7.4 Control of Gene Expression in Prokaryotes [HY]

Question 1

Operon

Answer 1

• When genes are transcribed in a group by sharing a single common promoter region on a DNA sequence

Question 2

Jacob–Monod model

Answer 2

• used to describe the structure and function of operons
• In this model, operons contain structural genes, an operator site, a promoter site, and a regulator gene
• structural gene codes for the protein of interest

Question 3

Operator site

Answer 3

a nontranscribable region of DNA that is capable of binding a repressor protein

Question 4

Promoter site

Answer 4

• similar in function to promoters in eukaryotes: it provides a place for RNA polymerase to bind

Question 5

Regulator gene

Answer 5

• codes for a protein known as the repressor. There are two types of operons: inducible systems and repressible systems.

Question 6

Inducible systems

Answer 6

• the repressor is bonded tightly to the operator system and thereby acts as a roadblock

Question 7

Negative control

Answer 7

• binding of a protein reduces transcriptional activity
• To remove that block, an inducer must bind the repressor protein so that RNA polymerase can move down the gene

Question 8

Inducible system

Answer 8

• The system is normally “off” but can be made to turn “on,” given a particular signal (Inducer [lac])
• Lac operon negative inducible
  system

Question 9

Positive control

Answer 9

• The binding of a protein to DNA increases transcription

Question 10

Negative control

Answer 10

• The binding of a protein to DNA stops transcription

Question 11

Repressible system

Answer 11

• The system is normally “on” but can be made to turn “off,” given a particular signal (corepressor [trp])
• trp operon is a negative repressible system

Question 12

Catabolite activator protein
(CAP)

Answer 12

• Assists binding of the lac operon
• a transcriptional activator used by E. coli when glucose levels are low to signal that alternative carbon sources should be used
• Falling levels of glucose cause an increase in the signaling molecule cyclic AMP
  (cAMP), which binds to CAP. This induces a conformational change in CAP that allows it to bind the promoter region of the operon, further increasing transcription of the lactase gene

Question 13

Positive control mechanisms

Answer 13

When binding of a molecule increases transcription of a gene

Question 14

Repressible systems

Answer 14

• allow constant production of a protein product. In contrast to the inducible system, the repressor made by the regulator gene is inactive until it binds to a corepressor
• Repressible systems tend to serve as negative feedback; often, the final structural product can serve as a corepressor. Thus, as its levels increase, it can bind the repressor, and the complex will attach to the operator region to prevent further transcription of the same gene.

Question 15

trp operon mechansim

Answer 15

• operates as a negative repressible system
• When tryptophan is high in the local environment, it acts as a corepressor
• The binding of two molecules of tryptophan to the repressor causes the repressor to bind to the operator site.