Q: Which of the following equations correctly represents the formation of a phosphodiester bond by DNA ligase?

$$5'\text{-}PO_4$ + 3'\text{-}OH \xrightarrow{\text{DNA ligase}} 5'\text{-}O\text{-}P\text{-}O\text{-}3'$$

Question 1

Which of the following best describes the role of a corepressor in the regulation of a repressible operon such as the tryptophan operon?

Accepted Answer

It binds to the repressor protein, activating it so it can bind to the operator and block transcription.

Question 2

In the absence of tryptophan, what is the state of the repressor protein in the tryptophan operon, and what is the outcome for gene expression?

Accepted Answer

The repressor is inactive and genes are transcribed.

Question 3

Which of the following equations correctly represents the formation of a phosphodiester bond by DNA ligase?

Accepted Answer

$$5'	ext{-}PO_4$ + 3'	ext{-}OH \xrightarrow{	ext{DNA ligase}} 5'	ext{-}O	ext{-}P	ext{-}O	ext{-}3'$$

Question 4

A bacterium is exposed to a new sugar. The genes for metabolizing this sugar are organized in an inducible operon. Which of the following best describes what happens when the sugar is present?

Accepted Answer

The sugar acts as an inducer, inactivating the repressor protein and allowing transcription.

Question 5

Which of the following is a key difference between an operon and a regulon?

Accepted Answer

An operon is a set of genes transcribed as a polycistronic message, while a regulon is a set of genes controlled by the same regulatory protein but transcribed separately.

Question 6

Which of the following best describes the mechanism of noncompetitive (allosteric) inhibition in enzyme regulation?

Accepted Answer

The inhibitor binds to a site other than the active site, changing the enzyme's shape and preventing substrate binding.

Question 7

During bacterial transformation, which protein is essential for the exchange of homologous DNA segments between donor and recipient DNA?

Accepted Answer

Rec A protein

Question 8

Which of the following scenarios best illustrates generalized transduction?

Accepted Answer

A lytic phage accidentally packages a random fragment of bacterial DNA and transfers it to another bacterium.

Question 9

Which of the following is a direct consequence of Hfr conjugation in bacteria?

Accepted Answer

The recipient receives some chromosomal DNA but usually does not become F+.

Question 10

A restriction endonuclease recognizes the following palindromic sequence: $$5'	ext{-}GAATTC	ext{-}3'. $$After cleavage, what type of ends are produced?

Accepted Answer

Sticky (cohesive) ends

Question 11

Which of the following best explains why resistance plasmid (R-plasmid) conjugation is a major clinical concern?

Accepted Answer

It enables rapid spread of multiple antibiotic resistance among pathogenic bacteria.

Question 12

Which of the following is true regarding feedback inhibition in metabolic pathways?

Accepted Answer

The end product binds to an allosteric site, inhibiting the first enzyme and shutting down the pathway.

Question 13

Which of the following statements about competitive inhibition is correct?

Accepted Answer

The inhibitor competes with the substrate for binding to the active site.

Question 14

Which of the following best describes the function of antisense RNA in bacterial translational control?

Accepted Answer

It binds to mRNA, preventing translation and enzyme synthesis.

Question 15

A mutation in the operator region of the lactose operon prevents the repressor from binding. What is the most likely outcome?

Accepted Answer

The operon will be constitutively expressed, regardless of lactose presence.

Question 16

Which of the following is a necessary step for successful transformation in bacteria?

Accepted Answer

The recipient must be competent to take up DNA.

Question 17

Which of the following best describes the role of an inducer in positive control of gene expression?

Accepted Answer

It binds to an activator protein, enabling it to bind to the activator-binding site and promote transcription.

Question 18

Which of the following is the correct sequence of events in the integration of donor DNA into recipient DNA during homologous recombination?

Accepted Answer

DNA binding, Rec A-mediated strand exchange, integration

Question 19

Which of the following is a real-world consequence of the spread of R-plasmids among Gram-negative bacteria in hospitals?

Accepted Answer

Increased incidence of multidrug-resistant infections such as urinary tract infections and pneumonia.

Question 20

Which of the following best explains why restriction endonucleases do not degrade the host bacterium's own DNA?

Accepted Answer

The host DNA is methylated at the recognition sites, protecting it from cleavage.

Enzyme Regulation and Genetic Recombination in Bacteria

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