The Operon

Question 1

What are the different types of mRNA?

Answer 1

1. Monocistronic

2. Polycistronic

Question 2

What is monocistronic mRNA?

Answer 2

encodes only 1 protein

 Coding region: ORF (open reading frame) is the “gene”

Question 3

What is polycistronic mRNA?

Answer 3

encodes for more than 1 protein
 Hence, more than 1 ORF
 Each ORF has a start and stop codon

Question 4

What types are mRNA are predominantly found in bacteria?

Answer 4

there are few monocistronic mRNA, mostly polycistronic

Question 5

What is a UTR?

Answer 5

untranslated sequence

o All mRNA have a certain length of 5’UTR before the start codon and 3’ UTR after the stop codon

Question 6

What is an operon?

Answer 6

transcription unit (“genes”) that encodes polycistronic mRNA in which all encoded proteins have a related function
o	Related function: e.g. enzymes of a signle pathway
o	1 promoter and 1 set of control elements for all the genes of the operon (single polycistronic mRNA)

Question 7

How is an operon an element of control?

Answer 7

o Need to turn genes on/off when needed/not needed
o Control elements at promoter
 Binding site for protein that turn on and off
o 1 promoter and 1 set of control elements for all the genes of the operon (single polycistronic mRNA)

Question 8

What is the transcription regulatory circuit for operons?

Answer 8

1. Trans-acting factor: regulator proteins

2. Cis-acting sequence: target site

Question 9

What are regulator genes?

Answer 9

Encodes regulator proteins

Question 10

What are structural genes?

Answer 10

All genes that don’t encode for regulator proteins

Question 11

What are the levels of regulation in operons?

Answer 11

o Regulator proteins

o Regulatory molecules

Question 12

What is a trans-acting factor?

Answer 12

• Regulator proteins that regulate or modify the expression of some other genes
• Functions at a site other than where it is synthesized (target gene located elsewhere)
• Can be
o Repressor protein: prevents (represses) expression of target gene
o Activator protein: required for (activates) expression of target gene

Question 13

What is a cis-acting sequence?

Answer 13

• Functions as a DNA sequence
• Physically linked (adjacent) to gene that will be regulated
• Binding site for regulator proteins

Question 14

What is the 1st level of regulation in operons?

Answer 14

Regulation at transcription initiation
• 2 types of regulation by regulator proteins
o Negative control (repression)
 By default: gene always expressed (default state is “on”)
 Specific regulatory protein i.e. repressor turns the gene “off”
o Positive control (activation)
 By default the gene is not expressed (off)
 Regulator protein is an activator that turns the gene on
• Helps RNA Pol to bind to a promoter
• Help needed because the promoter is anon-conserved sequence (at -10 and/or -35)
• The regulator protein (activator) helps to recognized the non-conserved promoter