Question

Which of the following is an example of positive gene regulation?
A. The tryptophan gene, in which the presence of tryptophan functions as a co-repressor to prevent its own transcription.
B. The lac gene, in which the repressor prevents the synthesis of the enzyme that degrades lactose is deactivated by the presence of lactose.
C. CAP, which acts as a required transcription factor for the enzymes that degrade lactose.
D. All of the above.

Answer 1

Hint: In positive gene regulation, the binding of specific protein / activator is required to begin the transcription. The operon will get turned off when the positive regulatory protein is absent or inactivated.

Complete Answer:
- The tryptophan consists of repressible Operon which contains five structural genes (trp E, D, C, B and A) in which when the end product tryptophan is present in sufficient amount it's molecules act as co-repressor and the tryptophan combines with inactive repressor to form the active trp repressor which now binds to the operator, preventing the transcription of the operon. This is called feedback repression or positive regulation.
- Inducible operon contains genes that encode enzymes involved in metabolic pathways.Lac operon is its example which require three structural gene:
1. Beta-galactose -hydrolyses lactose into glucose and galactose.
2. Lactose permease – Transports lactose into the cell of the body.
3. Thiogalactoside transacetylase- also hydrolyses lactose.

The correct answer is (A).

Note: There are broadly three levels of regulating gene expression:
- Transcriptional control (checks whether and how much a gene is transcribed into mRNA).
- Translational control (checks whether and how much an mRNA is translated into protein).
- Post-translational control (checks whether the protein is in an active or inactive form and whether the protein is stable or degraded).