Biology Questions and Answers – Molecular Basis of Inheritance – Lac Operon


This set of Biology Multiple Choice Questions & Answers (MCQs) focuses on “Molecular Basis of Inheritance – Lac Operon”.

1. Which polycistronic structural gene is regulated by a common promoter and regulatory gene?
a) Trp operon
b) Lac operon
c) Ara operon
d) His operon
View Answer

Answer: b
Explanation: Lac (lactose) operon has a polycistronic structural gene which is regulated by a common promoter and regulatory gene. It is mainly present in prokaryotes like bacteria. This type of an arrangement is called as an operon in general.

2. How many structural genes are present in a lac operon?
a) One
b) Five
c) Three
d) Seven
View Answer

Answer: c
Explanation: A lac operon consists of one regulatory gene (i) and three structural genes (z, y and a). The “i” in regulatory gene is derived from the word “inhibitor”.

3. What does the structural gene (y) of a lac operon code for?
a) β-galactosidase
b) Transacetylase
c) Permease
d) Glucagon
View Answer

Answer: c
Explanation: The structural gene (z) of the lac operon codes for β-galactosidase. It is responsible for the hydrolysis of polysaccharides. The ‘y’ genes code for permease. It increases the permeability of a cell to β-galactosidase. The ‘a’ genes code for transacetylase.

4. Which of the following is responsible for the switching on and off of the lac operon?
a) Lactose
b) Ethanol
c) Malate
d) Fructose
View Answer

Answer: a
Explanation: The substrate for the enzyme β-galactosidase is lactose. The lac operon can either be in its switched on or off position. This operation is being regulated by lactose which is also known as the inducer.

5. Which of the following is the necessary condition for the entry of lactose into the cell?
a) High level expression of lac operon
b) Low level expression of lac operon
c) Absence of lac operon in the cell
d) Absence of proteins in the cell
View Answer

Answer: b
Explanation: The action of the enzyme protease helps lactose to enter into the cell. But this can be achieved only if a low-level expression of the lac operon is present inside the cell. Otherwise, lactose cannot enter the cell.

6. What is the regulation of a lac operon by a repressor known as?
a) Neutral regulation
b) Positive regulation
c) Mixed regulation
d) Negative regulation
View Answer

Answer: d
Explanation: The regulation of a lac operon by the repressor is known as negative regulation. At rare occasions, lac operons are also observed to be under the control of positive regulation. In negative regulation, the operon cannot transcribe the RNA polymerase enzyme.

Sanfoundry Global Education & Learning Series – Biology – Class 12.


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