What is Operon ? Define types of genes which make up an operon.

Explanation

An operon is a part of genetic.material (or DNA) which acts as a single regulated unit having one or more structural genes, an operator gene, a promoter gene, a regulator gene, a represser and an inducer or corepressor (from outside). Operator, promoter and regulator genes constitute the regulatory region. Operon systems are common in prokarytoes. The first operon lac-operon was discovered by Jacob and Monad (1961), later on a number of such operons were discovered, e.g., trp-operon,ara-operon, his-operon, vol-operon. Operons are of two types, inducible and repressible.
(a) Structural genes. These genes which actually synthesise m-RNA by transcription, which forms proteins on the ribosomes.

An operon has one or more structural genes. The lactose or lac-operon of Escherichia coli contains three structural genes (Z, Y, A). They transcribe a polycistronic m-RNA molecule that helps in the synthesis of three enzymes-B-galactosidase, for hydrolysing lactose or galactoside, lactose: or galactoside permease for allowing entry of lactose from outside and thiogalactoside acetylase or transacetylase for metabolising toxic thiogalactosides which are also allowed entry by lactose permease. A very low level of expression of lac-operon and consequently its enzymes is always present. Initial entry of lactose into bacterium would occur only due to this activity.
Operator Gene. It is a gene which directly controls the synthesis of mRNAs over the structural It is switched off by the presence of a represser. An inducer can take away the represser and switch on the gene. The gene then directs the structural genes to transcribe. Operator gene of lac operon is made of only 27 base pairs.
Promoter Gene. It acts as an initiation signal which functions as recognition centre for RNA polymerase provided the operator gene is switched on. RNA polymerase is bound to the promoter gene. When the operator gene is functional, the polymerase moves over it and reacnes the structural genes to perform transcription.
Regulator Gene (Lac I Gene). In lac-operon it is called i-gene because it produces an inhibitor or represser. The represser binds to operator gene and stops the working of the latter. It exerts a negative control over the working of structural genes.