Question

Choose the correct answer from the alternatives given :
Repressible operon system is usually found in _(i)_ pathways. The pathways end product serves as a _(ii)_ to activate the repressor, turn off enzyme synthesis and prevent overproduction of the end product of the pathway. Genes for this operon are usually switched _(iii)_ and the repressor is synthesised in an _(iv)_ form.

• A. (i) anabolic, (ii) corepressor, (iii) on, (iv) - inactive
• B. (i) anabolic, (ii) inducer, (iii) off, (iv) - active
• C. (i) catabolic, (ii) inducer, (iii) off, (iv) - active
• D. (i) catabolic, (ii) corepressor, (iii) on, (iv) - inactive

Answer 1

The correct option is A (i) anabolic, (ii) corepressor, (iii) on, (iv) - inactive

Repressible operons are generally involved in anabolic pathways, or the synthesis of an essential component, while inducible operons are generally involved in catabolic pathways or the breakdown of a nutrient. Positive control of an operon is when gene expression is stimulated by the presence of a regulatory protein. The repressible operon work on the feedback inhibition mechanism which is the phenomenon where the output of a process is used as an input to control the behavior of the process itself and the co-repressor serves as to activate the repressor. Genes of a repressible operon are switched off as they are required to synthesize the structural/functional genes in the absence of the repressor. The presence of the repressor can stop this process and hence the repressor is present in the inactive form.

So, the correct option is '(i) anabolic, (ii) corepressor, (iii) on, (iv) - inactive'.