GRE Subject Test: Biology : Understanding Bacteriophages

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #2 : Microbiology

Which of the following is the best description of a bacteriophage?

Possible Answers:

A non-living organism

An obligate intracellular parasite

A living organism

A fungus

A prokaryote

Correct answer:

An obligate intracellular parasite

Explanation:

A bacteriophage is a virus that infects bacteria. They are not considered living (becuase they cannot replicate on their own) organisms, nor are they techincally considered non-living organisms. They are called obligate intracellular organisms, because they are parasites (kill the cell) that require a host in order to replicate. 

Example Question #1 : Understanding Bacteriophages

A lytic bacteriophage produces "substance X," which causes a host cell to lyse. At which point would the gene for substance X most likely be expressed?

Possible Answers:

After integration into the host chromosome

Immediately after viral infection

Prior to viral infection

Late after viral infection

After all viruses are released

Correct answer:

Late after viral infection

Explanation:

Substance X would logically be released in the later part of viral infection. This would be referred to as a "late gene." The reason is that the virus needs time to replicate and assemble its progeny in the host. Lysing the cell immediately after infection would be futile since no progeny could be created. Lytic viruses use the lysis event to escape from the cell, so it cannot occur after viral release. Finally, lytic viruses typically do not integrate into the host chromosome, lysogenic viruses do.

Example Question #2 : Understanding Bacteriophages

What allows bacteriophages to only infect bacteria cells?

Possible Answers:

They carry out the lytic cycle 

They attach to lipopolysaccharide and teichoic acid surface receptors 

They are retroviruses

They harbor mosaic genomes due to site-specific recombination or illegitimate recombination

They carry out the lysogenic cycle

Correct answer:

They attach to lipopolysaccharide and teichoic acid surface receptors 

Explanation:

The cell surface receptors that a virus recognizes and binds provides specificity for the organism(s) that it infects. Both lipopolysaccharides and teichoic acids are cell surface proteins of bacteria. Bacteriophages have very diverse mosaic genomes and replicate through both the lytic and lysogenic cycles. Retroviruses are RNA-positive strand viruses, which uses reverse transcriptase to synthesize DNA from an RNA template. Its DNA form of its genome is integrated into the host cell's genome.

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