This set of Tough Molecular Biology Questions and Answers focuses on “Regulation at the Level of Transcription Involves Several Steps”.
1. In case of transcription which is the rate limiting step?
a) Binding of RNA polymerase
b) Unwinding of DNA duplex
c) Promoter escape
d) Formation of the open complex
Explanation: When polymerase binds it spontaneously undergoes a transition to the open complex and initiates transcription. This gives a low level of constitutive expression called the basal level. Thus binding of RNA polymerase is the rate limiting step.
2. The site where repressor binds the DNA is the ______________
Explanation: Operator is the repressor binding site. Promoter is the polymerase binding site. Terminator is the site for transcription termination. ORF is the open reading frame in the mRNA used by the ribosome for translation.
3. Recruitment of RNA polymerase to the promoter is a ___________
a) Covalent binding
b) Cooperative binding
c) Protein – protein interaction
d) Van der Waals forces
Explanation: The activator uses one surface to bind to a site on the DNA near the promoter; with another surface, the activator simultaneously interacts with the RNA polymerase, bringing the enzyme close to the promoter. This mechanism is known as the recruitment of polymerase and is an example of cooperative binding of proteins to DNA.
4. The proteins of which of the following genes are regularly needed for cellular activity?
a) Regular genes
b) Smart genes
c) Structural genes
d) Housekeeping genes
Explanation: Housekeeping genes are those genes that are typically constitutive genes. They are generally required for the maintenance of basic cellular functions. They are thus expressed in all cells of an organism under normal conditions.
5. Which of the following are the products of constitutive gene?
i. Involved in the synthesis of structural elements of the cell
ii. Involved in catabolic pathway
iii. Involved in anabolic pathway
iv. Synthesized continuously
a) i and ii
b) ii and iii
c) iii and iv
d) i, ii, iii and iv
Explanation: The products of the constitutive genes are generally used all the time for different cellular processes. They include all metabolic, that is, catabolic and anabolic processes and production of structural elements of the cell. Thus they are synthesized continuously within the cell.
6. Which of the following is not an example of allosteric regulation?
a) Inactivation of nitrogenase by ADP ribosylation
b) Catabolite repression by CAP in E. coli
c) Regulation of phosphofructokinase activity
d) Regulation of lac operon by allolactose
Explanation: When the nitrogenase product, ammonium, becomes available, the ADP ribosyltransferase covalently links to the ADP-ribose moiety to a specific arginine residue on nitrogenase switching-off nitrogenase activity. Thus as it involves covalent reaction and is not an example of allosteric regulation.
7. Operons are ___________
a) of approximately uniform size
b) found in some eukaryotes
c) not able to bind to proteins
d) smaller in lower eukaryotes and longer in higher eukaryotes
Explanation: Operons are generally of varied lengths as the number of genes differs from operon to operon. They are able to bind to different proteins such as activators, repressors, polymerases, etc. their size does not depend on the genetic complexity of the eukaryotes and are generally not found in higher eukaryotes.
8. Which of the following are correct according to operons?
i) Consists of multiple genes organized between a promoter and a terminator
ii) Member genes regulate with coordination
iii) Usually controlled by autogenous transcription
iv) Contains genes for stress tolerance exclusively
a) i and ii
b) ii and iii
c) iii and iv
d) i, ii, iii and iv
Explanation: An operon is a functionating unit of genomic DNA containing a cluster of genes under the control of a single promoter. Thus the two ends of an operon are marked by the promoter and terminator on either side. As they are regulated by a single promoter, they work in coordination.
9. Choose the correct pair among the following.
a) The DNA sequence to which stigma factor binds – enhancer
b) The DNA sequence to which RNA polymerase binds – operator
c) The DNA sequence that codes for a protein – structural gene
d) The DNA sequence to which repressors binds – promoter
Explanation: The correct pairs are:
i) The DNA sequence to which stigma factor binds – promoter
ii) The DNA sequence to which RNA polymerase binds – promoter
iii) The DNA sequence to which repressors binds – operator.
10. Which of the following is not present in the β-galactosidase structural gene and thus is not transcribed by the same promoter?
a) Lac A
b) Lac I
c) Lac Z
d) Lac Y
Explanation: β-galactosidase structural gene consists of three genes, lac A, lac Z and lac Y. They synthesize β-galactoside transacetylase, β-galactosidase and β-galactoside permease respectively.
11. Glucose, as the carbon source, is the first choice by bacteria even if other sugars are available. The mechanism behind this selectivity is ___________
a) Operon repression
b) Glucose utilization
c) Enzyme repression
d) Catabolite repression
Explanation: Catabolite repression allows bacteria to adapt quickly to a preference carbon and energy source first. As the primary source of carbon for bacteria is glucose, thus it takes up glucose even if other sugars are present in abundance.
12. The lac operon is regulated by cAMP by ___________
a) Binding to lac repressor
b) Binding to operator
c) Binding to promoter
d) Binding to catabolite activator protein
Explanation: The cAMP binds to catabolite activator protein to regulate the activity of the lac operon. This enables the formation of a cAMP-activator complex that turns on transcription after it binds near the promoter.
13. Tryptophan operon in E. coli is a repressible operon.
Explanation: Tryptophan operon is a repressible negative regulated operon. Within the operon’s regulatory sequence, the operator is blocked by the repressor protein in the presence of tryptophan and is liberated in tryptophan’s absence. The process of attenuation complements this regulatory action.
14. Which of the following is true about tryptophan operon?
a) Tetramer of identical subunits
b) The RNA product is very stable
c) The Trp repressor binds to the tryptophan
d) The Trp repressor is the product of the Trp operon
Explanation: The tryptophan operon contains a gene trp R which synthesizes mRNA for a regulatory protein. This protein binds to tryptophan molecule and further interacts with the operator of the trp operon, thus effectively blocking transcription of the gene.
15. The expression of the tryptophan operon is independent of the availability of tryptophan in culture media.
Explanation: The expression of the tryptophan operon is dependent of the availability of tryptophan in culture media. This type of regulatory process is termed as attenuation that is, the operator is blocked by the repressor protein in the presence of tryptophan and is liberated in the presence of tryptophan.
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