Lactose Operon

   

(a) Process when Lactose absent
(b) Process when Lactose present

Regulator gene ( R ) : contain genetic information for the synthesis of protein called repressor .
Promoter ( P )           : Site for the attachment of RNA polymerase 
Operator ( O )           : site for the attachment of the repressor protein .
Structural Genes (S) : One to several genes determining the amino acids sequence of the protein

• (1) : lacZ - Produces B- galactosidase that functional to converts lactose to glucose & galactose          

• (2) : lacY - Produces Lactose permease that functional as transport lactose into the cell 

• (3) : lacA - Produce transacetylase that  used to transfer an aceytl group CO-A to B -galactosidase      

Operon                       : A group of structural genes and regulating genes that  function as a unit .
                                         ( OPERON = Structural gene + Operator + Promoter )

(a) Process when Lactose absent 

1. The repressor attaches itself to the operator and blocked part of promoter .
2. RNA Polymerase cannot bind to the promoter
3. The operon is not active because transcription is blocked by repressor-operator complex.
4.  No transcription occur and the 3 structural enzymes cannot be synthesized.

(b) Process when Lactose Present 

1. Lactose enter to E.coli and converted to Allolactose ( isomer of lactose )
2. Allolactose binds to the repressor protein which is produced by regulator gene.
3. The repressor protein changes shapes and it cannot bind to the operator that causes the operon is activated.
4. RNA polymerase binds to promoter .
5. Transcription of structural genes takes place to form mRNA
6. The translation of mRNA occur
7. The enzymes B-galactosidase , permease and transacetylase are produced
8. Lactose is hydrolysed into galactose and glucose .