Chapter 18 - Regulation of Gene Expression

Question 1

Types of Genes

Answer 1

Constituted genes: genes that are vital and expressed all the time

Regulated Genes: genes expressed only when needed

Question 1

Purpose of Regulation

Answer 1

• Regulates development of organism
• Responsible for differentiation between cell types

Question 2

Ways Metabolic Paths can be Controlled

Answer 2

1. feedback inhibition where the first enzyme in a pathway is inhibited by the presence of the pathways end product
   ○ a fairly rapid physiological response relying on the sensitivity of many enzymes to chemical cues that either increase or decrease its catalytic activity
2. Cells can regulate the expression of the genes encoding for the enzymes at the level of transcription
   ○ A slower but more long term suppression by not allowing the creation of the gene in the first place

Question 3

Operon

Answer 3

a unit of genetic function found in bacteria which consists of a promoter, an operator and a coordinately regulated cluster of genes whose products function in a common pathway

Question 4

Benefits of Operons

Answer 4

• Coordinately Controlled(all controlled) by a single on-off switch known as an operator
  ○ Positioned within or between promotor and enzyme coding genes and controls access of RNA polymerase to the gene

Question 5

Regulatory Genes

Answer 5

a gene which codes for a protein that controls the transcription of another gene, or group of genes
- E.g. trpR gene codes for trp repressor

Question 6

Trp Operon

Answer 6

• By itself, the switch is turned on allowing RNA polymerase to bind to the promoter and transcribe the genes
  ○ Turned off when a specific repressor protein, tryptophan, binds to the operator and prevents RNA polymerase from transcribing genes
  
  • The trp operon is not switched off permanently due to the binding of repressors to operators being irreversible
    ○ Operators alternate between two states; bound by a repressor and unbound
    ○ The trp repressor protein is an allosteric protein with two alternative shapes; active and inactive; based on whether tryptophan has bound to the allosteric site or not
  • The trp genes code for proteins which are responsible for catalyzing the formation of tryptophan

Question 7

Corepressor

Answer 7

a small molecule which binds to the repressor protein and influences its shape and therefore whether the operator is repressed or not.
○ E.g. The more tryptophan present, the greater chance it binds to trp repressor molecule and shuts down the production of tryptophan

Question 8

Repressible Operon

Answer 8

transcription is usually on but can be inhibited when a specific small molecule binds allosterically to a regulator protein
- E.g. Trp Operon

Question 9

Inducible Operon

Answer 9

when transcription is usually off but can be stimulated on when a specific small molecule interacts with a different regulatory(activator) protein
- e.g. lac operon

Question 10

Process of Lac Operon

Answer 10

• When E.coli metabolise lactose, it begins with hydrolysis of the disaccharide into is glucose and galactose monosaccharides, a reaction catalyzed by B-galactosidase
  
  • Normally there is only a few B-galactosidase in the cell however that number 1000x folds within 15 mins of lactose in the bacterium’s environment
  • The lacZ gene coding for b-galactosidase is located on the lac operon and the regulatory gene, lacI is located outside the lac operon which codes for the allosteric repressor
  • Unlike trp, lac repressor protein is active by itself and normally inhibits expression of lacZ, therefore an inducer needs to inactivate the repressor for b-galactosidase production.
    
    • This inducer is allolactose, an isomer of lactose
  • In the presence of allolactose, it binds to the lac repressor, altering its shape and allowing the genes to be expressed, producing b-galactosidase

Question 11

Inducible Enzyme

Answer 11

enzymes functioning in catabolic pathways in which their synthesis is induced by a chemical signal
- E.g. allolactose for b-galactosidase

Question 12

Repressible Enzyme

Answer 12

enzymes which function in anabolic pathways and synthesize essential end products from precursors(raw materials)

Question 13

negative control of genes

Answer 13

• Regulation of trp and lac operons involve negative control of genes as their operons are switched off by the active form of their respective repressor proteins
  • Gene regulation is only positive when a reg protein interacts to increase transcription.

Question 14

Negative REgulation of Lac Operon

Answer 14

• Once glucose in the cell increases in concentration, cAMP concentration falls, resulting in CRP detaching
  - This results in RNA polymerase binding less efficiently to the promoter, and transcription of lac operon proceeds at a lower level