
- there’s no inhibition of adenlayte cyclase to make cAMP which will bind to CRP being a positive regulator and increase RNA pol activity
- the inducer allolactose will be produced and bind to the lac repressor so now expression occurs

Question 29

what happens when both glucose and lactose are present

Answer 29

low levels of expression because
- glucose inhibits production of cAMP
- the allolactose will still be produced so the lac repressor will be removed

Question 30

what happens when both glucose and lactose are gone

Answer 30

no expression
- the lac repressor is on the operon cause there no allolactose
- cAMP is high but that doesn’t matter

Question 31

how is the trp operon an example of a negative regulation by a repressor in need of a co repressor?

Answer 31

Tryophan is the co repressor needed for its operon. When it is present, it binds to the repressor so it doesn’t make more. When it is absent, the repressor won’t be binding so transcription occurs