Question

In the lac operon model, as proposed by Jacob and Monod, how many structural genes are controlled by a single operator gene?
A.1
B.2
C.3
D.None of the above

Answer 1

Hint: Regulation of gene expression is name given to very broad term that may occur at Various level we consider the gene expression that result in the formation of a polypeptide and it can be regulated at several levels in eukaryotes the regulation of gene expression can be exerted at 4 levels that is, transcriptional level processing level, transport of m-RNA and translational level.

Complete answer:
The genes in a cell are expressed to perform a particular function or a set of functions. In eukaryotes, functionally related genes do not represent an operon but are present on different sites, chromosomes. Here structural gene is called split gene which is a mosaic of exons and introns, i.e., the base triplet - amino acid matching is not continuous. The entire split gene is transcribed to form a continuous strip of mRNA. The removal of non-coding intronic parts and fusion of exonic coding parts of RNA is called RNA splicing. About 50-90% of primary transcribed RNA is discarded during processing. The development and differentiation of embryos into adult organisms are also a result of the coordinated regulation of expression of several sets of genes.
It is a metabolic, physiological or environmental condition that regulates the expression of genes.
Francois Jacob (a geneticist) and Jacques Monod (a biochemist) proposed a model of gene regulation, known as operon model in bacteria. Operon is a coordinated group of genes such as structural gene, operator gene, promoter gene, regulator gene which function together and regulate a metabolic pathway as a unit, e.g., lac operon that is lactose operon, trp operon that is tryptophan operon, ara operon, his operon, val operon etc. consider the lac operon:
The lac operon (lac refers to lactose) consists of one regulatory gene or inhibitor gene (), one promoter gene, one operator gene and three structural genes. A polycistronic structural gene is regulated by a common promoter and regulatory gene.

1. Structural genes: Three structural genes are:
(i) lac z: The z gene codes for B-galactosidase which is primarily responsible for the hydrolysis of the disaccharide, lactose into its monomeric units’ galactose and glucose.
(i) lac y: The y gene codes for permease, which increases permeability of the cell to B-galactosides.
(iii) lac a: A gene code for transacetylase which can transfer acetyl group to B-galactoside.
2. Operator gene: it interacts with a protein molecule or regulator molecule, which prevents the transcription of structural genes.
Promoter gene: The gene contains the site for RNA polymerase attachment.
Regulator gene (i): The gene is used and codes for a protein known as repressor protein, it is synthesised all the time from the i-gene, that’s why it is constitutive gene which is functional always. switched off when repressor protein produced by regulatory or inhibitor gene binds to operator So our required answer is operator gene

Hence, the correct answer is option (C)

Note: In E. coli, breakdown of lactose requires three enzymes. These enzymes are synthesised together in a coordinated manner by a functional unit of DNA i.e., lac operon. Since the addition of lactose itself stimulates the production of required enzymes, thus it is called an inducible system.