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Operon Concept (Level : Moderate)

Description: proteins
Number of Questions: 15
Created by:
Tags: proteins Gene Expression and Regulation Regulation of Gene Expression
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What happens in lac operon when glucose is present?

  1. Cyclic AMP is high, the catabolite activator protein (CAP) binds to the activator binding site and transcription of lactose is turned on.

  2. Cyclic AMP is low, CAP does not bind to the activator binding site and transcription is turned off.

  3. Cyclic AMP is low, CAP binds to the activator binding site and transcription of lactose is turned on.

  4. Cyclic AMP is high, CAP does not bind to the activator binding site and transcription of lactose is turned on.

Show answer
Correct Option: B
Explanation:

When glucose is present, the concentration of cyclic AMP is low, CAP is unable to bind to the activator binding site and transcription cannot take place.

What happens in lac operon when both glucose and lactose are present in the cell?

  1. CAP binds to the activator binding site.

  2. RNA polymerase binds to the promoter.

  3. There is little or insufficient cyclic AMP.

  4. Lactose binds to the repressor and transcription takes place.

Show answer
Correct Option: C
Explanation:

There is insufficient cyclic AMP when equal amounts of both glucose and lactose are present in the cell.

What is the function of lac repressor?

  1. It binds to the CAP site and prevents transcription.

  2. Lac repressor binds to the CAP site and facilitates transcription.

  3. Lac repressor binds to the operator and prevents transcription.

  4. Lac repressor binds to the operator and facilitates transcription.

Show answer
Correct Option: C
Explanation:

When lac repressor binds to the operator, transcription is blocked.

In lac operon, RNA polymerase binds to

  1. operator

  2. promoter

  3. activator

  4. structural proteins

Show answer
Correct Option: B
Explanation:

In lac operon, RNA polymerase binds to the promoter and carries out transcription.

The lacY gene of lac operon codes for

  1. β-galactosidase

  2. lactose permease

  3. transacetylase

  4. RNA polymerase

Show answer
Correct Option: B
Explanation:

Lactose permease is encoded by Y-gene of lac operon.

Repressor proteins are encoded by

  1. promoter genes

  2. operator genes

  3. regulatory genes

  4. structural genes

Show answer
Correct Option: C
Explanation:

Repressor proteins are encoded by regulatory genes. 

Which of the following structural genes encodes for enzyme that is responsible for cleavage of lactose into glucose and galactose?

  1. LacY

  2. LacZ

  3. LacA

  4. LacL

Show answer
Correct Option: B
Explanation:

LacZ encodes for enzyme β-galactosidase that cleaves the disaccharide lactose into glucose and galactose.

Which of the following enzymes is not produced by E. coli during lactose metabolism?

  1. Lactose dehydrogenase

  2. Galactoside O-acetyltransferase

  3. Beta-galactosidase

  4. Lactose permease

Show answer
Correct Option: A
Explanation:

Lactose dehydrogenase is not produced by E. coli during lactose metabolism.

An operon is a

  1. functioning unit of chromosome containing a cluster of DNA molecules under the control of a single promoter

  2. functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter

  3. functioning unit of genomic DNA containing a cluster of genes under the control of a single operator

  4. functioning unit of chromosome containing a cluster of DNA molecules under the control of a single operator

Show answer
Correct Option: B
Explanation:

An operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single promoter.

Lac operon is an example of

  1. negative repressible operons

  2. negative inducible operons

  3. positive inducible operons

  4. positive repressible operons

Show answer
Correct Option: B
Explanation:

The lac operon is a negatively-controlled inducible operon, where the inducer molecule is allolactose.

Which of the following proteins is not regulated by the repressor in lac operon of E. coli?

  1. Beta-galactosidase

  2. Lactose permease

  3. Tryptophan

  4. Transacetylase

Show answer
Correct Option: C
Explanation:

Tryptophan is not regulated by repressor in lac operon. This protein is not induced in lac operon.

A polycistronic mRNA

  1. carries the information for more than one type of protein

  2. carries the information for a single type of protein

  3. contains codons of more than one cistron

  4. Both (1) and (3)

Show answer
Correct Option: D
Explanation:

Polycistronic mRNA contains codons of more than one cistron. 

Which of the following statements is true?

  1. The lac repressor binds with lactose and transcription occurs when both glucose and lactose are present in the cell.

  2. When both glucose and lactose are absent, transcription occurs.

  3. In the absence of glucose, when lactose is present, it combines with the repressor, allowing RNA polymerase to carry on transcription.

  4. When glucose is present, cyclic AMP is high, CAP does not bind to the activator binding site and transcription of lactose is turned on.

Show answer
Correct Option: C
Explanation:

This is a true statement.

The lac operator is located

  1. adjacent to activator

  2. partially within the promoter

  3. within the structural genes

  4. just before terminator

Show answer
Correct Option: B
Explanation:

The lac operator lies partially within the promoter and interacts with a regulatory protein that controls the transcription of the operon.

Who got the Nobel Prize in 1965 for the discovery of operon?

  1. Beadle and Tatum

  2. Temin and Baltimore

  3. Jacob and Monad

  4. John Otto

Show answer
Correct Option: C
Explanation:

Jacob and Monad got the Nobel Prize in 1965 for the discovery of operon.

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