lecture 8: operons

Question 1

what type of regulation do operons conduct?

Answer 1

regulation at the gene level

Question 2

what type of cells do operons exist in?

Answer 2

prokaryotic cells

Question 3

what are prokaryotic genes organized into?

Answer 3

organized into operons (the genes are the operons)

Question 4

what are the 2 elements to operons?

Answer 4

1) promoter: contains an on/off switch called operator

2) genes that will be under control of the operator (code for proteins involved in the same metabolic pathway)

Question 5

what is a repressible operon?

Answer 5

-operon for synthesizing (anabolic)
-repressor protein is made in an inactive form (will be activated by presence of metabolite)
-trasncription is ON by default

Question 6

what acts as a co-repressor for repressible operons?

Answer 6

metabolite that will bind to repressor and put it in its active form

Question 7

are repressible operons on or off by default?

Answer 7

on by default and repressor proteins deactivates them

Question 8

what is an inducible operon?

Answer 8

-operon for breaking something down
-repressor is made in an active form (will be deactivated by present of metabolite)
-transcription is off by default

Question 9

how are the genes of the operon transcribed?

Answer 9

as one single mRNA molecule

Question 10

what happens when an operon is turned off?

Answer 10

-repressor protein binds to the operator (thats why its off)
-Rna polymerase is prevented from binding to the promoter and transcription is blocked

Question 10

what happens when an operon is turned on?

Answer 10

-RNA polymerase binds to the promoter
-all genes of the operon are transcribed as one long mRNA
-each gene on the mRNA is translated into a polypeptide
-repressor protein is off

Question 11

true or false, a repressor protein is always transcribed and translated.

Answer 11

true

Question 12

what is a repressor protein?

Answer 12

-product of a regulatory gene
-gene is always being transcribed

-binding of repressor protein is reversible
-contains an allosteric site: allosteric regulators are metabolites in the cell

Question 13

how do repressor proteins function?

Answer 13

they have allosteric regulators, which are metabolites in the cell, and make them vary between inactive and active repressors.

Question 14

what are the 2 types of operons?

Answer 14

repressible and inducible

Question 15

what is a repressible operon? give an example.

Answer 15

transcription is turned off by the product of a metabolic pathway. An example of this would be the trp operon. this type of operon is made for synthesis

Question 16

what is an inducible operon? give an example.

Answer 16

transcription is turned ON by the presence of a reactant in a metabolic pathway. an example of this is the lac operon

Question 17

what is the tryptophan operon?

Answer 17

-produces enzymes involved in the production of tryptophan
-repressor protein is active when bound to tryptophan

Question 18

what happens if there’s low tryptophan?

Answer 18

-repressor protein is inactive and does not bind to operator
-operon is turned on and mRNA is produced

Question 19

what happens if there’s high tryptophan?

Answer 19

-repressor protein is active and binds to operator
-operon is turned off and mRNA is not produced

Question 20

what is lactose metabolism?

Answer 20

the breakdown of lactose to make ATP

Question 21

what happens to the repressor protein when lactose is present?

Answer 21

-repressor protein is inactive when lactose is present
-allolactose acts as an inducer
-lactose is converted into allolactose in the cell

Question 22

what happens to lac operon when lactose is not present?

Answer 22

-repressor protein is active and binds to operator
-operon is turned off

Question 23

what happens to lac operon when lactose is present?

Answer 23

-allolactose binds to receptor and inactivates it
-repressor protein is inactive and does not bind to operator
-operon is turned on and mRNA is produced

Question 24

what is allolactose?

Answer 24

-it acts as an inducer (induces activity of metabolic pathway)
-its present when lactose is present, cause lactose is converted into allolactose in the cell

Question 25

what is negative control of gene expression? give an example.

Answer 25

-operon can be turned off by repressor
-repressible and inducible operons

example: lac and trp operon (repressor)

Question 26

what are the two types of gene expression controls?

Answer 26

negative and positive control

Question 27

what is the positive control of gene expression? give an example.

Answer 27

transcription of operon can be increased by an activator

-example: lac operon (CAP-cAMP)

Question 28

what is the difference between positive and negative control of lac operon?

Answer 28

positive control adjusts levels of transcription (doesn’t turn it on/off), while negative controls turns it on/off via a repressor protein

Question 29

if the glucose concentration level is high, why are the transcription levels of the lac operon very low even when lactose is present?

Answer 29

-glucose determines the transcription levels (positive control)
-glucose is a better/more efficient source of energy (dont need to break down lactose to form energy)

Question 30

for the lac operon, if glucose is low, do we want high levels of transcription?

Answer 30

yes

Question 31

explain what CAP/cAMP is

Answer 31

-in addition to an operator, the lac operon promoter also had a CAP-binding site

-when CAP binds to the promoter, it stimulates transcription
-CAP is only in its active form when bound by cyclic-AMP (cAMP)
-cAMP levels are high when glucose concentration is low
-

Question 32

why are cAMP levels high when glucose concentration is low?

Answer 32

glucose inactivates the enzyme adenylyl cyclase, which is the enzymes that turns ATP into cAMP

Question 33

what happens when an active cAMP binds to the CAP-binding site?

Answer 33

transcription levels increase

Question 34

will transcription be turned on if there is no glucose and no lactose in the cell?

Answer 34

look at lactose first: because only lactose tells us if transcription is on/off, then glucose tells us about how fast transcription is.

Question 35

what will the level of transcription be like if lactose and glucose are present? what type of regulation is happening?

Answer 35

transcription is on, but because of the presence of glucose, we have low level of transcription. we have both positive and negative regulation occurring at the same time.