GRE Subject Test: Biology : Understanding Viral Life Cycles and Replication

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

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Example Questions

Example Question #1 : Understanding Viral Life Cycles And Replication

Which of the following structures will never be found in a virus?

Possible Answers:

RNA

Ribosome

DNA

Lipids

Correct answer:

Ribosome

Explanation:

A virus will either contain RNA or DNA; never both. These nucleic acids are essential for carrying and transmitting the viral genome. Some viruses can also have a lipid-rich envelope surrounding them.

Since viruses hijack cellular ribosomes in order to create their needed proteins, there will never be a virus containing ribosomes.

Example Question #1 : Viruses

A mad scientist is working with a rare retrovirus under the hood, when he accidentally spills the viral sample over three live tissue cultures. Assuming the cultures were composed of stem cells, epithelial cells, and muscle cells, in which sample would the mad scientist expect to find the lowest viral titer?

Possible Answers:

Stem cells

There would be no significant difference in viral titer between the three cell types

Muscle cells

Epithelial cells

Correct answer:

Muscle cells

Explanation:

This question requires knowledge of two concepts: first, that retroviruses integrate their own genetic material into the host and are propagated during normal cell division; second, that some cell types do not continue to divide once they have reached maturity (muscle and nerve).

Example Question #2 : Viruses

Prions are the suspected cause of a wide variety of neurodegenerative diseases in mammals. According to prevailing theory, prions are infectious particles made only of protein and found in high concentrations in the brains of infected animals. All mammals produce normal prion protein, PrPC, a transmembrane protein whose function remains unclear. 

Infectious prions, PrPRes, induce conformational changes in the existing PrPC proteins according to the following reaction:

PrPC  + PrPRes  → PrPRes + PrPRes

The PrPRes is then suspected to accumulate in the nervous tissue of infected patients and cause disease. This model of transmission generates replicated proteins, but does so bypassing the standard model of the central dogma of molecular biology. Transcription and translation apparently do not play a role in this replication process.

This theory is a major departure from previously established biological dogma. A scientist decides to test the protein-only theory of prion propagation. He establishes his experiment as follows:

Homogenized brain matter of infected rabbits is injected into the brains of healthy rabbits, as per the following table:

Rabbit 1 and 2: injected with normal saline on days 1 and 2

The above trials serve as controls.

Rabbit 3 and 4: injected with homogenized brain matter on days 1 and 2

The above trials use unmodified brain matter.

Rabbit 5 and 6: injected with irradiated homogenized brain matter on days 1 and 2

The above trials use brain matter that has been irradiated to destroy nucleic acids in the homogenate.

Rabbit 7 and 8: injected with protein-free centrifuged homogenized brain matter on days 1 and 2

The above trials use brain matter that has been centrifuged to generate a protein-free homogenate and a protein-rich homogenate based on molecular weight.

Rabbit 9 and 10: injected with boiled homogenized brain matter on days 1 and 2

The above trials use brain matter that have been boiled to destroy any bacterial contaminants in the homogenate.

Some scientists argue that there must be a virus or bacterium that cause infectious diseases, and claim that there is likely a heretofore undiscovered microbe causing neurodegeneration that most scientists claim are caused by PrPRes. What is a key way for these scientists to distinguish viruses from bacteria?

Possible Answers:

Bacteria have mitochondria, viruses do not.

Viruses have ribosomes, bacteria do not.

Bacteria have nuclear pores, viruses do not.

Bacteria have true membrane bound organelles, viruses do not.

Bacteria have ribosomes, viruses do not.

Correct answer:

Bacteria have ribosomes, viruses do not.

Explanation:

Bacteria have ribosomes to facilitate protein synthesis, but lack other membrane bound organelles such as nuclei, Golgi, or mitochondrion. Viruses lack all of these, including ribosomes.

Example Question #2 : Understanding Viral Life Cycles And Replication

Which of the following is not true of retroviruses? 

Possible Answers:

Harbor their own reverse transcriptase

Contain single stranded postive sense RNA

Integrate DNA into host genome via integrase

All of the answers are characteristic of retroviruses

Harbor their own DNA polymerase

Correct answer:

Harbor their own DNA polymerase

Explanation:

Retroviruses do not harbor their own DNA polymerase. Once inside a host cell, the viral reverse transcriptase reverse transcribes its ssRNA to DNA. At this point, integrase incorporates its synthesized DNA into the host genome. Expression of this viral DNA utilizes host cellular machinery. 

Example Question #3 : Understanding Viral Life Cycles And Replication

Which of the following is not part of the lysogenic cycle?

Possible Answers:

Integration into the host chromosome

Utilization of host machinery by the virus

Reproduction of the host cell

Formation of a prophage

Insertion of viral material into a host cell

Correct answer:

Utilization of host machinery by the virus

Explanation:

The lytic cycle is a nondestructive viral cycle in which viral genetic code integrates into the host chromosome but allows the host to live normally. The virus does not take over cell machinery (in order to churn out new viral parts) until it enters the destructive lytic cycle. All other answer choices are indeed parts of the lysogenic cycle.

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