Lac operon

Question 1

What is an operon?

Answer 1

• group of genes that are under the control of the same regulatory mechanism
• are expressed at the same time

Question 2

Where are operons more common?

Answer 2

• far more common in prokaryotes than eukaryotes
• owing to the smaller and simpler structure of their genomes

Question 3

Why are operons useful?

Answer 3

• efficeient way of saving resources
• if certain gene products are not needed, then all of the genes involved in their production can be switched off

Question 4

What do bacteria metabolise?

Answer 4

• glucose is easiest to metabolise and is the preferred respiratory substrate of Escherichida coli and many other bacteria
• if glucose is in short supply, lactose can be used as a respiratory substrate
• different enzymes are needed to metabolise lactose

Question 5

What is the lac operon?

Answer 5

• group of 3 genes:
  
  • lacZ, lacY, lacA

Question 6

WHat is the function of the lac operon?

Answer 6

• involved in the metabolism of lactose
• they are structural genes as they code for three enzymes

Question 7

How does the lac operon work?

Answer 7

• they are transcribed onto a single long molecule of mRNA
• a regulatory gene, lacI, is located near the opperon and codes for a repressor protein
  
  • prevents the transcription of the structural genes in the absence of lactose

Question 8

WHat does the repressor protein do?

Answer 8

• constantly produced
• binds to an area called the operator which is also close to the structural genes
• the binding of this protein prevents RNA polymerase binding to DNA and beginning transcription
• this is called down regulation
• the section of DNA that is the binding site for RNA polymerase is called the promoter

Question 9

What happens when lactose is present?

Answer 9

• it binds to the repressor protein
  
  • causing it to change shape
  • no longer can bind to the operator
• RNA polymerase can then bind to the promoter
• the three structural genes are transcribed and the enzymes are synthesised

Question 10

Why does cyclic AMP need to be bound?

Answer 10

• the binding of RNA polymerase still only results in a relatively slow rate of transcription
  
  • needs to be sped up or up-regulated to produce the required quantity of enzymes to metabolise lactose efficiently
• this is acheived by binding of cAMP receptor protein (CRP)
  
  • that is only possible when CRP is bound to cAMP (secondary messenger)

Question 11

What does the transport of glucose into an E. coli result in?

Answer 11

• decreases levels of cAMP
• reduces the transcription of the genes responsible for the metabolism of lactose
  *

Question 12

What happens if both glucose and lactose are present ?

Answer 12

it will still be glucose

the preffered respiratory substrate