Test: Regulation of Gene Expression (NCERT) - NEET MCQ

Promoter gene acts as an initiation signal which functions as recognition center for RNA polyrmerase 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 reaches the structural genes to perform transcription.

Griffith performed the transformation experiment on S.pneumoniae to prove that the genetic material was inheritable. Hershey and Chase performed the transduction experiment which is the infection of a bacteria by a virus called bacteriophage to prove that DNA and not proteins were the genetic material. Meselson and Stahl the experiment on E.coli to establish that replication was semi-conservative (half of parent and half synthesized). Jacob and Monad elucidated the Lac operon a transcription-ally regulated system. 

The accessibility of promoter regions of prokaryotic DNA is regulated by interaction of proteins with sequences called as operators. Repressor molecules that repress gene expression bind to the operator sits and as a result the promoter cannot initiate gene transcription. Promoter is the site for the binding of the RNA polymerase to initiate the process of transcription. It refers to a conserved sequence that is specific for the binding of the RNA polymerase that can bring the expression genes for various functions. Regulator gene is the gene of an operon that codes for a repressor molecule that can bind to the operator sequence and stop gene expression. Structural gene codes for proteins or enzymes that are essential for the cell growth and survival 
So, the correct option is 'A - (ii), B - (i), C - (iv), D - (iii)'.

The lac operon consists of three structural genes (lac Z, lac Y, lac A). Lac Z codes for 13-galactosidase which hydrolyses lactose to glucose and galactose. LacY codes for lac permease, a membrane-bound protein constituent of the lactose transport system which increase permeability of the cell to 13-galactosides. Lac A codes of thiogalactoside transacetylase, an enzyme of uncertain metabolic function. The appearance of ~3-galactosidase is coordinated with the production of permease and transacetylase. Operator gene is a gene which directly controls the synthesis of mRNA over the structural genes. Repressor is regulator protein synthesised all the time (constitutively) by the regulator igene. Repressor is meant for blocking the operator gene so that the structural genes are unable to form mRNAs. After coming in contact with inducer (e.g., lactose) the repressor undergoes conformational change in such a, way that it is unable to combine with operator. Thus, formation of inducer-repressor complex switches on the lac operon.

Regulation of gene expression can be exerted at four levels:
(i) transcriptional level during formation of primary transcript,
(ii) processing like splicing, terminal additions or modifications,
(iii) transport of mRNAs from nucleus to the cytoplasm and
(iv) translational level.