Exam 2

Question 1

What are Operons?

Answer 1

functional gene clusters “sets of genes”- bacteria are clustered into operons

Question 2

What are Operons composed of? (3 parts)

Answer 2

1. promoter: a site for RNA polymerase to bind and initiate transcription
2. Operator: turns on or off and is located in DNA within the promoter or between the promoter and the first gene
3. The Genes: when on, all genes in Operon get transcribed into RNA

Question 3

is the regulatory gene a part of the Operons?

Answer 3

no it’s not part of the operon but binds to the operator

Question 4

How is gene expression regulated in bacteria? (5 parts)

Answer 4

1. Energy conservation: only make proteins when needed at a specific time
2. (Non-Regulated gene expression) Constitutive genes: Enzymes that are always needed
3. Negative gene regulation: the repressor “turns off” the expression of those genes (blocks RNA polymerase)
4. Positive gene regulation: activator protein binds to separate site but near the promoter and enhances activity of RNA polymerase
5. Regulon: multiple operons regulated by the same regulator

Question 5

By default, is the operon usually turned on or off?

Answer 5

On

Question 6

describe how gene expression works when operon is on

Answer 6

RNA polymerase binds to the promoter region and proceeds to transcribe

Question 7

describe how gene expression works when the operon is off

Answer 7

a repressor binds to the operator blocking RNA polymerase from binding, therefore, turning off gene expression

Question 8

What does a repressor need for it to inhibit RNA polymerase?

Answer 8

a corepressor bind allosterically to a repressor protein and turns the repressor on

Question 9

how does a repressor protein get turned off?

Answer 9

A inducer molecule binds allosterically to the repressor protein → turning off the repressor protein

Question 10

What does the corepressor do?

Answer 10

turn operons off → by turning repressor protein on

Question 11

What do inducers do?

Answer 11

turn operons on → by turning repressor protein off

Question 12

What is the Tryptophan operon?

Answer 12

the operon that turns on the 5 genes that synthesize tryptophan
Tryptophan is always made (always there) but remains inactive until needed → when in the inactive form it won’t bind to the operator therefore the operon will be expressed and Tryptophan will be made
when in the environment synthesis of tryptophan will be turned off since the cell does not need it
→ Tryptophan is not only the final product it is also the corepressor

Question 13

What are inducible operons?

Answer 13

(breaking down) initial substrate stimulates operon → it turns on the transcription of genes that enables the cell to use that substrate

Question 14

What is an example of an inducible operon? How does it work

Answer 14

Lactose metabolism in E. coli
Lac Operon is an inducible operon that induces the synthesis of the enzymes to transport lactose (no lactose? no need to make the enzymes to transport it)

Question 15

Describe the 3 genes and their functions of the Lac operon

Answer 15

Z ⇒ β-galactosidase: an enzyme that cleves lactose to glucose (enzyme that uses lactose)
Y ⇒ permease: a transporter that allows lactose to get inside the cell
A ⇒ transacetylase: detoxifies the side products of permease “guard protein”

Question 16

what happens when lactose is in media?

Answer 16

lactose binds to the repressor → changes the shape of the repressor→ changed repressor cant bind to the operator → therefore RNA polymerase can bind to the promotor → (operon genes get expressed) → therefore enzymes for lactose can be produced→ lactose can enter the cell and be metabolized into glucose

Question 17

What is Positive Gene Regulation?

Answer 17

Stimulatory activator protein binds separate binding site near promoter → Enhances RNA polymerase activity and Increases gene expression & enzyme synthesis

Question 18

What is the Catabolite Activator Protein (CAP)?

Answer 18

Activates many catabolic pathways

ex: lac operon

Question 19

Describe the positive regulation of the lac operon

Answer 19

When available glucose decreases (ATP is low) and intracellular cAMP (cyclic-adenosine monophosphate) increases (to stimulate ATP production) → cAMP binds to CAP, causing it to change into the active configuration (now CAP will bind to the sites that will activate the promoter to stimulate the productions of the enzymes for these alt. pathways) → Active CAP enhances operon promoters for alternative catabolic pathways, including the lac operon

Question 20

What is the preferred energy source for making ATP?

Answer 20

glucose is preferred over lactose

Question 21

What happens under low lactose conditions?

Answer 21

Repressor active → operon suppressed

Question 22

What happens under High lactose/high glucose conditions?

Answer 22

Repressor inactive but activator

inactive → low operon activity

Question 23

What happens under High lactose/low glucose conditions?

Answer 23

Repressor inactive and activator active

→ high operon activity → enhanced lactose catabolism

Question 24

In prokaryotes, when does transcription & translation happen?

Answer 24

At the same time (Simultaneous transcription & translation)

Question 25

What is the GC hairpin and what does it do?

Answer 25

it is the form of termination in prokaryotes and works by Transcribed RNA forms a GC hairpin and the Hairpin causes RNA poly to fall off, terminating transcription.

Question 26

What does the leader do in prokaryotes?

Answer 26

The leader peptide functions to terminate transcripts before the RNA polymerase reaches the first structural gene of the operon