GRE Subject Test: Biology : Divisions of Life and Viruses

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

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Example Questions

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Example Question #1 : Viruses

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

Possible Answers:

A fungus

An obligate intracellular parasite

A non-living organism

A prokaryote

A living organism

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 : Viruses

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 all viruses are released

Late after viral infection

Immediately after viral infection

Prior to viral infection

After integration into the host chromosome

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 #1 : Divisions Of Life And Viruses

What allows bacteriophages to only infect bacteria cells?

Possible Answers:

They are retroviruses

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

They carry out the lysogenic cycle

They attach to lipopolysaccharide and teichoic acid surface receptors 

They carry out the lytic 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.

Example Question #1 : Divisions Of Life And Viruses

Which of the following statements describes a lysogenic infection?

Possible Answers:

A lysogenic infection can only be done by DNA viruses

The virus incorporates its nucleic acid into the host's genome

The virus will inject its genome into the host cell through its tail

The virus enters the cell, hijacks host reproductive machinery, and creates new viruses that will destroy the cell

Correct answer:

The virus incorporates its nucleic acid into the host's genome

Explanation:

A lysogenic infection involves the virus entering the cell and incorporating its nucleic acid into the host genome. RNA viruses are capable of this type of infection, but must first have their RNA be converted to DNA using the enzyme reverse transcriptase. The DNA genome remains dormant in the cell as it undergoes cellular divisions, increasing the number of cells that contain the viral DNA. This stage is known as the latent period.

Eventually, the viral genome will be stimulated and enter into the lytic viral cycle. During this phase, the virus hijacks the cell ribosomes and produces large quantities of the viral genome and proteins. Eventually the virus causes the cell to lyse, releasing more virions into the host.

Example Question #2 : Viruses

Which of the following is not a component of HIV?

Possible Answers:

Ribosomes

RNA

Reverse transcriptase

Proteases

Protein

Correct answer:

Ribosomes

Explanation:

HIV is a retrovirus, meaning that it contains RNA as its genetic material, and thus reverse transcriptase to code for DNA from the RNA template. It also has a protein coat, and must use proteases in order to degrade the host cell membrane to inject its RNA. It does not, however, have ribosomes. It will eventually use the host ribosomes to translate its own genetic material.

Example Question #1 : Microbiology

Which of the following is not a described type of virus?

I.  A virus containing double strand DNA

II.  A virus containing single strand DNA

III.  A virus containing single strand RNA and single strand DNA

IV.  A virus containing single strand RNA

V.  A virus containing double strand RNA

Possible Answers:

III

V

I

IV

II

Correct answer:

III

Explanation:

The question addresses the Baltimore classification system for viral genetic information.  Viruses contain some form of DNA or RNA, but never both.

Example Question #3 : Viruses

What molecule would you not expect to find in a retrovirus?

Possible Answers:

Adenine

Cytosine

Uracil

Thymine

Guanine

Correct answer:

Thymine

Explanation:

Retroviruses carry RNA and when they infect a host cell, they use their own reverse transciptase to made DNA from that RNA; therefore, within the virus, there are only the molecules that make up RNA. Uracil is used in RNA place of thymine, which can be found in DNA. Cytosine, guanine, and adenine can be found in both DNA and RNA.

Example Question #2 : Microbiology

Sexually transmitted diseases are a common problem among young people in the United States. One of the more common diseases is caused by the bacterium Neisseria gonorrhoeae, which leads to inflammation and purulent discharge in the male and female reproductive tracts.

The bacterium has a number of systems to evade host defenses. Upon infection, it uses pili to adhere to host epithelium. The bacterium also uses an enzyme, gonococcal sialyltransferase, to transfer a sialyic acid residue to a gonococcal surface lipooligosaccharide (LOS). A depiction of this can be seen in Figure 1. The sialyic acid residue mimics the protective capsule found on other bacterial species.

Once infection is established, Neisseria preferentially infects columnar epithelial cells in the female reproductive tract, and leads to a loss of cilia on these cells. Damage to the reproductive tract can result in pelvic inflammatory disease, which can complicate pregnancies later in the life of the woman.

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Which of the following is true of Neisseria, but not true of a virus?

Possible Answers:

Neisseria has ribosomes; viruses do not have ribosomes

Neisseria does not have flagella; all viruses have flagella

Neisseria infects only one cell type; viruses have wide variety in their cellular targets

Neisseria contains DNA; all viruses contain RNA

Neisseria is enveloped; no viruses are enveloped

Correct answer:

Neisseria has ribosomes; viruses do not have ribosomes

Explanation:

Neisseria synthesizes proteins as a free living prokaryote. Viruses are generally dependent on host cellular machinery, and synthesize their proteins upon hijacking of a host ribosome.

Example Question #1 : Viruses

What component is common for all viruses?

Possible Answers:

A protein coat

DNA

RNA

A protective envelope

Correct answer:

A protein coat

Explanation:

All viruses contain a protein coat (or capsule) that protects the genetic material (which can be either DNA or RNA). In some instances (such as herpes simplex), the virus can also contain an lipid envelope that serves as a membrane.

Example Question #2 : Divisions Of Life And Viruses

Which of the following best describes retroviral DNA that has been incorporated into a host genome? 

Possible Answers:

Exogenous retrovirus 

Provirus

Retroelements

Virion

Endogenous retrovirus

Correct answer:

Provirus

Explanation:

The correct answer is provirus. A retrovirus uses its own reverse transcriptase to synthesize DNA from its RNA. This DNA is then incorporated into the host genome via the integrase enzyme. The viral DNA in the host genome is referred to as a provirus; the host cell transcribes and translates the provirus to produce new copies of the virus, along with its own genome.

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