Lesson 2.3 - lac operon

Question 1

What is the flow of information?

Answer 1

DNA to mRNA to protein (polypeptide)

Question 2

What are genes?

Answer 2

DNA sequence that codes for a product (include protein, rRNA, tRNA)

Question 3

What does RNA polymerase look for on mRNA?

Answer 3

An available 3’ -OH

Question 4

mRNA is ___ and ___ to the template strand

Answer 4

complementary; antiparallel

Question 5

______ result in polycistronic mRNA

Answer 5

Operons

Question 6

Why use operons?

Answer 6

• Single switch can turn on or off a cluster of functionally related genes
  
  • Saves time and energy, makes lots of protein
• Bacteria & Archaea, rarely Eukarya

Question 7

mRNA has ribosome binding sites in _____

Answer 7

5’ UTR

Question 8

Define polysome.

Answer 8

Multiple ribosomes on a single mRNA

Question 9

Role of homeostasis

Answer 9

• Maintain internal stability by expressing and repressing different genes

Question 10

What does the lac operon create?

Answer 10

• Enzymes needed for catabolism of lactose
  
  • ​Disaccharide aka milk sugar

Question 11

What is the default state of the lac operon?

Answer 11

Off; only expressed when needed (inducible)

Question 12

Function of lacZ

Answer 12

• Codes for ß-galactosidase
• Cuts lactose

Question 13

Function of lacY

Answer 13

• Codes for permease
  
  • Membrane transport protein for lactose

Question 14

Function of lacA

Answer 14

• Codes for transacetylase
  
  • Enzyme that transfers acetyl group (C-C) from acetyl-COA to ß-galactosides
  • Function in operon unclear

Question 15

(2) Functions of ß-galactosidase

Answer 15

• Cuts lactose to galactose & glucose
  
  • Enter glycolysis
• Converts lactose to allolactose
  
  • Inducer of lac operon

Question 16

What is the lacI gene?

Answer 16

• Located on side of chromosome; not next to lac protein
• Codes for repressor protein LacI

Question 17

What is the LacI protein?

Answer 17

• Binds to operator sequence & prevents binding of RNA poly. & transcription of lacZ,Y, and A
• Fits major groove of operator
  
  • Head pieces bind succssive turns
  • Inducer changes orientation of head piece (allosteric site)

Question 18

What occurs when lactose is absent?

Answer 18

The lac operon is OFF. Repressor protein stops 96% of transcription.

Question 19

What occurs when lactose is present?

Answer 19

• LacI (repressor) detaches and allows RNA poly. to initiate transcription.
  
  • Lactose converted to allolactose (inducer) and binds to repressor protein.

Question 20

Lac operator (O_lac)

Answer 20

• Overlaps promoter; binding site for repressor
• Inverted repeat (snug fit)
  
  • Match b/t operator & repressor

Question 21

Catabolite repression

Answer 21

Turning off of genes due to plenty of glucose (E. coli prefers glucose before other C sources)

Question 22

What is cAMP?

Answer 22

• Binds to cAMP receptor protein (CAP) & hyper-stimulates lac operon transcription
  
  • CAP - cAMP complex binds to promoter
• Increased when [glucose] decreases

Question 23

When glucose is low, positive control of lac operon results in…

Answer 23

Increase in transcription when cAMP - CAP complex binds to prmoter (unless repressor is bound to operator)

Question 24

When glucose is high, positive control of lac operon results in…

Answer 24

catabolite repression; glucose inhibits cAMP synthesis

Question 25

Conditions affecting homeostasis (table)

Answer 25