Question

Match column with column Il and select the correct option from the given codes.

Column I	Column II
A. Operator site	(i) Binding site for RNA polymerase
B. Promoter site	(ii) Binding site for repressor molecule
C. Regulator gene	(iii) Codes for protein / enzyme
D. Structural gene	(iv) Codes for repressor molecule

Answer 1

Hint: Related genes in bacteria are often found in a cluster on the chromosome, on which they are transcribed from one promoter as a single unit. Such a gene cluster under control of a single promoter is known as an operon. The given question is about a typical operon and its different regions which act as either binding sites or different codes for molecules or proteins.

Complete answer:
Operators are sequences which regulate the accessibility of promoter regions of a prokaryotic DNA by interaction of proteins. Repressor molecules which repress gene expression bind to the operator site and as a result the promoter cannot initiate gene transcription. Hence (A) operator site in column I matches to (ii) binding sites for repressor molecules in column II.
Promoter site is the site where RNA polymerase binds to initiate the process of transcription. It refers to a conserved sequence which is specific for the binding of the RNA polymerase resulting in the expression genes for various functions. Hence (B) promoter site matches to (i) binding site for RNA polymerase.
Regulator gene codes for a repressor molecule which can bind to the operator sequence which stops gene expression. Hence (C) regulator gene matches to (iv) codes for repressor molecules.
Structural genes are genes that code for proteins or enzymes which are necessary for cell growth and survival. Hence (D) structural genes match to (iii) codes for protein/enzyme.

Hence, correct match to this question is A – (ii), B – (i), C – (iv), D – (iii).

Notes:
The expression of one or more gene is controlled by the regulator genes. The genes of the operator are responsible for the number of Rh antigens, while its qualitative characteristics are caused by structural genes. In prokaryotes, structural genes of related function are typically found adjacent to each other on a single strand of DNA, which forms an operon enabling to permit simpler regulation of gene expression.