This set of Cell Biology Multiple Choice Questions & Answers (MCQs) focuses on “Gene Expression Control – Bacteria”.
1. Lactose can be a nutrient source for bacteria, it is a _____________________
a) monosaccharide
b) lipid
c) disaccharide
d) polysaccharide
View Answer
Explanation: Lactose is a disaccharide comprised of glucose and galactose. Oxidation of this disaccharide can provide a bacterial cell with metabolic intermediates and energy.
2. The first step in catabolism of lactose by the bacteria is ________________ of a linkage bond.
a) hydrolysis
b) oxidation
c) reduction
d) alkylation
View Answer
Explanation: The two sugars, glucose and galactose are linked through a β-galactosidase linkage. The first step in lactose catabolism is the hydrolysis of this linkage by the bacterial enzyme β-galactosidase.
3. Under minimal conditions, the cell has fewer than 5 copies of enzyme β-galactosidase.
a) True
b) False
View Answer
Explanation: Under minimal conditions, when glucose is present as an energy source there is no need for the cell to utilize lactose. Under such conditions an average cell contains fewer than 5 copies of enzyme β-galactosidase and single copy of associated mRNA.
4. Enzymes of __________________________ are clustered together in a bacterial operon.
a) metabolic pathway
b) transcription
c) transfusion
d) transformation
View Answer
Explanation: The genes that encode enzymes for metabolic pathways are clustered together on the chromosome in a function complex called a bacterial operon.
5. When was the operation mechanism of a bacterial operon first elucidated?
a) 1961
b) 1971
c) 1981
d) 1991
View Answer
Explanation: The mechanism by which all the genes of an operon operate, was first elucidated in 1961 by Francois Jacob and Jacques Monod of the Pasteur Institute in Paris. An operon consists of structural genes, a promoter region, an operator region and a regulator gene.
6. In a bacterial operon, which is located downstream of the structural genes?
a) operator
b) inducer
c) promoter
d) regulatory gene
View Answer
Explanation: In a bacterial operon, promoter region and operator region are located upstream of the cluster of structural genes, and the regulatory genes are located downstream of the structural genes.
7. The structural genes in an operon lie adjacent to one another.
a) True
b) False
View Answer
Explanation: The structural genes code for the enzymes involved in metabolic pathways. The genes are located adjacent to each other and the RNA polymerase moves through the structural genes one after the other, transcribing all of the genes into a single mRNA.
8. Which is a DNA-binding protein?
a) repressor
b) operator
c) luciferase
d) thymidine
View Answer
Explanation: Repressor is a DNA-binding protein that recognizes a specific sequence of base pairs within the DNA and binds to the sequence with high affinity. The binding site for the repressor is called the operator region.
9. The repressor protein is encoded by _________________
a) regulatory gene
b) structural gene
c) transfer-DNA
d) ribosomal-RNA
View Answer
Explanation: The repressor proteins control the transcription of genes located downstream of the operator regions. The repressor protein is encoded by the regulatory gene which is present downstream of the structural genes.
10. The capability of the repressor to bind the operator depends upon _____________
a) pH
b) conformation
c) temperature
d) moisture
View Answer
Explanation: The capability of repressor to bind to the operator region and prevent the transcription of structural genes depends on the conformation of the repressor protein. The conformation is regulated by compounds of metabolic pathways such as lactose and trypyophan.
11. The lac operon consists of ____ structural genes.
a) 1
b) 2
c) 3
d) 4
View Answer
Explanation: The lac operon is an inducible operon that consists of 3 tandem structural genes namely the z gene, y gene and the gene; encoding the β-galactosidase, galactoside permease and thiogalactoside transacetylase respectively.
12. Which of the following acts as an inducer in the lac operon?
a) glucose
b) tryptophan
c) lactose
d) galactose
View Answer
Explanation: When lactose is present in the medium, it binds to the repressor protein changing its conformation while preventing the former’s binding at the operator region. Thus the structural genes are transcribed and lactose is catabolized, therefore lactose acts as an inducer in the lac operon.
13. The lac operon is under positive control, a phenomenon called _________________
a) regulation
b) mutation
c) lactose-effect
d) glucose-effect
View Answer
Explanation: The lac operon is negatively regulated by repressor proteins. There is also a positive control that prevails in the lac operon, called the glucose-effect. When glucose is present in the medium along with other substrates, the catabolism of the letter is suppressed.
14. Which of the following acts as a co-repressor in tryptophan operon?
a) tryptophan
b) tyrosine
c) glucose
d) lactose
View Answer
Explanation: Unlike lactose operon which is an inducible operon, the tryptophan operon is a repressible operon. Presence of tryptophan prevents the transcription of synthesis machinery requisite for the production of tryptophan by the cell. Tryptophan in this case is therefore a co-repressor.
15. In bacteria, mRNAs bound to small metabolites are called ______________
a) euchromatin
b) riboswitches
c) heterochromatin
d) nucleosome
View Answer
Explanation: In bacteria, mRNAs bound to small metabolites such as glucosamine and adenine are called riboswitches that can alter the expression of a gene involved in the production of bound metabolite.
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